Hole s Essentials of Human Anatomy & Physiology

Transcription

1 Hole s Essentials of Human Anatomy & Physiology David Shier Jackie Butler Ricki Lewis Created by Dr. Melissa Eisenhauer Trevecca Nazarene University Amended by John Crocker Chapter 9 1

2 Chapter 9 Nervous System 2

3 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) What are the functions of the nervous system? Describe each of the five types of neuroglial cells. Describe the structure of a nerve fiber. Describe the groupings of neurons by structural differences.and by functional differences. Compare and contrast resting potential and action potential. How does a nerve impulse travel the length of a nerve? How is this process different in myelenated fibers and unmyelinated fibers? Describe the process of synaptic transmission. Compare and contrast exitatory and inhibitory actions by neurons. What are neurotransmitters? Where are they synthesized and stored? How do they act? compare and contrast facilitatation, convergence, and divergence. What is a reflex arc? What steps are involved? What are reflex behaviors? What are meninges? Describe the three types. Describe the structure of the spinal chord. Compare and contrast white matter and gray matter. What are the functions of the spinal chord? Describe each of the functional divisions of the brain. Describe the functional regions of the cerebral cortex. What is hemisphere dominance? What functions are performed by the dominant hemisphere? What functions are performed by the non-dominant hemisphere? What are the functions of the thalamus and hypothalamus? What is the limbic system and what does it do? List the components of the brain stem and describe the functions of each. What are the functions of the cerebellum. What are plexuses? 3 Compare and contrast the sympathetic and parasympathetic nervous systems.

4 Introduction: A. The nervous system is composed of neurons and neuroglia. 1. Neurons are typically divided into: a) cell body b) axon c) dendrites 2. Neurons transmit nerve impulses along nerve fibers to other neurons. 3. Neuroglia carry out a variety of functions to aid and protect components of the nervous system. 4. Nerves are made up of bundles of nerve fibers. 4

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6 A. Organs of the nervous system can be divided into 1. Central nervous system (CNS) a) Brain b) Spinal cord 2. Peripheral nervous system (PNS) a) Peripheral nerves b) Connect the CNS to the rest of the body B. The nervous system provides 1. Sensory 2. Integrative 3. Motor functions a) Somatic nervous system - consciously controlled b) Autonomic system - unconscious 6

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8 General Functions of the Nervous System A. Sensory receptors 1. At the ends of peripheral nerves 2. Gather information 3. Convert it into nerve impulses B. Sensory impulses are integrated in the brain as perceptions (integrative function) C. Conscious or subconscious decisions follow D. Lead to motor functions via effectors 8

9 Supporting cells A. Function of Neuroglial Cells 1. Fill spaces 2. Support neurons 3. Provide structural frameworks 4. Produce myelin 5. Carry on phagocytosis B. Classification of Neuroglial Cells 1. Four are in the CNS and the last in the PNS 2. Microglial cells a) Small b) Phagocytize bacterial cells and cellular debris 9

10 1. Oligodendrocytes a) Form myelin in the brain and spinal cord 2. Astrocytes a) Near blood vessels and support structures b) Aid in metabolism c) Respond to brain injury by filling in spaces. 3. Ependyma a) Cover the inside of ventricles b) Form choroid plexuses within the ventricles. 4. Schwann cells a) Peripheral nervous system only b) Produce myelin 10

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12 Neuron Structure A. Cell body contains: 1. Mitochondria 2. Lysosomes 3. Golgi apparatus 4. Chromatophilic substances called Nissl bodies containing: a) rough endoplasmic reticulum b) neurofibrils 12

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14 A. Nerve fibers include: A. Numerous dendrites 1. Branching 2. Carry impulses from other neurons (or from receptors) toward the cell body B. A solitary axon 1. Transmits the impulse away from the axonal hillock of the cell body 2. May give off side branches 14

15 1. Larger axons are enclosed by sheaths of myelin a) Provided by Schwann cells b) Outer layer of myelin surrounded by a neurilemma (neurilemmal sheath) made up of the cytoplasm and nuclei of the Schwann cell. c) Narrow gaps in the myelin sheath between Schwann cells are called nodes of Ranvier. 2. The smallest axons lack a myelin sheath and so are called unmyelinated fibers. 15

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17 1. White matter in the CNS is due to myelin sheaths in this area 2. Unmyelinated nerve tissue in the CNS comprise the gray matter. 3. Peripheral neurons are able to regenerate because of the neurilemma 4. CNS axons are myelinated by oligodendrocytes thus lacking neurilemma and usually do not regenerate 17

18 Classification of Neurons A. Neurons can be grouped in two ways: 1. Structural differences a) Bipolar neurons a) found in the eyes, nose, and ears b) have a single axon and a single dendrite extending from opposite sides of the cell body. b) Unipolar neurons a) found in ganglia outside the CNS b) have an axon and a dendrite arising from a single short fiber extending from the cell body c) Multipolar neurons a) found in the brain and spinal cord b) have many nerve fibers arising from their cell bodies 18

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20 1. Functional differences a) Sensory neurons (afferent neurons) a) usually unipolar b) some bipolar c) conduct impulses from peripheral receptors to the CNS b) Interneurons a) multipolar neurons b) within the CNS c) form links between other neurons c) Motor neurons a) multipolar neurons b) conduct impulses from the CNS to effectors 20

21 Cell Membrane Potential A. A cell membrane is usually polarized 1. Excess of negative charges on the inside of the membrane 2. Polarization is important to the conduction of nerve impulses 21

22 A. Distribution of Ions 1. Distribution of ions is determined by the membrane channel proteins 2. MCPs selective for certain ions 3. Potassium ions (K+) pass through the membrane more readily than do sodium ions (Na+) 4. Potassium ions a major contributor to membrane polarization 22

23 A. Resting Potential 1. Due to active transport a) Cells maintain a greater concentration of sodium ions outside b) Greater concentration of potassium ions inside the membrane 2. Inside the membrane - excess negative charge 3. Outside - excess positive charge 4. This separation of charge, or potential difference, is called the resting potential 23

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25 A. Potential Changes 1. Stimulation of a membrane can locally affect its resting potential 2. When the membrane potential becomes less negative, the membrane is depolarized 3. If sufficiently strong depolarization occurs, a threshold potential is achieved as ion channels open 4. At threshold, an action potential is reached 5. Action potentials may be reached when a series of subthreshold stimuli combine to reach threshold 25

26 A. Action Potential 1. At threshold potential a) Na+ channels open b) Membrane permeability to Na+ quickly changes in the region of stimulation. c) Na+ rushes in d) Membrane potential changes and becomes depolarized. e) K+ channels open allowing K+ to leave the cell f) The membrane becomes repolarized g) Resting potential is reestablished. 2. This rapid sequence of events is the action potential. 3. Active transport mechanisms then restore the original concentrations of Na+ and K+ 26

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28 Nerve Impulse A. A nerve impulse is conducted as an action potential is reached at the trigger zone. B. This spreads by a local current flowing down the fiber, and adjacent areas of the membrane reach action potential. 28

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30 A. Impulse Conduction 1. Unmyelinated fibers conduct impulses over their entire membrane surface. 2. Myelinated fibers conduct impulses from one Node of Ranvier to the next (saltatory conduction) 3. Saltatory conduction is many times faster conduction over the complete membrane surface A. All-or-None Response 1. Nerve fibers respond to a stimulus completely by conducting an impulse or not at all 2. Greater intensity of stimulation triggers more impulses per second, not stronger impulses. 30

31 The Synapse A. Nerve impulses travel from neuron to neuron along complex nerve pathways. B. The junction between two communicating neurons is called a synapse C. A synaptic cleft is the space between them across which the impulse must be conveyed. 31

32 A. Synaptic Transmission 1. When an impulse reaches the synaptic knobs of an axon, synaptic vesicles release a neurotransmitter into the synaptic cleft 2. The neurotransmitter from the presynaptic neuron reacts with specific receptors on the postsynaptic membrane 3. This process by which the impulse in the presynaptic neuron is transmitted across the synaptic cleft to the postsynaptic neuron is called synaptic transmission 32

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34 A. Excitatory and Inhibitory Actions 1. Neurotransmitters that increase postsynaptic membrane permeability to Na+ may trigger impulses and are thus excitatory 2. Other neurotransmitters may decrease membrane permeability to Na+, reducing the chance that it will reach threshold, and are thus inhibitory 3. The effect on the postsynaptic neuron depends on which presynaptic knobs are activated 34

35 A. Neurotransmitters different neurotransmitters are produced by the nervous system a) most are synthesized in the cytoplasm of synaptic knobs b) stored in synaptic vesicles. 2. When an action potential reaches the synaptic knob a) calcium ions rush inward b) in response some synaptic vesicles fuse with the membrane releasing their contents to the synaptic cleft 3. Synaptic cleft and on postsynaptic membrane enzymes rapidly decompose the neurotransmitters 4. Removal of the neurotransmitter prevents continuous stimulation of the postsynaptic neuron 35

36 Impulse Processing A. How impulses are processed is dependent upon how neurons are organized in the brain and spinal cord. B. Neuronal Pools 1. Neurons within the CNS are organized into neuronal pools with varying numbers of cells. 2. Each pool receives input from afferent nerves and processes the information according to the special characteristics of the pool. 36

37 A. Facilitation 1. A particular neuron of a pool may receive excitatory or inhibitory stimulation 2. If the net effect is excitatory but subthreshold, the neuron becomes more excitable to incoming stimulation (facilitation). B. Convergence 1. A single neuron within a pool may receive impulses from two or more fibers (convergence) 2. Makes it possible for the neuron to summate (combine) impulses from different sources. 37

38 A. Divergence 1. Impulses leaving a neuron in a pool may be passed into several output fibers (divergence) 2. This pattern serves to amplify an impulse. 38

39 Convergence Divergence 39

40 Types of Nerves A. A nerve is a bundle of nerve fibers held together by layers of connective tissue. B. Nerves can be: 1. Sensory 2. Motor 3. Mixed, carrying both sensory and motor fibers. 40

41 Nerve Pathways A. The routes nerve impulses travel are called pathways, the simplest of which is a reflex arc. B. Reflex Arcs include: 1. Sensory receptors 2. Sensory neurons 3. Interneurons in the spinal cord 4. Motor neurons 5. Effectors 41

42 A. Reflex Behavior 1. Reflexes are automatic subconscious responses to stimuli 2. Help maintain homeostasis (heart rate, blood pressure, etc.) 3. Carry out automatic responses (vomiting, sneezing, swallowing, etc.) 4. The knee-jerk reflex (patellar tendon reflex) is an example of a monosynaptic reflex (no interneuron) 5. The withdrawal reflex involves a) b) c) d) Sensory neurons Interneurons Motor neurons Simultaneously the antagonistic extensor muscles are inhibited 42

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44 Meninges A. The brain and spinal cord are surrounded by membranes called meninges B. Meninges lie between the bone and the soft tissues. C. Dura mater 1. Outermost meninx 2. Tough, white dense connective tissue 3. Contains many blood vessels 4. Forms the inner periosteum of the skull bones. 5. Forms partitions between lobes of the brain 6. Forms dural sinuses. 7. The sheath around the spinal cord is separated from the vertebrae by an epidural space. 44

45 A. Arachnoid mater 1. The middle meninx 2. Thin and lacks blood vessels 3. Does not follow the convolutions of the brain 4. Subarachnoid space a) Between the arachnoid and pia mater b) Contains cerebrospinal fluid B. Pia mater 1. Innermost 2. Thin and contains many blood vessels and nerves 3. Attached to the surface of the brain and spinal cord 4. Follows the contours of the brain and spinal cord 45

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48 Spinal Cord A. The spinal cord begins at the base of the brain and extends as a slender cord to the level of the intervertebral disk between the first and second lumbar vertebrae. B. Structure of the Spinal Cord 1. The spinal cord consists of 31 segments 2. Each gives rise to a pair of spinal nerves 3. Cervical enlargement gives rise to nerves leading to the upper limbs 4. Lumbar enlargement gives rise to nerves innervating the lower limbs 48

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50 1. Two deep longitudinal grooves divide the cord into right and left halves a) anterior median fissure b) posterior median sulcus 2. White matter a) Bundles of myelinated nerve fibers (nerve tracts) b) Surrounds a butterfly-shaped core of gray matter housing interneurons 3. A central canal contains cerebrospinal fluid 50

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52 A. Functions of the Spinal Cord 1. The spinal cord has two major functions: a) to transmit impulses to and from the brain b) to house spinal reflexes 2. Nerve tracts are identified by the origin and termination of the fibers in the tract 3. Ascending tracts tracts carry sensory information to the brain 4. Descending tracts carry motor information from the brain 5. Many spinal reflexes also pass through the spinal cord. 52

53 Brain A. The brain is the largest most complex portion of the nervous system B. Contains 100 billion multipolar neurons C. Can be divided into the: 1. Cerebrum a) largest portion b) associated with higher mental functions 2. Diencephalon - processes sensory input 3. Cerebellum - coordinates muscular activity 4. Brain stem - coordinates and regulates visceral activities 53

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56 A. Structure of the Cerebrum 1. Largest portion of the mature brain 2. Consists of two cerebral hemispheres. 3. The corpus callosum, a deep ridge of nerve fibers, connects the hemispheres. 4. The surface of the brain is marked by convolutions, sulci, and fissures. 5. Lobes of the brain named according to the bones they underlie a) b) c) d) e) frontal lobe parietal lobe temporal lobe occipital lobe insula 56

57 1. The cerebral cortex a) A thin layer of gray matter b) Lies on the outside of the cerebrum c) Contains 75% of the cell bodies in the nervous system 2. Beneath the cortex lies a mass of white matter a) Made up of myelinated nerve fibers b) Connects the cell bodies of the cortex with the rest of the nervous system 57

58 A. Functions of the Cerebrum - higher brain functions 1. Interpretation of sensory input 2. Initiating voluntary muscular movements 3. Memory 4. Integrating information for reasoning 58

59 A. Functional Regions of the Cerebral Cortex 1. The functional areas of the brain overlap 2. The cortex can, however, generally be divided into a) Motor areas b) Sensory areas c) Association areas 3. The primary motor areas lie in the frontal lobes, anterior to the central sulcus and in its anterior wall. 4. Broca s area a) Anterior to the primary motor cortex b) Coordinates muscular activity for speech 59

60 1. The frontal eye field a) Above Broca s area b) Controls the voluntary movements of the eyes and eyelids 2. The sensory areas a) located in several areas of the cerebrum b) interpret sensory input, producing feelings or sensations 3. Sensory areas for sight lie within the occipital lobe 4. Sensory and motor fibers cross over in the spinal cord or brain stem a) centers in the right hemisphere interpret and control the left side of the body b) centers in the left hemisphere interpret and control the right side of the body 60

61 1. The various association areas of the brain a) Analyze and interpret sensory impulses b) Function in reasoning, judgment, emotions, verbalizing ideas, and storing memory 2. Association areas of the frontal lobe control a number of higher intellectual processes 3. A general interpretive area found at the junction of the parietal, temporal, and occipital lobes plays the primary role in complex thought processing 61

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63 A. Hemisphere Dominance 1. Both cerebral hemispheres function in receiving and analyzing sensory input and sending motor impulses to the opposite side of the body. 2. Most people exhibit hemisphere dominance for the language-related activities of speech, writing, and reading. 3. The left hemisphere is dominant in 90% of the population 4. The non-dominant hemisphere specializes in nonverbal functions and controls emotions and intuitive thinking 63

64 1. Basal ganglia a) b) c) d) Masses of gray matter Located deep within the cerebral hemispheres Relay motor impulses from the cerebrum Help to control motor activities by producing inhibitory dopamine 2. Basal ganglia include: a) Caudate nucleus b) Putamen c) Globus pallidus. 64

65 A. Ventricles and Cerebrospinal Fluid 1. Ventricles a) Series of connected cavities within the cerebral hemispheres and brain stem b) Continuous with the central canal of the spinal cord c) filled with cerebrospinal fluid 2. Choroid plexuses a) b) c) d) Specialized capillaries from the pia mater Secrete cerebrospinal fluid Most cerebrospinal fluid arises in the lateral ventricles. Cerebrospinal fluid has nutritive as well as protective (cushioning) functions 65

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67 A. Diencephalon 1. Lies above the brain stem 2. Composed of the: a) Thalamus b) hypothalamus c) optic tracts d) optic chiasma e) infundibulum (attachment for the pituitary) f) posterior pituitary g) mammillary bodies h) pineal gland 67

68 1. The thalamus a) functions in sorting and directing sensory information arriving from other parts of the nervous system b) performing the services of both messenger and editor 2. The hypothalamus a) maintains homeostasis by regulating a wide variety of visceral activities including: 1) 2) 3) 4) 5) 6) 7) heart rate and arterial blood pressure body temperature water and electrolyte balance hunger and body weight movements and secretions of the digestive tract growth and reproduction sleep and wakefulness b) Links the endocrine system with the nervous system 68

69 1. The limbic system a) In the area of the diencephalon b) Controls emotional experience and expression c) Generates pleasant or unpleasant feelings about experiences d) Guides behavior that may enhance the chance of survival using these feelings 69

70 A. Brain Stem 1. Lies at the base of the cerebrum 2. Connects the brain to the spinal cord 3. Consists of the: a) Midbrain b) Pons c) Medulla oblongata 70

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72 1. Midbrain a) Located between the diencephalon and pons b) Contains bundles of myelinated axons that convey impulses to and from higher parts of the brain c) Masses of gray matter that serve as reflex centers, including centers for auditory and visual reflexes. 2. Pons a) lies between the midbrain and medulla oblongata b) transmits impulses between the brain and spinal cord c) contains centers that regulate the rate and depth of breathing 72

73 1. Medulla Oblongata a) transmits all ascending and descending impulses between the brain and spinal cord b) houses nuclei that control visceral functions, including 1) the cardiac center that controls heart rate 2) the vasomotor center for blood pressure control 3) the respiratory center that works, along with the pons, to control the rate and depth of breathing 4) Other nuclei associated with coughing, sneezing, swallowing, and vomiting 73

74 1. Reticular Formation a) Throughout the brain stem, hypothalamus, cerebrum, cerebellum, and basal ganglia, is a complex network of nerve fibers connecting tiny islands of gray matter; this network is the reticular formation. b) Decreased activity in the reticular formation results in sleep; increased activity results in wakefulness. c) The reticular formation filters incoming sensory impulses 74

75 A. Cerebellum 1. Two hemispheres connected by a vermis 2. The cerebellar cortex is a thin layer of gray matter that lies outside a core of white matter 3. Communicates with other parts of the CNS via cerebellar peduncles 4. Integrates sensory information about the position of body parts 5. Coordinates skeletal muscle activity and maintains posture 75

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77 Peripheral Nervous System A. The peripheral nervous system (PNS) consists of the cranial and spinal nerves that arise from the central nervous system and travel to the remainder of the body. B. Includes two components 1. The somatic nervous system that oversees voluntary activities 2. The autonomic nervous system that controls involuntary activities. 77

78 A Cranial Nerves arise from the underside of the brain most are mixed nerves Twelve pairs designated by number and name include the: I. II. III. IV. V. VI. Olfactory nerves Optic nerves Oculomotor nerves Trochlear nerves Trigenimal nerves Abducens nerves VII. Facial nerves VIII. Vestibulocochlear nerves IX. Glossopharyngeal nerves X. Vagus nerves XI. Accessory nerves XII. Hypoglossal nerves 78

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81 A. Spinal Nerves 1. Thirty-one pairs of mixed nerves 2. Grouped according to the level from which they arise 3. Numbered in sequence beginning in the cervical region 4. Each spinal nerve arises from two roots: a) a dorsal root (sensory root) b) a ventral root (motor root) 81

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83 1. The main branches of some spinal nerves form plexuses. a) Cervical plexuses 1) Lie on either side of the neck 2) Supply muscles and skin of the neck b) Brachial plexuses 1) Arise from lower cervical and upper thoracic nerves 2) Lead to the upper limbs c) Lumbrosacral plexuses 1) arise from the lower spinal cord 2) Lead to the lower abdomen, external genitalia, buttocks, and legs 83

84 Autonomic Nervous System A. The autonomic nervous system (ANS) maintains the homeostasis of visceral activities without conscious effort B. General Characteristics 1. Motor pathways include two fiber types: a) preganglionic fibers leave the CNS b) postganglionic fibers innervate the effectors 84

85 1. Includes two divisions exerting opposing effects on target organs: a) sympathetic 1) operates under conditions of stress or emergency 2) Fibers arise from the thoracic and lumbar regions of the spinal cord 3) synapse in paravertebral ganglia close to the vertebral column 4) Postganglionic axons lead to an effector organ b) Parasympathetic 1) operates under normal conditions 2) Fibers arise from the brainstem and sacral region of the spinal cord 3) synapse in ganglia close to the effector organ 85

86 Sympathetic Parasympathetic 86

87 1. Autonomic Neurotransmitters a) Preganglionic fibers of both sympathetic and parasympathetic divisions release acetylcholine. b) Parasympathetic postganglionic fibers are cholinergic fibers and release acetylcholine c) Sympathetic postganglionic fibers are adrenergic and release norepinephrine d) The effects of these two divisions are generally antagonistic due to the effects of releasing different neurotransmitters to the effector 87

88 1. Control of Autonomic Activity a) largely controlled by reflex centers in the brain and spinal cord b) The limbic system and cerebral cortex alter the reactions of the autonomic nervous system through emotional influence 88