Integrative Physiology I: Control of Body Movement

Transcription

1 3 Integrative Physiology I: Control of Body Movement Neural Reflexes Neural Reflex Pathways Can Be Classified in Different Ways Autonomic Reflexes Skeletal Muscle Reflexes Muscle Spindles Respond to Muscle Stretch Golgi Tendon Organs Respond to Muscle Tension Stretch Reflexes and Reciprocal Inhibition Control Movement Around a Joint Flexion Reflexes Pull Limbs away from Painful Stimuli The Integrated Control of Body Movement Movement Can Be Classified as Reflex, Voluntary, or Rhythmic The CNS Integrates Movement Symptoms of Parkinson s Disease Reflect Basal Ganglia Function Control of Movement in Visceral Muscles Extracting signals directly from the brain to directly control robotic devices has been a science fiction theme that seems destined to become fact. Dr. Eberhard E. Fetz, Science News 56: 4, 8/8/99 Background Basics Reflex pathways Central nervous system Summation of action potentials Isometric contraction Sensory pathways and receptors Graded potentials Tonic control Tendons Each dot of a microarray represents one gene. Genes that are active show up in bright colors. From Chapter 3 of Human Physiology: An Integrated Approach, Sixth Edition. Dee Unglaub Silverthorn. Copyright 03 by Pearson Education, Inc. All rights reserved. 465

2 Think of a baseball pitcher standing on the mound. As he looks at the first batter, he receives sensory information from multiple sources: the sound of the crowd, the sight of the batter and the catcher, the smell of grass, the feel of the ball in his hand, and the alignment of his body as he begins his windup. Sensory receptors code this information and send it to the central nervous system (CNS), where it is integrated. Th e pitcher acts consciously on some of the information: he decides to throw a fastball. But he processes other information at the subconscious level and acts on it without conscious thought. As he thinks about starting his motion, for instance, he shifts his weight to offset the impending movement of his arm. The integration of sensory information into an involuntary response is the hallmark of a reflex. Neural Reflexes All neural reflexes begin with a stimulus that activates a sensory receptor. The sensor sends information in the form of action potentials through sensory afferent neurons to the CNS. The CNS is the integrating center that evaluates all incoming information and selects an appropriate response. It then initiates action potentials in efferent neurons to direct the response of muscles and glands the targets. A key feature of many reflex pathways is negative feedback. Feedback signals from muscle and joint receptors keep the CNS continuously informed of changing body position. Some reflexes have a feedforward component that allows the body to anticipate a stimulus and begin the response. Bracing yourself in anticipation of a collision is an example of a feedforward response. Neural Reflex Pathways Can Be Classified in Different Ways Reflex pathways in the nervous system consist of chains or networks of neurons that link sensory receptors to muscles or glands. Neural reflexes can be classified in several ways ( Tbl. 3. ): By the efferent division of the nervous system that controls the response. Reflexes that involve somatic motor neurons and skeletal muscles are known as somatic reflexes. Reflexes whose responses are controlled by autonomic neurons are called autonomic reflexes. By the CNS location where the reflex is integrated. Spinal reflexes are integrated in the spinal cord. These reflexes may be modulated by higher input from the brain, but they can occur without that input. Reflexes integrated in the brain are called cranial reflexes. 3 By whether the reflex is innate or learned. Many reflexes are innate ; in other words, we are born with them, and they are genetically determined. One example is the knee jerk, or patellar tendon reflex: when the patellar tendon at the lower edge of the kneecap is stretched with a tap from a reflex hammer, the lower leg kicks out. Other reflexes are acquired RUNNING PROBLEM Tetanus She hasn t been able to talk to us. We re afraid she may have had a stroke. That is how her neighbors described 77-yearold Cecile Evans when they brought her to the emergency room. But when a neurological examination revealed no problems other than Mrs. Evans s inability to open her mouth and stiffness in her neck, emergency room physician Dr. Doris Ling began to consider other diagnoses. She noticed some scratches healing on Mrs. Evans s arms and legs and asked the neighbors if they knew what had caused them. Oh, yes. She told us a few days ago that her dog jumped up and knocked her against the barbed wire fence. At that point, Dr. Ling realized she was probably dealing with her first case of tetanus. through experience. The example of Pavlov s dogs salivating upon hearing a bell is the classic example of a learned reflex, also referred to as a conditioned reflex. 4 By the number of neurons in the reflex pathway. The simplest reflex is a monosynaptic reflex, named for the single synapse between the two neurons in the pathway: a sensory afferent neuron (often just called a sensory afferent ) and an efferent somatic motor neuron ( Fig. 3. a). These Classification of Neural Reflexes Neural Reflexes Can Be Classified by: Table 3.. Efferent division that controls the effector a. Somatic motor neurons control skeletal muscles. b. Autonomic neurons control smooth and cardiac muscle, glands, and adipose tissue.. Integrating region within the central nervous system a. Spinal reflexes do not require input from the brain. b. Cranial reflexes are integrated within the brain. 3. Time at which the reflex develops a. Innate (inborn) reflexes are genetically determined. b. Learned (conditioned) reflexes are acquired through experience. 4. The number of neurons in the reflex pathway a. Monosynaptic reflexes have only two neurons: one afferent (sensory) and one efferent. Only somatic motor reflexes can be monosynaptic. b. Polysynaptic reflexes include one or more interneurons between the afferent and efferent neurons. All autonomic reflexes are polysynaptic because they have three neurons: one afferent and two efferent. 466

3 Fig. 3. ESSENTIALS Neural Reflexes SKELETAL MUSCLE REFLEXES (a) A monosynaptic reflex has a single synapse between the afferent and efferent neurons. Stimulus Receptor Skeletal muscle Sensory neuron Spinal cord integrating center Somatic motor neuron Response Target cell Efferent neuron One synapse (b) Polysynaptic reflexes have two or more synapses. This somatic motor reflex has both synapses in the CNS. Stimulus Receptor Sensory neuron Interneuron Synapse Spinal cord integrating center Response Target cell Efferent neuron Synapse AUTONOMIC REFLEXES (c) All autonomic reflexes are polysynaptic, with at least one synapse in the CNS and another in the autonomic ganglion. Stimulus Receptor Sensory neuron CNS integrating center Response Target cell Postganglionic autonomic neuron Autonomic ganglion Preganglionic autonomic neuron 467

4 RUNNING PROBLEM Tetanus {tetanus, a muscle spasm}, also known as lockjaw, is a devastating disease caused by the bacterium Clostridium tetani. These bacteria are commonly found in soil and enter the human body through a cut or wound. As the bacteria reproduce in the tissues, they release a protein neurotoxin. This toxin, called tetanospasmin, is taken up by somatic motor neurons at the axon terminals. Tetanospasmin then travels along the axons until it reaches the nerve cell body in the spinal cord. Q: a. Tetanospasmin is a protein. By what process is it taken up into neurons? b. By what process does it travel up the axon to the nerve cell body? two neurons synapse in the spinal cord, allowing a signal initiated at the receptor to go directly from the sensory neuron to the motor neuron. (The synapse between the somatic motor neuron and its muscle target is ignored.) Most reflexes have three or more neurons in the pathway (and at least two synapses), leading to their designation as polysynaptic reflexes ( Fig. 3. b, c). Polysynaptic reflexes may be quite complex, with extensive branching in the CNS to form networks involving multiple interneurons. Divergence of pathways allows a single stimulus to affect multiple targets. Convergence integrates the input from multiple sources to modify the response. The modification in polysynaptic pathways may involve excitation or inhibition. Autonomic Reflexes Autonomic reflexes are also known as visceral reflexes because they often involve the internal organs of the body. Some visceral reflexes, such as urination and defecation, are spinal reflexes that can take place without input from the brain. However, spinal reflexes are often modulated by excitatory or inhibitory signals from the brain, carried by descending tracts from higher brain centers. For example, urination may be voluntarily initiated by conscious thought. Or it may be inhibited by emotion or a stressful situation, such as the presence of other people (a syndrome known as bashful bladder ). Often, the higher control of a spinal reflex is a learned response. The toilet training we master as toddlers is an example of a learned reflex that the CNS uses to modulate the simple spinal reflex of urination. Other autonomic reflexes are integrated in the brain, primarily in the hypothalamus, thalamus, and brain stem. These regions contain centers that coordinate body functions needed to maintain homeostasis, such as heart rate, blood pressure, breathing, eating, water balance, and maintenance of body temperature. The brain stem also contains the integrating centers for autonomic reflexes such as salivating, vomiting, sneezing, coughing, swallowing, and gagging. An interesting type of autonomic reflex is the conversion of emotional stimuli into visceral responses. The limbic system the site of primitive drives such as sex, fear, rage, aggression, and hunger has been called the visceral brain because of its role in these emotionally driven reflexes. We speak of gut feelings and butterflies in the stomach all transformations of emotion into somatic sensation and visceral function. Other emotion-linked autonomic reflexes include urination, defecation, blushing, blanching, and piloerection, in which tiny muscles in the hair follicles pull the shaft of the hair erect ( I was so scared my hair stood on end! ). Autonomic reflexes are all polysynaptic, with at least one synapse in the CNS between the sensory neuron and the preganglionic autonomic neuron, and an additional synapse in the ganglion between the preganglionic and postganglionic neurons ( Fig. 3. c). Many autonomic reflexes are characterized by tonic activity, a continuous stream of action potentials that creates ongoing activity in the effector. For example, the tonic control of blood vessels is an example of a continuously active autonomic reflex. You will encounter many autonomic reflexes as you continue your study of the systems of the body. Concept Check Answers: End of Chapter. List the general steps of a reflex pathway, including the anatomical structures in the nervous system that correspond to each step.. If a cell hyperpolarizes, does its membrane potential become more positive or more negative? Does the potential move closer to threshold or farther from threshold? Skeletal Muscle Reflexes Although we are not always aware of them, skeletal muscle reflexes are involved in almost everything we do. Receptors that sense changes in joint movements, muscle tension, and muscle length feed this information to the CNS, which responds in one of two ways. If muscle contraction is the appropriate response, the CNS activates somatic motor neurons to the muscle fibers. If a muscle needs to be relaxed to achieve the response, sensory input activates inhibitory interneurons in the CNS, and these interneurons inhibit activity in somatic motor neurons controlling the muscle. 468

5 Excitation of somatic motor neurons always causes contraction in skeletal muscle. There is no inhibitory neuron that synapses on skeletal muscles to cause them to relax. Instead, relaxation results from the absence of excitatory input by the somatic motor neuron. Inhibition and excitation of somatic motor neurons and their associated skeletal muscles must occur at synapses within the CNS. Skeletal muscle reflexes have the following components: Sensory receptors, known as proprioceptors, are located in skeletal muscles, joint capsules, and ligaments. Proprioceptors monitor the position of our limbs in space, our movements, and the effort we exert in lifting objects. The input signal from proprioceptors goes to the CNS through sensory neurons. The central nervous system integrates the input signal using networks and pathways of excitatory and inhibitory interneurons. In a reflex, sensory information is integrated and acted on subconsciously. However, some sensory information may be integrated in the cerebral cortex and become perception, and some reflexes can be modulated by conscious input. 3 Somatic motor neurons carry the output signal. The somatic motor neurons that innervate skeletal muscle contractile fibers are called alpha motor neurons ( Fig. 3. a). MUSCLE SPINDLES AND GOLGI TENDON ORGANS (a) Muscle spindle sends information about muscle stretch to the CNS. Muscle spindles are buried among the extrafusal fibers of the muscle. Extrafusal muscle fibers are normal contractile fibers. Alpha motor neuron innervates extrafusal muscle fibers. Golgi tendon organ links the muscle and the tendon. Tendon Central region lacks myofibrils. Muscle spindle Gamma motor neurons from CNS innervate intrafusal fibers. To CNS Tonically active sensory neurons send information to CNS. Gamma motor neurons from CNS control contraction in intrafusal fibers. 3 Intrafusal fibers are found in muscle spindles. Extrafusal fiber (b) Golgi tendon organ consists of sensory nerve endings interwoven among collagen fibers. Extrafusal muscle fibers FIGURE QUESTIONS. When the muscle shown in (a) is relaxed, which neurons are firing? (a) muscle spindle gamma motor neuron (b) muscle spindle sensory neuron (c) Golgi tendon organ sensory neuron (d) none of the above. Which neuron fires to cause contraction of the extrafusal muscle fibers? (a) muscle alpha motor neuron (b) muscle spindle gamma motor neuron (c) muscle spindle sensory neuron (d) Golgi tendon organ sensory neuron (e) none of the above Capsule Tendon Collagen fiber Sensory neuron Fig

6 4 The effectors are contractile skeletal muscle fibers, also known as extrafusal muscle fibers. Action potentials in alpha motor neurons cause extrafusal fibers to contract. Th ree types of proprioceptors are found in the body: muscle spindles, Golgi tendon organs, and joint receptors. Joint receptors are found in the capsules and ligaments around joints in the body. They are stimulated by mechanical distortion that accompanies changes in the relative positioning of bones linked by flexible joints. Sensory information from joint receptors is integrated primarily in the cerebellum. In the next two sections we examine the function of muscle spindles and Golgi tendon organs, two interesting and unique receptors. These receptors lie inside skeletal muscles and sense changes in muscle length and tension. Their sensory output activates muscle reflexes. Muscle Spindles Respond to Muscle Stretch Muscle spindles are stretch receptors that send information to the spinal cord and brain about muscle length and changes in muscle length. They are small, elongated structures scattered among and arranged parallel to the contractile extrafusal muscle fibers ( Fig. 3. a). With the exception of one muscle in the jaw, every skeletal muscle in the body has many muscle spindles. For example, a small muscle in the index finger of a newborn human has on average about 50 spindles. Each muscle spindle consists of a connective tissue capsule that encloses a group of small muscle fibers known as intrafusal fibers { intra-, within + fusus, spindle}. Intrafusal muscle fibers CLINICAL FOCUS Reflexes and Muscle Tone Clinicians use reflexes to investigate the condition of the nervous system and the muscles. For a reflex to be normal, there must be normal conduction through all neurons in the pathway, normal synaptic transmission at the neuromuscular junction, and normal muscle contraction. A reflex that is absent, abnormally slow, or greater than normal (hyperactive) suggests the presence of a pathology. Interestingly, not all abnormal reflexes are caused by neuromuscular disorders. For example, slowed relaxation of the ankle flexion reflex suggests hypothyroidism. (The cellular mechanism linking low thyroid to slow reflexes is not known.) Besides testing reflexes, clinicians assess muscle tone. Even when relaxed and at rest, muscles have a certain resistance to stretch that is the result of continuous (tonic) output by alpha motor neurons. The absence of muscle tone or increased muscle resistance to being stretched by an examiner (increased tone) indicates a problem with the pathways that control muscle contraction. are modified so that the ends are contractile but the central region lacks myofibrils ( Fig. 3. ). The contractile ends of the intrafusal fibers have their own innervation from gamma motor neurons. The noncontractile central region of each intrafusal fiber is wrapped by sensory nerve endings that are stimulated by stretch. These sensory neurons project to the spinal cord and synapse directly on alpha motor neurons innervating the muscle in which the spindles lie. When a muscle is at its resting length, the central region of each muscle spindle is stretched enough to activate the sensory fibers ( Fig. 3.3 a). As a result, sensory neurons from the spindles are tonically active, sending a steady stream of action potentials to the CNS. Because of this tonic activity, even a muscle at rest maintains a certain level of tension, known as muscle tone. Muscle spindles are anchored in parallel to the extrafusal muscle fibers. Any movement that increases muscle length also stretches the muscle spindles and causes their sensory fibers to fire more rapidly ( Fig. 3.3 b). This creates a reflex contraction of the muscle, which prevents damage from overstretching. The reflex pathway in which muscle stretch initiates a contraction response is known as a stretch reflex. Concept Check Answers:EndofChapter 3. Using the standard steps of a reflex pathway (stimulus, receptor, and so forth), draw a reflex map of the stretch reflex. Muscle stretch activates muscle spindles, but what happens to spindle activity when a resting muscle contracts and shortens? You might predict that the release of tension on the center of the intrafusal fibers in the absence of gamma motor neuron activity would cause the spindle afferents to slow their firing rate, as shown in Figure 3.4 b. However, the presence of gamma motor neurons in a normal muscle keeps the muscle spindles active, no matter what the muscle length is. When alpha motor neurons fire, the muscle shortens and releases tension on the muscle spindle capsule ( Fig. 3.4 a). Simultaneously, gamma motor neurons innervating the contractile ends of the muscle spindle fire, which causes the ends of intrafusal fibers to contract and shorten. Contraction of the spindle ends lengthens the central region of the spindle and maintains stretch on the sensory nerve endings. As a result, the spindle remains active even when the muscle contracts. Excitation of gamma motor neurons and alpha motor neurons at the same time is a process known as alpha-gamma coactivation. An example of how muscle spindles work during a stretch reflex is shown in Figure 3.5 a c. You can demonstrate this yourself with an unsuspecting friend. Have your friend stand with eyes closed, one arm extended with the elbow at 90, and the hand palm up. Place a small book or other flat weight in the outstretched hand and watch the arm muscles contract to compensate for the added weight. 470

7 THE STRETCH REFLEX (a) Spindles are tonically active and firing even when muscle is relaxed. Sensory neuron endings Intrafusal fibers of muscle spindle Sensory neuron 3 Extrafusal muscle fibers at resting length Sensory neuron is tonically active. Alpha motor neuron 4 Spinal cord 3 4 Spinal cord integrates function. Alpha motor neurons to extrafusal fibers receive tonic input from muscle spindles. 5 5 Extrafusal fibers maintain a certain level of tension even at rest. (b) Muscle stretch can trigger a stretch reflex. When muscles stretch and lengthen, muscle spindle sensory afferent neurons fire more. The reflex response is muscle contraction to prevent damage from over-stretching. () () Muscle stretch Increased afferent signals to spinal cord Spinal cord Increased efferent output through alpha motor neurons Muscle contracts Firing rate of afferent sensory neuron decreases. 3 Negative feedback Time Muscle length Action potentials in spindle sensory neuron Fig. 3.3 Muscle is stretched. Muscle returns to initial length. 47

8 ALPHA-GAMMA COACTIVATION Gamma motor neurons innervate muscle fibers at the ends of muscle spindles. Alpha-gamma coactivation keeps the spindles stretched when the muscle contracts. (a) Alpha-gamma coactivation maintains spindle function when muscle contracts. Alpha motor neuron fires and gamma motor neuron fires. Muscle length Muscle shortens 3 3 Muscle and intrafusal fibers both contract. Stretch on centers of intrafusal fibers unchanged. Firing rate of afferent neuron remains constant. Action potentials of spindle sensory neuron Intrafusal fibers do not slacken so firing rate remains constant. Muscle shortens Time (b) Without gamma motor neurons, muscle contraction causes the spindle firing rate to decrease. Alpha motor neuron fires. Muscle shortens 3 Muscle contracts. Muscle length Less stretch on center of intrafusal fibers Firing rate of spindle sensory neuron decreases. Action potentials of spindle sensory neuron Less stretch on intrafusal fibers Muscle shortens Time Action potential Fig. 3.4 Now suddenly drop a heavier load, such as another book, onto the subject s hand. The added weight will send the hand downward, stretching the biceps muscle and activating its muscle spindles. Sensory input into the spinal cord then activates the alpha motor neurons of the biceps muscle. The biceps will contract, bringing the arm back to its original position. Golgi Tendon Organs Respond to Muscle Tension A second type of muscle proprioceptor is the Golgi tendon organ ( Fig. 3. b). These receptors are found at the junction of tendons and muscle fibers, placing them in series with the muscle fibers. Golgi tendon organs respond primarily to muscle tension created during an isometric contraction and are relatively insensitive to muscle stretch. Golgi tendon reflexes cause relaxation, the opposite of the reflex contraction caused by muscle spindle reflexes. Golgi tendon organs are composed of free nerve endings that wind between collagen fibers inside a connective tissue capsule ( Fig. 3. b). When a muscle contracts, its tendons act as an elastic component during the isometric phase of the contraction. Contraction pulls collagen fibers within the Golgi tendon organ tight, pinching sensory endings of the afferent neurons and causing them to fire. Afferent input from activation of the Golgi tendon organ excites inhibitory interneurons in the spinal cord. The interneurons inhibit alpha motor neurons innervating the muscle, and muscle contraction decreases or ceases. Under 47

9 MUSCLE REFLEXES HELP PREVENT DAMAGE TO THE MUSCLE Muscle spindle reflex: the addition of a load stretches the muscle and the spindles, creating a reflex contraction. Sensory neuron Spindle Spinal cord Motor neuron Muscle Add load (a) Load added to muscle. (b) Muscle and muscle spindle stretch as arm extends. (c) Reflex contraction initiated by muscle spindle restores arm position. Golgi tendon reflex protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load. Muscle contracts Inhibiting interneuron Neuron from Golgi tendon organ fires. Motor neuron 3 Motor neuron is inhibited. Golgi tendon organ 3 Muscle relaxes. 3 (d) Muscle contraction stretches Golgi tendon organ. (e) If excessive load is placed on muscle, Golgi tendon reflex causes relaxation, thus protecting muscle. 4 4 Load is dropped. Fig. 3.5 most circumstances, this reflex slows muscle contraction as the force of contraction increases. In other instances, the Golgi tendon organs prevent excessive contraction that might injure the muscle. Think back to the example of books placed on the outstretched hand. If supporting the added weight requires more tension than the muscle can develop, the Golgi tendon organ will respond as muscle tension nears its maximum. The Golgi tendon organ triggers reflex inhibition of the biceps motor neurons, causing the biceps to relax and the arm to fall. The person then drops the added weight before the muscle fibers can be damaged ( Fig. 3.5 d, e). Golgi tendon organ input is an important source of inhibition to alpha motor neurons. Concept Check Answers:EndofChapter 4. Using the standard steps of a reflex pathway, create a map showing alpha-gamma coactivation and the Golgi tendon reflex. Begin with the stimulus Alpha motor neuron fires. Stretch Reflexes and Reciprocal Inhibition Control Movement Around a Joint Movement around most flexible joints in the body is controlled by groups of synergistic and antagonistic muscles that act in a coordinated fashion. Sensory neurons from muscle receptors 473

10 THE PATELLAR TENDON (KNEE JERK) REFLEX The patellar tendon (knee jerk) reflex illustrates a monosynaptic stretch reflex and reciprocal inhibition of the antagonistic muscle. Stimulus: Tap to tendon stretches muscle. Receptor: Muscle spindle stretches and fires. Afferent path: Action potential travels through sensory neuron. Integrating center: Sensory neuron synapses in spinal cord. Efferent path : Somatic motor neuron onto Effector : Quadriceps muscle Efferent path : Interneuron inhibiting somatic motor neuron Response: Quadriceps contracts, swinging lower leg forward. Effector : Hamstring muscle Response: Hamstring stays relaxed, allowing extension of leg (reciprocal inhibition). Fig. 3.6 and efferent motor neurons that control the muscle are linked by diverging and converging pathways of interneurons within the spinal cord. The collection of pathways controlling a single joint is known as a myotatic unit {myo-, muscle + tasis, stretching}. The simplest reflex in a myotatic unit is the monosynaptic stretch reflex, which involves only two neurons: the sensory neuron from the muscle spindle and the somatic motor neuron to the muscle. The patellar tendon reflex is an example of a monosynaptic stretch reflex ( Fig. 3.6 ). To demonstrate this reflex, a person sits on the edge of a table so that the lower leg hangs relaxed. When the patellar tendon below the kneecap is tapped with a small rubber hammer, the tap stretches the quadriceps muscle, which runs up the front of the thigh. This stretching activates muscle spindles and sends action potentials via the sensory fibers to the spinal cord. The sensory neurons synapse directly onto the motor neurons that control contraction of the quadriceps muscle (a monosynaptic reflex). Excitation of the motor neurons causes motor units in the quadriceps to contract, and the lower leg swings forward. For muscle contraction to extend the leg, the antagonistic flexor muscles must relax ( reciprocal inhibition ). In the leg, this requires relaxation of the hamstring muscles running up the back of the thigh. The single stimulus of the tap to the tendon accomplishes both contraction of the quadriceps muscle and reciprocal inhibition of the hamstrings. The sensory fibers branch upon entering the spinal cord. Some of the branches activate motor neurons innervating the quadriceps, while the other branches synapse on inhibitory interneurons. The inhibitory interneurons suppress activity in the motor neurons controlling the hamstrings (a polysynaptic reflex). The result is a relaxation of the hamstrings that allows contraction of the quadriceps to proceed unopposed. 474

11 RUNNING PROBLEM Once in the spinal cord, tetanospasmin is released from the motor neuron. It then selectively blocks neurotransmitter release at inhibitory synapses. Patients with tetanus experience muscle spasms that begin in the jaw and may eventually affect the entire body. When the extremities become involved, the arms and legs may go into painful, rigid spasms. Q: Using the reflex pathways diagrammed in Figures 3.6 and 3.7, explain why inhibition of inhibitory interneurons might result in uncontrollable muscle spasms. Flexion Reflexes Pull Limbs away from Painful Stimuli Flexion reflexes are polysynaptic reflex pathways that cause an arm or leg to be pulled away from a noxious stimulus, such as a pinprick or a hot stove. These reflexes, like the reciprocal inhibition reflex just described, rely on divergent pathways in the spinal cord. Figure 3.7 uses the example of stepping on a tack to illustrate a flexion reflex. When the foot contacts the point of the tack, nociceptors (pain receptors) in the foot send sensory information to the spinal cord. Here the signal diverges, activating multiple excitatory interneurons. Some of these interneurons excite alpha motor neurons, leading to contraction of the flexor muscles of the stimulated limb. Other interneurons simultaneously activate inhibitory interneurons THE CROSSED EXTENSOR REFLEX A flexion reflex in one limb causes extension in the opposite limb. The coordination of reflexes with postural adjustments is essential for maintaining balance. Spinal cord Gray matter White matter Spinal cord Sensory neuron 3b - - Ascending pathways to brain 3a Painful stimulus activates nociceptor. Primary sensory neuron enters spinal cord and diverges. 3 Nociceptor 3c 3a One collateral activates ascending pathways for sensation (pain) and postural adjustment (shift in center of gravity). Painful stimulus Extensors inhibited Alpha motor neurons 3b 3c Withdrawal reflex pulls foot away from painful stimulus. Crossed extensor reflex supports body as weight shifts away from painful stimulus. Flexors contract, moving foot away from painful stimulus. Extensors contract as weight shifts to left leg. Flexors inhibited Fig

12 that cause relaxation of the antagonistic muscle groups. Because of this reciprocal inhibition, the limb is flexed, withdrawing it from the painful stimulus. This type of reflex requires more time than a stretch reflex (such as the knee jerk reflex) because it is a polysynaptic rather than a monosynaptic reflex. Concept Check 5. Draw a reflex map of the flexion reflex initiated by a painful stimulus to the sole of a foot. Flexion reflexes, particularly in the legs, are usually accompanied by the crossed extensor reflex. The crossed extensor reflex is a postural reflex that helps maintain balance when one foot is lifted from the ground. The quick withdrawal of the right foot from a painful stimulus (a tack) is matched by extension of the left leg so that it can support the sudden shift in weight ( Fig. 3.7 ). The extensors contract in the supporting left leg and relax in the withdrawing right leg, while the opposite occurs in the flexor muscles. Note in Figure 3.7 how the one sensory neuron synapses on multiple interneurons. Divergence of the sensory signal permits a single stimulus to control two sets of antagonistic muscle groups as well as to send sensory information to the brain. This type of complex reflex with multiple neuron interactions is more typical of our reflexes than the simple monosynaptic knee jerk stretch reflex. In the next section we look at how the CNS controls movements that range from involuntary reflexes to complex, voluntary movement patterns such as dancing, throwing a ball, or playing a musical instrument. Concept Check Answers: End of Chapter 6. Add the crossed extensor reflex in the supporting leg to the map you created in Concept Check As you pick up a heavy weight, which of the following are active in your biceps muscle: alpha motor neurons, gamma motor neurons, muscle spindle afferents, Golgi tendon organ afferent neurons? 8. What distinguishes a stretch reflex from a crossed extensor reflex? The Integrated Control of Body Movement Answers:EndofChapter Most of us never think about how our body translates thoughts into action. Even the simplest movement requires proper timing so that antagonistic and synergistic muscle groups contract in the appropriate sequence and to the appropriate degree. In addition, the body must continuously adjust its position to compensate for differences between the intended movement and RUNNING PROBLEM Dr. Ling admits Mrs. Evans to the intensive care unit. There Mrs. Evans is given tetanus antitoxin to deactivate any toxin that has not yet entered motor neurons. She also receives penicillin, an antibiotic that kills the bacteria, and drugs to help relax her muscles. Despite these treatments, by the third day Mrs. Evans is having difficulty breathing because of spasms in her chest muscles. Dr. Ling calls in the chief of anesthesiology to administer metocurine, a drug similar to curare. Curare and metocurine induce temporary paralysis of muscles by binding to ACh receptors on the motor end plate. Patients receiving metocurine must be placed on respirators that breathe for them. For people with tetanus, however, metocurine can temporarily halt the muscle spasms and allow the body to recover. Q3: a. Why does the binding of metocurine to ACh receptors on the motor end plate induce muscle paralysis? (Hint: what is the function of ACh in synaptic transmission?) b. Is metocurine an agonist or an antagonist of ACh? the actual one. For example, the baseball pitcher steps off the mound to field a ground ball but in doing so slips on a wet patch of grass. His brain quickly compensates for the unexpected change in position through reflex muscle activity, and he stays on his feet to intercept the ball. Skeletal muscles cannot communicate with one another directly, and so they send messages to the CNS, allowing the integrating centers to take charge and direct movement. Most body movements are highly integrated, coordinated responses that require input from multiple regions of the brain. Let s examine a few of the CNS integrating centers that are responsible for control of body movement. Movement Can Be Classified as Reflex, Voluntary, or Rhythmic Movement can be loosely classified into three categories: reflex movement, voluntary movement, and rhythmic movement ( Tbl. 3. ). Reflex movements are the least complex and are integrated primarily in the spinal cord (for example, see the knee jerk reflex in Fig. 3.6 ). However, like other spinal reflexes, reflex movements can be modulated by input from higher brain centers. In addition, the sensory input that initiates reflex movements, such as the input from muscle spindles and Golgi tendon organs, goes to the brain and participates in the coordination of voluntary movements and postural reflexes. Postural reflexes help us maintain body position as we stand or move through space. These reflexes are integrated in 476

13 Types of Movement Table 3. Reflex Voluntary Rhythmic Stimulus that initiates movement Primarily external via sensory receptors; minimally voluntary External stimuli or at will Initiation and termination voluntary Example Knee jerk, cough, postural reflexes Playing piano Walking, running Complexity Least complex; integrated at level of spinal cord or brain stem with higher center modulation Most complex; integrated in cerebral cortex Intermediate complexity; integrated in spinal cord with higher center input required Comments Inherent, rapid Learned movements that improve with practice; once learned, may become subconscious ( muscle memory ) Spinal circuits act as pattern generators; activation of these pathways requires input from brain stem the brain stem. They require continuous sensory input from visual and vestibular (inner ear) sensory systems and from the muscles themselves. Muscle, tendon, and joint receptors provide information about proprioception, the positions of various body parts relative to one another. You can tell if your arm is bent even when your eyes are closed because these receptors provide information about body position to the brain. Information from the vestibular apparatus of the ear and visual cues help us maintain our position in space. For example, we use the horizon to tell us our spatial orientation relative to the ground. In the absence of visual cues, we rely on tactile input. People trying to move in a dark room instinctively reach for a wall or piece of furniture to help orient themselves. Without visual and tactile cues, our orientation skills may fail. The lack of cues is what makes flying airplanes in clouds or fog impossible without instruments. The effect of gravity on the vestibular system is such a weak input when compared with visual or tactile cues that pilots may find themselves flying upside down relative to the ground. Voluntary movements are the most complex type of movement. They require integration at the cerebral cortex, and they can be initiated at will without external stimuli. Learned voluntary movements improve with practice, and some even become involuntary, like reflexes. Think about learning to ride a bicycle. It may have been difficult at first but once you learned to pedal smoothly and to keep your balance, the movements became automatic. Muscle memory is the name dancers and athletes give the ability of the unconscious brain to reproduce voluntary, learned movements and positions. Rhythmic movements, such as walking or running, are a combination of reflex movements and voluntary movements. Rhythmic movements are initiated and terminated by input from the cerebral cortex, but once activated, networks of CNS interneurons called central pattern generators (CPGs) maintain the spontaneous repetitive activity. Changes in rhythmic activity, such as changing from walking to skipping, are also initiated by input from the cerebral cortex. As an analogy, think of a battery-operated bunny. When the switch is thrown to on, the bunny begins to hop. It continues its repetitive hopping until someone turns it off (or until the battery runs down). In humans, rhythmic movements controlled by central pattern generators include locomotion and the unconscious rhythm of quiet breathing. An animal paralyzed by a spinal cord injury is unable to walk because damage to descending pathways blocks the start walking signal from the brain to the legs motor neurons in the spinal cord. However, these paralyzed animals can walk if they are supported on a moving treadmill and given an electrical stimulus to activate the spinal CPG governing that motion. As the treadmill moves the animal s legs, the CPG, reinforced by sensory signals from muscle spindles, drives contraction of the leg muscles. Th e ability of central pattern generators to sustain rhythmic movement without continued sensory input has proved important for research on spinal cord injuries. Researchers are trying to take advantage of CPGs and rhythmic reflexes in people with spinal cord injuries by artificially stimulating portions of the spinal cord to restore movement to formerly paralyzed limbs. The distinctions among reflex, voluntary, and rhythmic movements are not always clear-cut. The precision of voluntary movements improves with practice, but so does that of some reflexes. Voluntary movements, once learned, can become reflexive. In addition, most voluntary movements require continuous input from postural reflexes. Feedforward reflexes allow the body to prepare for a voluntary movement, and feedback mechanisms are used to create a smooth, continuous motion. Coordination of movement requires cooperation from many parts of the brain

14 Neural Control of Movement Table 3.3 Location Role Receives Input from: Sends Integrative Output to: Spinal cord Spinal reflexes; locomotor pattern generators Sensory receptors and brain Brain stem, cerebellum, thalamus/cerebral cortex Brain stem Posture, hand and eye movements Cerebellum, visual and vestibular sensory receptors Spinal cord Motor areas of Planning and coordinating complex movement Thalamus Brain stem, spinal cord (corticospinal tract), cerebellum, basal ganglia Cerebellum Monitors output signals from motor areas and adjusts movements Spinal cord (sensory), cerebral cortex (commands) Brain stem, cerebral cortex (Note: All output is inhibitory.) Thalamus Contains relay nuclei that modulate and pass messages to cerebral cortex Basal ganglia, cerebellum, spinal cord Cerebral cortex Basal nuclei Motor planning Cerebral cortex Cerebral cortex, brain stem The CNS Integrates Movement Th ree levels of the nervous system control movement: () the spinal cord, which integrates spinal reflexes and contains central Cerebrum pattern generators; () the brain stem and cerebellum, which control postural reflexes and hand and eye movements; and (3) the cerebral cortex and basal ganglia, which are responsible for voluntary movements. The thalamus relays and modifies signals as they pass from the spinal cord, basal ganglia, and cerebellum to the cerebral cortex ( Tbl. 3.3 ). Reflex movements do not require input from the cerebral cortex. Proprioceptors such as muscle spindles, Golgi tendon organs, and joint capsule receptors provide information to the spinal cord, brain stem, and cerebellum ( Fig. 3.8 ). The brain stem is in charge of postural reflexes and hand and eye movements. It also gets commands from the cerebellum, the part of the brain responsible for fine-tuning movement. The result is reflex movement. However, some sensory information is sent through ascending pathways to sensory areas of the cortex, where it can be used to plan voluntary movements. Voluntary movements require coordination between the cerebral cortex, cerebellum, and basal ganglia. The control of voluntary movement can be divided into three steps: () decisionmaking and planning, () initiating the movement, and (3) executing the movement ( Fig. 3.9 ). The cerebral cortex plays a key role in the first two steps. Behaviors such as movement require knowledge of the body s position in space (where am I?), a decision on what movement should be executed (what shall I do?), Fig. 3.8 INTEGRATION OF MUSCLE REFLEXES Postural reflexes, hand and eye movements Signal Cerebellum Sensory receptors Sensory areas of cerebral cortex Thalamus Feedback Sensory input ( ) from receptors goes to spinal cord, cerebral cortex, and cerebellum. Signals from the vestibular apparatus go directly to the cerebellum. Brain stem Spinal cord Muscle contraction and movement Postural and spinal reflexes do not require integration in the cortex. Output signals ( ) initiate movement without higher input. 478

15 PHASES OF VOLUNTARY MOVEMENT Voluntary movements can be divided into three phases: planning, initiation, and execution. Sensory feedback allows the brain to correct for any deviation between the planned movement and the actual movement. PLANNING MOVEMENT INITIATING MOVEMENT EXECUTING MOVEMENT Basal nuclei Idea Cortical association areas Motor cortex Movement Cerebellum Cerebellum KEY Feedback pathways Fig. 3.9 a plan for executing the movement (how shall I do it?), and the ability to hold the plan in memory long enough to carry it out (now, what was I just doing?). As with reflex movements, sensory feedback is used to continuously refine the process. Let s return to our baseball pitcher and trace the process as he decides whether to throw a fastball or a slow curve. Standing out on the mound, the pitcher is acutely aware of his surroundings: the other players on the field, the batter in the box, and the dirt beneath his feet. With the help of visual and somatosensory input to the sensory areas of the cortex, he is aware of his body position as he steadies himself for the pitch ( Fig. 3.0 ). Deciding which type of pitch to throw and anticipating the consequences CONTROL OF VOLUNTARY MOVEMENTS 3 Sensory input Prefrontal cortex Motor association areas Motor cortex Sensory cortex Planning and decision-making Basal ganglia Thalamus Coordination and timing: cerebellar input Brain stem Cerebellum Feedback 4 Execution: corticospinal tract to skeletal muscles Spinal cord 5 Execution: extrapyramidal influence on posture, balance, and gait KEY Input Output Feedback Muscle contraction and movement Sensory receptors 6 6 Continuous feedback Fig

16 occupy many pathways in his prefrontal cortex and association areas. These pathways loop down through the basal ganglia and thalamus for modulation before cycling back to the cortex. Once the pitcher makes the decision to throw a fastball, the motor cortex takes charge of organizing the execution of this complex movement. To initiate the movement, descending information travels from the motor association areas and motor cortex to the brain stem, the spinal cord, and the cerebellum 3 4. The cerebellum assists in making postural adjustments by integrating feedback from peripheral sensory receptors. The basal ganglia, which assisted the cortical motor areas in planning the pitch, also provide information about posture, balance, and gait to the brain stem 5. The pitcher s decision to throw a fastball now is translated into action potentials that travel down through the corticospinal tract, a group of interneurons controlling voluntary movement that run from the motor cortex to the spinal cord, where they synapse directly onto somatic motor neurons ( Fig. 3. ). Most of these descending pathways cross to the opposite side of the body in a region of the medulla known as the pyramids. Consequently, this pathway is sometimes called the pyramidal tract. Neurons from the basal ganglia also influence body movement. These neurons have multiple synapses in the CNS and make up what is sometimes called the extrapyramidal tract or the extrapyramidal system. It was once believed that the pyramidal and extrapyramidal pathways were separate systems, but we now know that they interact and are not as distinct in their function as was once believed. As the pitcher begins the pitch, feedforward postural reflexes adjust the body position, shifting weight slightly in anticipation EMERGING CONCEPTS Visualization Techniques in Sports Researchers now believe that presynaptic facilitation, in which modulatory input increases neurotransmitter release, is the physiological mechanism that underlies the success of visualization techniques in sports. Visualization, also known as guided imagery, enables athletes to maximize their performance by psyching themselves, picturing in their minds the perfect vault or the perfect fastball. By pathways that we still do not understand, the mental image conjured up by the cerebral cortex is translated into signals that find their way to the muscles. Guided imagery is also being used in medicine as adjunct (supplementary) therapy for cancer treatment and pain management. The ability of the conscious brain to alter physiological function is only one example of the many fascinating connections between the higher brain and the body. To learn more about this, go to and search for visualization. THE CORTICOSPINAL TRACT Interneurons run directly from the motor cortex to their synapses with somatic motor neurons. Most corticospinal neurons cross the midline at the pyramids. Cranial nerves to selected skeletal muscles Most corticospinal pathways cross to the opposite side of the body at the pyramids. Lateral corticospinal tract Somatic motor neurons to skeletal muscles Fig. 3. Feedforward reflexes and feedback of information during movement Brain initiates movement Fig. 3. Feedforward for anticipated postural disturbance Body moves Posture adjusted Primary motor cortex of left cerebral hemisphere MIDBRAIN MEDULLA OBLONGATA Pyramids Anterior corticospinal tract SPINAL CORD Posture is disturbed Feedback for unanticipated postural disturbance of the changes about to occur ( Fig. 3. ). Through the appropriate divergent pathways, action potentials race to the somatic motor neurons that control the muscles used for pitching: some are excited, others are inhibited. The neural circuitry allows 480

17 precise control over antagonistic muscle groups as the pitcher flexes and retracts his right arm. His weight shifts onto his right foot as his right arm moves back. Each of these movements activates sensory receptors that feed information back to the spinal cord, brain stem, and cerebellum, initiating postural reflexes. These reflexes adjust his body position so that the pitcher does not lose his balance and fall over backward. Finally, he releases the ball, catching his balance on the follow-through another example of postural reflexes mediated through sensory feedback. His head stays erect, and his eyes track the ball as it reaches the batter. Whack! Home run. As the pitcher s eyes follow the ball and he evaluates the result of his pitch, his brain is preparing for the next batter, hoping to use what it has learned from these pitches to improve those to come. Symptoms of Parkinson s Disease Reflect Basal Ganglia Function Our understanding of the role of the basal ganglia in the control of movement has been slow to develop because, for many years, animal experiments yielded little information. Randomly destroying portions of the basal ganglia did not appear to affect research animals. However, research focusing on Parkinson s disease (Parkinsonism) in humans has been more fruitful. From studying patients with Parkinson s, scientists have learned that the basal ganglia play a role in cognitive function and memory as well as in the coordination of movement. Parkinson s disease is a progressive neurological disorder characterized by abnormal movements, speech difficulties, and cognitive changes. These signs and symptoms are associated with loss of neurons in the basal ganglia that release the neurotransmitter dopamine. One abnormal sign that most Parkinson patients have is tremors in the hands, arms, and legs, particularly at rest. In addition, they have difficulty initiating movement and walk slowly with stooped posture and shuffling gait. They lose facial expression, fail to blink (the reptilian stare), and may develop depression, sleep disturbances, and personality changes. Th e cause of Parkinson s disease is usually not known and appears to be a combination of environmental factors and genetic susceptibility. However, a few years ago, a number of young drug users were diagnosed with Parkinsonism. Their disease was traced to the use of homemade heroin containing a toxic contaminant that destroyed dopaminergic (dopaminesecreting) neurons. This contaminant has been isolated and now enables researchers to induce Parkinson s disease in experimental animals so that we have an animal model on which to test new treatments. Th e primary current treatment for Parkinson s is administration of drugs designed to enhance dopamine activity in the brain. Dopamine cannot cross the blood-brain barrier, so patients take l -dopa, a precursor of dopamine that crosses the blood-brain barrier, then is metabolized to dopamine. Other RUNNING PROBLEM Four weeks later, Mrs. Evans is ready to go home, completely recovered and showing no signs of lingering effects. Once she could talk, Mrs. Evans, who was born on the farm where she still lived, was able to tell Dr. Ling that she had never had immunization shots for tetanus or any other diseases. Well, that made you one of only a handful of people in the United States who will develop tetanus this year, Dr. Ling told her. You ve been given your first two tetanus shots here in the hospital. Be sure to come back in six months for the last one so that this won t happen again. Because of national immunization programs begun in the 950s, tetanus is now a rare disease in the United States. However, in developing countries without immunization programs, tetanus is still a common and serious condition. Q4: On the basis of what you know about who receives immunization shots in the United States, predict the age and background of people who are most likely to develop tetanus this year. drug treatments include dopamine agonists and inhibitors of enzymes that break down dopamine, such as MAO. In severe cases, selected parts of the brain may be destroyed to reduce tremors and rigidity. Experimental treatments include transplants of dopaminesecreting neurons. Proponents of stem cell research feel that Parkinson s may be one of the conditions that would benefit from the transplant of stem cells into affected brains. For more information on Parkinson s treatments, see the National Parkinson Foundation. Control of Movement in Visceral Muscles Movement created by contracting smooth and cardiac muscles is very different from that created by skeletal muscles, in large part because smooth and cardiac muscle are not attached to bone. In the internal organs, or viscera, muscle contraction usually changes the shape of an organ, narrowing the lumen of a hollow organ or shortening the length of a tube. In many hollow internal organs, muscle contraction pushes material through the lumen of the organ: the heart pumps blood, the digestive tract moves food, the uterus expels a baby. Visceral muscle contraction is often reflexively controlled by the autonomic nervous system, but not always. Some types of smooth and cardiac muscle are capable of generating their own action potentials, independent of an external signal. Both 3 48