1. NERVOUS SYSTEM FUNCTIONS OF THE NERVOUS SYSTEM. FUNCTION The major function of the nervous system can be summarized as follows (Figure 1-1).

Transcription

1 1. NERVOUS SYSTEM FUNCTION The major function of the nervous system can be summarized as follows (Figure 1-1). Sensory input. Multiple signals from both, internal and external environment are detected by specialized cell located throughout the organism and sent for processing at different levels of the nervous system. Some inputs result in sensations that are consciously recognized or become aware of, such as images that are seen or food that is tested. Other inputs are dealt within an unconscious level such as maintaining blood pressure, within normal range or moving food through the digestive tract and secreting the proper digestive enzymes. FUNCTIONS OF THE NERVOUS SYSTEM Figure 1-1. Role of the nervous system Integration Within the central nervous system (brain and spinal cord) the sensory inputs are processed and the result could be an active response (moving away); could be stored as a memory (the smell of a meal) or could be discarded (the feeling of a breeze). Control of muscles and glands Most skeletal muscles contract to create body movement after being stimulated through the nervous system. Some of this activity is done as a reflex (to maintain standing position) while other are done voluntarily (running to catch a pray). Smooth muscle contract in response to either hormonal action (smooth muscles) or in response to direct nervous impulse (muscles in the wall of blood vessels). The activity of many secretory glands (sweat, salivary, digestive) is controlled by the nervous system. Homeostasis Most of the activities of the nervous system are aimed at maintaining homeostatic conditions. This requires the synchronization of the trillions of cells that an animal has. All of this is coordinated by the nervous system. Mental activity Thinking, storing and recalling memories, emotional responses, awareness or consciousness are all conducted within the brain. V BS 122 Physiology I 1 Class of 2013

2 STRUCTURE AND DIVISIONS OF THE NERVOUS SYSTEM The basic unit of the nervous system is the neuron. The basic structure of a neuron is presented in figure 1-2. There are three basic types of neurons (Fig, 1-3). The multipolar neurons are characterized for having a highly branched dendritic terminal and a single axon capable or interacting with a very large number of other neurons and are found principally in the central nervous system (CNS). The bipolar neuron has one dendritic process and one axon. They are part of a sequence of neurons that convey action potentials to the CNS. The monopolar neuron consists of a single process leaving the cell body which branches into two components that functional act as a single axon. The branch that reaches the periphery has receptor like dendrites which generate action potential that are transmitted through the second branch to the CNS. The body of the monopolar neurons resides in dorsal root ganglia and are part of the sensory or afferent division. Figure 1-2. Structure of a neuron The nervous system is divided Figure 1-3. Types of neurons in two main components, (Fig. 1-4) the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain, which is located in the skull, and the spinal cord which is located within the vertebral canal. V BS 122 Physiology I 2 Class of 2013

3 The peripheral nervous system is made up of all components located outside the CNS. These include ganglia, which are groups of neurons cell bodies located outside the CNS; plexuses, which are large networks of axons and neuron s cell bodies located outside the CNS; sensory receptors which are either specialized cells connected to afferent neurons or the nerve ending of efferent neurons; and nerves which are made of axons and their respective sheaths. PERIPHERAL N. SYSTEM A Sensory (Afferent) NERVOUS SYSTEM B Somatic (Efferent) Figure 1-4. Components and divisions of the nervous system The PNS is divided into the set of components bringing information into the CNS which is the sensory or afferent division and the set of components sending instructions, in the form of action potentials, to the different parts of the body and it is called the motor or efferent division. C Autonomic (Efferent) The sensory division is made of neurons which have their bodies located in the dorsal root ganglion and they are the receptor or connect to the specific receptor to bring the information in the form of an action potential to the CNS (Fig. 1-5A). The motor or efferent division is further divided into the somatic nervous system and the autonomic nervous system (ANS). The somatic nervous system specifically Figure 1-5. Components and divisions of the peripheral nervous system V BS 122 Physiology I 3 Class of 2013

4 connects the CNS with skeletal muscles (Fig. 1-5B). It is characterized for having all the neurons cell bodies located within the CNS, specifically in the spinal cord, and their axons leave the spinal cord through the ventral root of a given spinal nerve and connect directly with the skeletal muscle through a synapse. The autonomic nervous system sends information in the form of action potentials between the CNS and smooth muscle, cardiac muscles and several secretory glands. All the activity of the ANS takes place in an involuntary or subconscious manner. The ANS takes care of the ongoing functioning of the organism such as respiration, digestion, cardiac function, etc. Two neurons complete the connection between the CNS and the effector organ. The first neuron is located within the CNS and its axon leaves through the ventral root of a spinal nerve and synapse with another neuron, the second motor neuron whose body is located in an autonomic ganglion and the axon synapse with the effector organ (Fig. 1-5C). The ANS in turn is divided in two large divisions, the sympathetic and parasympathetic divisions and an entirely separate system called the enteric nervous system. You have dealt with the ANS early and will deal with the enteric nervous system next semester. Now we will concentrate in the central nervous system. GLIAL CELLS OF THE CNS CENTRAL NERVOUS SYSTEM The central nervous system is made up of the brain and the spinal cord. The CNS is characterized for being cover or constraint by a multilayer protective membrane of connective tissue called the meninges. The CNS is supported by a variety of cells which perform very specific functions to protect, or to enhance its functioning (Fig. 1-6). Astrocytes, ependymal and microglia cells contribute Figure 1-6. Supportive cells of the CNS and their to protection of the CNS by establishing a blood respective roles barrier, forming the choroid plexus which secretes cerebrospinal fluid and cleaning inflamed tissue by phagocytic action respectively. Oligodendrocytes provide enhancement in function by providing myelin sheaths to axon bundles. Brain The brain is divided into the telencephalon or cerebrum, diencephalon, cerebellum, brainstem and reticular formation (Figs. 1-7; 8). The telencephalon or cerebrum is the largest component of the brain. It is divided into cortex (grey matter, because is made up of unmyelinated neurons) and cerebral medulla (white matter, because is made up of myelinated neurons). The cortex is responsible for memory, awareness, perception, language, consciousness and thought. The medulla is made of nervous tracts connecting different areas of the brain and the rest of the CNS. The brain is also made of two hemispheres and several lobes, each of which has a specific function. The frontal lobe is mostly concerned with motivation, aggression, mood, voluntary motor activity and some of the sense of V BS 122 Physiology I 4 Class of 2013

5 smell. The parietal lobe is mostly in charge of sensory information except for special senses such as vision, which is processed in the occipital lobe as well as smell and hearing which are processed in the temporal lobe. The diencephalon is located between the brainstem and the cerebrum. It contains the thalamus which synapse all sensory information (except olfactory) before sending information to the different parts of the cortex, thus is considered the sensory relay for the brain. Emotions such as rage or fear are processed in the thalamus thus influencing integrating sensory information and determining mood. ANATOMY OF THE BRAIN The hypothalamus is the lowest component of the diencephalon. It contains a variety of nuclei and Figure 1-7 Structures of the brain nervous tracts. Some of these are involved in responding to olfactory stimulus but the majority is involved in controlling and regulating the endocrine system in conjunction with the STRUCTURES OF THE BRAIN AND THEIR FUNCTION hypophysis. Through these nuclei the animal controls its body temperature, thirst, hunger, sex drive, blood volume, renal function and many productive functions such as growth and milk production. The epithalamus which includes the pineal gland influence several biological rhythms including reproductive ones in seasonal breeders. It also has the habenular nuclei which participate in innate Figure 1-8 Role of the different areas of the brain response to odours. V BS 122 Physiology I 5 Class of 2013

6 The cerebellum interacts with the brainstem and other components of the CNS. The most complex cells in the cerebellum are Purkinje cells. These are capable of receiving around 200,000 synapses. The cerebellum is responsible for eye movement, posture, locomotion and fine motor coordination. Complex movements are learned in collaboration with the frontal lobe of the cerebral cortex. The brainstem is made up by the midbrain, pons and medulla oblongata. The midbrain controls the movement of the head, body and eyes in response to sound, texture or sight. The pons serves as a relay of information between the cerebrum and the cerebellum. Furthermore, within this area are integrated some of the component of the sleep and respiratory center of the medulla oblongata. Within the medulla, different nuclei control different aspects such as conscious control of skeletal muscles that permits the control of balance. In the medulla the nerve fibres cross from one hemisphere of the brain to the opposite side of the PNS. The reticular formation is made of several nuclei distributed throughout the brainstem. These nuclei regulate or controls several cyclic activities such as wake-sleep. The brain give rise to 12 pairs of cranial nerves, two connect to the cerebrum, nine to the brainstem and one to the spinal cord. The function of the cranial nerves can be sensory (afferent), somatic motor (efferent) or parasympathetic (efferent). Spinal cord The spinal cord plays the fundamental role of linking the brain to the peripheral nervous system. The spinal cord carries out a significant amount of regulatory activity through reflexes. It extends from the foramen magnum where it joins the brain to the second lumbar vertebra. The name of the sections of the spinal cord corresponds to the names of the vertebra from which the corresponding nerves leave or enter. Therefore there is a cervical, thoracic, lumbar, and sacral segment of the spinal cord. A total of 31 pairs of spinal nerves connect the spinal cord with different organs and tissues of the body. There are 8 cervical, 12 thoracic, 5 lumbar, 6 sacral which includes the coccygeal nerves. Sensory division of the peripheral nervous system All signals conveying information from the internal or external environment are Figure 1-9. Synaptic connection generated by a stimulus which can be light, touch, heat, vibration, chemical, etc. The stimulus is sensed or detected by a receptor and, if it is of enough strength, it will be converted to an action potential and sent to the CNS where it can reach different levels (spinal V BS 122 Physiology I 6 Class of 2013

7 cord, mid brain, cerebrum). Once the action potential reaches its destination it is processed to determine if and what type of response is warranted, if it can be stored as a memory or simply ignored. The transmission of an action potential from one cell to CLASSIFICATION OF RECEPTOR TYPES another is carried out by a synaptic connection (Fig. 1-9). A couple of conditions have to be fulfilled for a stimulus to trigger an action potential. The stimulus has to be the appropriate for the receptor to be stimulated (a thermo receptor does not respond to pressure, it only respond to certain temperatures or changes in temperature). Secondly, it has to be of enough strength to reach a threshold which causes the depolarization. There is a variety of receptors capable of detecting different types of stimulus (Fig 1-10). A single stimulus causes initially a graded potential or local potential (Fig. 1-11). A graded potential ranges from very weak, thus unable to reach GRADED POTENTIALS threshold, in which case the signal is not transmitted and the stimulus is not felt. The other alternative is that the stimulus is strong enough to generate a depolarization which reaches the threshold and triggers an action potential. If the graded potential is not strong enough to trigger an action potential it will spread over the plasma membrane with decreasing strength and decay overtime until it disappears. It is possible, however, that a local or graded potential is followed by a second stimulus before the first potential has decayed. This causes the summation of the stimulus and may result in a local depolarization that will trigger an action potential. Figure Receptor types, the sensation that they identify and the triggering stimulus Figure Events taking place during a graded potential and it consequences V BS 122 Physiology I 7 Class of 2013

8 The organism is able to disregard any stimulus that is continuous but does not present a threat to the wellbeing of the animal. Therefore, stimuli that initially may cause awareness of their existence over time disappear as such. An example is a dog s collar. The first time you put it on the animal it is uncomfortable and the dog tries to remove it. After a period of time the animal does not feel it any longer. The same happen with rings in fingers or with sound that eventually become white sound. In summary the organism adapts to these stimulus. As a protection mechanism, however, the organism does not adapt to any noxious stimuli. Specifically pain receptors keep sending action potentials as long as the noxious stimulus persists, making the animal aware of a problem regardless of the duration. The intensity of a given stimulus is determined by the frequency code of stimulation and the population code STIMULUS STRENGHT OR INTENSITY Figure Factors determining stimulus strength of receptors in the area (Fig. 1-12). The frequency is determined by the number of action potentials taking place in a given unit of time and it is quantified in Hertz (Hz). One Hz is one stimulus per second. Therefore if a receptor fires at a rate of 4 Hz it is being stimulated four times stronger than a receptor firing at a rate of 1 Hz. The population code refers to the number of receptors in a given tissue. The more receptors there are, the larger the possibility that more than one is stimulated, thus the stimulus is perceived in a stronger manner. The lips are very sensitive because they have many receptors but the arms have less receptors, thus, they are more difficult to stimulate. Sensitivity. The sensitivity of a tissue to stimuli depends on the number of receptors in the area (population code) and the size covered by each receptor. Based on these the ability of the tissues to discriminate the number and location of stimulus varies. You can touch your back with a two point compass separated 3 or 4 cm but you feel only one contact point. On the other hand you can touch your lips at a distance of 2 mm and you are able to recognize that there are two points of contact. Integration or summation of graded potential. If the axons of several presynaptic neurons deliver their action potential to a single postsynaptic neuron, this is called spatial summation. If an axon fires repeatedly (faster than the time required for the graded potential to disappear) this will trigger temporal summation. In both cases it is possible that, as a result of the summation, the graded potential reaches a threshold capable of generating an action potential in the postsynaptic neuron. V BS 122 Physiology I 8 Class of 2013