Chapter 9. Nervous System

Transcription

1 Chapter 9 Nervous System 1

2 9.1 Introduction A. The nervous system is composed of neurons and neuroglia. 1. Neurons transmit nerve impulses along nerve fibers to other neurons. 2. Neurons typically have a cell body, an axon, and dendrites. 3. Nerves are made up of bundles of nerve fibers (axons). 4. Neuroglia carry out a variety of functions to aid and protect components of the nervous system. 2

3 B. The synapse 1. A small space lies between a neuron and the cell it is communicating with. 2. The nerve impulse can not jump over this space. 3. Neurotransmitters are released and convey the information across the synapse. 3

4 C. Organs of the nervous system can be divided into the central nervous system (CNS), made up of the brain and spinal cord, and the peripheral nervous system (PNS), made up of nerves that connect the CNS to the rest of the body. D. General functions of the nervous system sensory, integrative, and motor 4

5 9.2 General functions of the nervous system A. Sensory receptors at the ends of peripheral nerves gather information and convert it into nerve impulses. B. When sensory impulses are brought together, create sensation, add to memory, or produce thought in the brain as perceptions, this is the integrative function of the nervous system. C. Conscious or subconscious decisions follow, leading to motor functions via effectors (muscles or glands). D. Motor functions can be divided into the consciously controlled somatic nervous system and the unconscious autonomic system. 5

6 9.3 Neuroglia A. Neuroglial cells fill spaces, support neurons, provide structural frameworks, produce myelin, and carry on phagocytosis; they have the ability to divide. B. Central nervous system neuroglia 1. Microglial cells are small cells that phagocytize bacterial cells and cellular debris. 2. Oligodendrocytes form the myelin sheath in the brain and spinal cord. 6

7 Neuroglia, cont 3. Astrocytes are between blood vessels and neurons a. Provide support b. Regulate nutrients and ions c. Respond to brain injury by filling in spaces with scar tissue. d. Form the blood-brain barrier that protects brain tissue from chemical fluctuation and prevents entry of many substances 4. Ependymal cells line the inside of ventricles and covers choroid plexuses within the ventricles. C. Schwann cells are the myelin-producing neuroglia of the peripheral nervous system. 7

8 Fluid-filled cavity of the brain or spinal cord Neuron Ependymal cell Oligodendrocyte Astrocyte Microglial cell Axon Myelin sheath (cut) Capillary Node 8

9 9.4 Neurons A. Neuron Structure 1. A neuron contains a cell body, tubular cytoplasm filled dendrites, and an axon a. The granular cell body contains mitochondria, lysosomes, a Golgi apparatus, chromatophilic substance (Nissl bodies similar to rough ER), neurofibrils, and a large nucleus with a nucleolus. b. Short, branching dendrites carry impulses from other neurons (or from receptors) toward the cell body. 9

10 Neuron structure, cont. c. The axon transmits the impulse away from the axon hillock of the cell body and may give off side branches. 2. Myelination of axons a. Larger axons are enclosed by myelin sheaths produced by Schwann cells and are called myelinated fibers. b. The outer layer of myelin is surrounded by a neurilemma (neurilemmal sheath) made up of the cytoplasm and nuclei of the Schwann cell. 10

11 Myelinated axons, cont. c. Narrow gaps in the myelin sheath between Schwann cells are called nodes of Ranvier. d. The smallest axons lack a myelin sheath and are unmyelinated fibers. e. White matter in the CNS is due to myelin sheaths in this area. f. Unmyelinated nerve tissue in the CNS comprise the gray matter. 11

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13 Myelinated axons, cont. g. Peripheral neurons are able to regenerate because of the neurilemma but the CNS axons are myelinated by oligodendrocytes thus lacking neurilemma and usually do not regenerate. h. Neural stem cells in the hippocampus and near the ventricles can divide to produce new neurons or neuroglia. 13

14 B. Classification of Neurons 1. Structural classification a. Multipolar neurons have many dendrites and one axon arising from their cell bodies and are commonly found in the CNS. b. Bipolar neurons are found in the eyes, nose, and ears, and have a single axon and a single dendrite extending from opposite sides of the cell body. c. Unipolar neurons are found in ganglia outside the CNS and have one axon that divides; the peripheral process has dendrites near a peripheral body part and a central process that runs into the CNS. 14

15 2. Classification by function a. Sensory neurons (afferent neurons) conduct impulses from peripheral receptors to the CNS and are usually unipolar, although some are bipolar neurons. b. Interneurons (association or internuncial neurons) are multipolar neurons lying within the CNS that form links between other neurons. The cell bodies of some interneurons aggregate in specialized masses called nuclei. c. Motor neurons (efferent neurons) are multipolar neurons that conduct impulses from the CNS to effectors. 15

16 Peripheral Nervous System Sensory receptor

17 Peripheral Nervous System Sensory neuron Sensory (afferent) neurons transfer sensory information to neurons in the CNS Sensory receptor

18 Central Nervous System Interneuron Interneuron Interneurons transfer information from one part of the CNS to another Peripheral Nervous System Sensory neuron Sensory (afferent) neurons transfer sensory information to neurons in the CNS Sensory receptor

19 Central Nervous System Interneuron Interneuron Motor neuron Interneurons transfer information from one part of the CNS to another Peripheral Nervous System Sensory neuron Sensory (afferent) neurons transfer sensory information to neurons in the CNS Motor (efferent) neurons transfer instructions from the CNS to effectors Sensory receptor

20 Central Nervous System Interneuron Interneuron Motor neuron Interneurons transfer information from one part of the CNS to another Peripheral Nervous System Sensory neuron Sensory (afferent) neurons transfer sensory information to neurons in the CNS Motor (efferent) neurons transfer instructions from the CNS to effectors Sensory receptor Effector (muscle or gland)

21 9.5 The synapse A. The synapse is the junction between two communicating neurons; there exists a synaptic cleft between them across which the impulse must be conveyed. 1. The neuron sending the impulse is the presynaptic neuron. 2. The neuron receiving the impulse is the postsynaptic neuron. 21

22 B. Synaptic transmission is the process whereby the message crosses the synaptic cleft. 1. Neurotransmitters are biochemicals that carry out the synaptic transmission process in the synaptic cleft. 2. Distal end of axons have extensions called synaptic knobs which contain synaptic vesicles filled with neurotransmitters. 3. The action of the neurotransmitters are either excitatory or inhibitory. 22

23 9.6 Cell membrane potential A. A cell membrane is usually polarized, with an excess of negative charges on the inside of the membrane; polarization is important to the conduction of nerve impulses (action potentials). 23

24 B. Distribution of Ions 1. The distribution of ions is determined by the membrane channel proteins that are selective for certain ions. 2. Potassium ions pass through the membrane more readily than do sodium ions, making potassium ions a major contributor to membrane polarization. 3. There is a greater concentration of sodium ions on the outside and a greater concentration of potassium ions, along with large negatively charged 24 ions on the inside.

25 C. Resting Potential 1. Due to active transport, the cell maintains a greater concentration of sodium ions outside and a greater concentration of potassium ions inside the membrane. 2. The inside of the membrane has excess negative charges, while the outside has more positive charges. 3. This separation of charge, or potential difference, is called the resting potential and has a value of -70mV. 4. The Na + /K + pump actively transports Na + out and K + in to maintain the resting potential. 25

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27 D. 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. (-55mV) 5. Local potentials are graded the magnitude of change to the resting potential is directly proportional to the intensity of the stimulation. 27

28 E. Action Potential 1. At threshold potential, membrane permeability to sodium suddenly changes at the axon hillock, called the trigger zone. 2. As sodium channels open, sodium ions rush in, and the membrane potential changes and becomes depolarized. 3. At the same time, potassium channels open to allow potassium ions to leave the cell, the membrane becomes repolarized, and resting potential is reestablished. 4. The membrane may become overly negative for a short time which is hyperpolarization. 28

29 Action potential, cont. 5. This rapid sequence of events is the action potential. 6. The active transport mechanism then works to maintain the original concentrations of sodium and potassium ions. 29

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31 9.7 Impulse conduction A. Only axons are capable of action potentials 1. A nerve impulse is conducted as an action potential is reached at the trigger zone. 2. An action potential at the trigger zone causes a bioelectrical current to flow to adjacent regions of the axon membrane. 3. This spreads by a local current flowing down the fiber that stimulates the next region continuing down the axon to its end. 31

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33 B. 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, a phenomenon called saltatory conduction. 3. Saltatory conduction is many times faster than conduction on unmyelinated neurons. 4. Speed of impulse conduction is proportional to the diameter of the axon a. Thick, myelinated motor axons conduct at 120 m/s b. Thin, unmyelinated sensory axons conduct at 0.5 m/s 33

34 C. All-or-None Response 1. If a nerve fiber responds at all to a stimulus of threshold or above, it responds completely by conducting an impulse (all-or-none response). 2. All action potentials are of the same strength. 3. Greater intensity of stimulation triggers more impulses per second, not stronger impulses. 4. Refractory period short period of time after an action potential when a threshold stimulus will NOT trigger another action potential. a. Limits frequency b. Ensures the impulse is only transmitted in one direction down the axon. 34

35 9.8 Synaptic transmission A. Excitatory and Inhibitory Actions 1. Neurotransmitters that increase postsynaptic membrane permeability to sodium ions may trigger impulses and are thus excitatory. 2. Other neurotransmitters may decrease membrane permeability to sodium ions, reducing the chance that it will reach threshold, and are thus inhibitory. 3. When an impulse reaches the synaptic knobs of an axon, synaptic vesicles release a neurotransmitter into the synaptic cleft. 4. The neurotransmitter reacts with specific receptors on the postsynaptic membrane. 35

36 B. Neurotransmitters 1. More than 100 neurotransmitters are produced by the nervous system, most of which are synthesized in the cytoplasm of the synaptic knobs and stored in synaptic vesicles. 2. Types: acetylcholine, biogenic amines, amino acids, and more than 50 neuropeptides. 36

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38 C. Synaptic transmission 1. When an action potential reaches the synaptic knob, calcium ions rush inward and, in response, some synaptic vesicles fuse with the membrane and release their contents to the synaptic cleft. 2. Enzymes in synaptic clefts and on postsynaptic membranes rapidly decompose the neurotransmitters after their release. (Acetylcholinesterase breaks down acetylcholine) 3. Destruction or removal of the neurotransmitter prevents 38 continuous stimulation of the postsynaptic neuron.

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40 9.9 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 that work together. 2. Each pool receives input from afferent nerves and processes the information according to the special characteristics of the pool. 40

41 C. Facilitation 1. A particular neuron of a pool may receive excitatory or inhibitory stimulation; if the net effect is excitatory but subthreshold, the neuron becomes more excitable to incoming stimulation (a condition called facilitation). 2. This second stimulus may bring the postsynaptic cell to threshold and an action potential is created. 41

42 D. Convergence 1. A single neuron within a pool may receive impulses from two or more fibers (convergence), which makes it possible for the neuron to summate impulses from different sources. E. Divergence 1. Impulses leaving a neuron in a pool may be passed into several output fibers (divergence), a pattern that serves to amplify an impulse. 42

43 9.10 Types of nerves A. A nerve is a bundle of nerve fibers (axons) held together by layers of connective tissue. B. Connective tissue wraps 1. Epineurium outer covering of a nerve 2. Perineurium covering of fascicles of nerve fibers (bundles) 3. Endoneurium covering of individual nerve fibers (axons) 43

44 C. Types of nerves 1. Sensory or afferent nerves bring information to the CNS. 2. Motor or efferent nerves carry impulses from the CNS to effectors. 3. Mixed nerves carry sensory and motor fibers; most nerves are of this type. 44

45 9.11 Nerve pathways A. The routes nerve impulses travel are called pathways, the simplest of which is a reflex arc. B. Reflex arc components 1. A sensory receptor that detects changes. 2. Sensory neuron that carries the information toward the CNS 3. An interneuron in the CNS (reflex center) 4. Motor neuron that carries a response to effectors 5. An effector that responds to the initial change. 45

46 C. Reflex Behavior 1. Reflexes are automatic responses to changes (stimuli) in or out of the body, that help maintain homeostasis (heart rate, blood pressure, etc.) and carry out automatic responses (vomiting, sneezing, swallowing, etc.). 2. The patellar reflex (knee-jerk reflex) tendon is an example of a simple reflex (no interneuron). a. Helps maintain upright posture 46

47 Spinal cord Receptor associated with dendrites of sensory neuron Patella Patellar ligament

48 Axon of sensory neuron Spinal cord Cell body of sensory neuron Receptor associated with dendrites of sensory neuron Patella Direction of impulse conduction Patellar ligament

49 Axon of sensory neuron Cell body of sensory neuron Spinal cord Axon of motor neuron Cell body of motor neuron Receptor associated with dendrites of sensory neuron Patella Direction of impulse conduction Patellar ligament

50 Axon of sensory neuron Cell body of sensory neuron Spinal cord Axon of motor neuron Cell body of motor neuron Effector (extensor muscles contract and cause a kicking motion) Receptor associated with dendrites of sensory neuron Patella Direction of impulse conduction Patellar ligament

51 Reflex behavior, cont. 3. The withdrawal reflex involves sensory neurons, interneurons, and motor neurons. a. Motor signals are sent to flexor muscles to contract. b. At the same time, the antagonistic extensor muscles are inhibited and a message is sent to the brain. c. Serves a protective function 51

52 Cell body of sensory neuron Direction of impulse conduction Axon of sensory neuron Spinal cord Dendrite of sensory neuron Pain receptor in skin Tack

53 Cell body of sensory neuron Direction of impulse conduction Axon of sensory neuron Interneuron Spinal cord Dendrite of sensory neuron Pain receptor in skin Tack

54 Cell body of sensory neuron Direction of impulse conduction Axon of sensory neuron Interneuron Dendrite of sensory neuron Effector (flexor muscles contract and withdraw part being stimulated) Axon of motor neuron Spinal cord Cell body of motor neuron Pain receptor in skin Tack

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56 9.12 Meninges A. The brain and spinal cord are surrounded by membranes called meninges that lie between the bone and the soft tissues. B. The outermost meninx is made up of tough, white dense connective tissue, contains many blood vessels, and is called the dura mater. 1. It forms the inner periosteum of the skull bones. 2. In some areas, the dura mater forms partitions between lobes of the brain, and in others, it forms dural sinuses. 3. The sheath around the spinal cord is separated from the vertebrae by an epidural space. 56

57 C. The middle meninx, the arachnoid mater, is thin and lacks blood vessels. 1. It does not follow the convolutions of the brain. 2. Between the arachnoid and pia mater is the subarachnoid space containing cerebrospinal fluid (CSF). D. The innermost pia mater is thin and contains many blood vessels and nerves. 1. It is attached to the surface of the brain and spinal cord and follows their contours.

58 9.13 Spinal cord A. The spinal cord begins at the base of the brain at the foramen magnum and extends as a slender cord to the level of the intervertebral disc between the first and second lumbar vertebrae.

59 B. Structure of the Spinal Cord 1. The spinal cord consists of 31 segments, each of which gives rise to a pair of spinal nerves. 2. A cervical enlargement gives rise to nerves leading to the upper limbs, and a lumbar enlargement gives rise to those innervating the lower limbs. 3. Two deep longitudinal grooves (anterior median fissure and posterior median sulcus) divide the cord into right and left halves. 4. White matter, made up of bundles of myelinated nerve fibers (nerve tracts), surrounds a butterfly-shaped core of gray matter housing interneurons and cell bodies. 59

60 Spinal cord structure, cont. 5. The upper and lower wings of gray matter are the posterior and anterior horns and between them is the lateral horn. 6. The gray matter divides the white matter into three regions - anterior, lateral and posterior funiculi (columns), each consisting of longitudinal bundles called tracts. 7. A central canal contains cerebrospinal fluid. 60

61 C. Functions of the Spinal Cord 1. The spinal cord has two major functions: to transmit impulses to and from the brain, and to house spinal reflexes. 2. Tracts carrying sensory information to the brain are called ascending tracts; descending tracts carry motor information from the brain. 3. The names that identify nerve tracts identify the origin and termination of the fibers in the tract. a. Spinothalamic tracts carry sensory information to the thalamus b. Corticospinal tracts (pyramidal tracts) carry motor impulses from the cerebral cortex, down the spinal cord 61

62 Tracts of the Spinal Cord c. Corticospinal tracts are also called pyramidal tracts because they pass through and cross in the pyramids of the medulla oblongata. d. Other descending tracts dealing with posture and balance are extra-pyramidal tracts and do not pass through the medullary pyramids. 62

63 9.14 The brain A. Introduction 1. The brain is the largest, most complex portion of the nervous system, containing 100 billion multipolar neurons. 2. The brain can be divided into the cerebrum (largest portion and associated with higher mental functions), the diencephalon (processes sensory input and controls many homeostatic processes), the cerebellum (coordinates muscular activity), and the brain stem (coordinates and regulates visceral activities). 63

64 B. Structure of the Cerebrum 1. The cerebrum is the largest portion of the mature brain, consisting of two cerebral hemispheres. 2. A flat bundle of nerve fibers called the corpus callosum connects the hemispheres. 3. The surface of the brain is marked by gyri, sulci, and fissures (longitudinal, transverse). 64

65 Structure of the cerebrum, cont. 4. Four lobes of the brain are named according to the bones they underlie and include the frontal lobe, parietal lobe, temporal lobe, occipital lobe, and the fifth lobe is the insula. 5. A thin layer of gray matter, the cerebral cortex, lies on the outside of the cerebrum and contains 75% of the cell bodies in the nervous system. 6. Beneath the cortex lies a mass of white matter made up of myelinated nerve fibers connecting the cell bodies of the cerebral cortex with the rest of the nervous system. 65

66 C. Functions of the Cerebrum 1. The cerebrum provides higher brain functions, such as interpretation of sensory input, initiating voluntary muscular movements, memory, and integrating information for reasoning. 66

67 2. Functional Regions of the Cerebral Cortex a. The functional areas of the brain overlap, but the cortex can generally be divided into motor, sensory, and association areas. b. The sensory areas are located in several areas of the cerebrum and interpret sensory input, producing feelings or sensations. 1) Cutaneous senses anterior parietal lobe 2) Visual area posterior occipital lobe 3) Auditory area temporal lobe 4) Taste area base of central sulcus 5) Smell area - insula 67

68 Functional regions, cont. c. The various association areas of the brain analyze and interpret sensory impulses and function in reasoning, judgment, emotions, verbalizing ideas, and storing memory. 1) Association areas of the frontal lobe control a number of higher intellectual processes. 2) Association areas next to sensory areas are important for analyzing that sensory input 3) A general interpretive area is found at the junction of the parietal, temporal, and occipital lobes, and plays a primary role in complex thought processing. 68

69 Functional areas, cont. d. The primary motor areas lie in the posterior frontal lobes, anterior to the central sulcus. 1) This region includes the pyramidal cells that are also called upper motor neurons. They synapse with lower motor neurons that exit the spinal cord and reach the skeletal muscles. 2) Broca s motor speech area is in the frontal lobe. 3) Frontal eye field controls voluntary eye movements 69

70 Functional areas, cont. e. Sensory and motor fibers alike cross over in the spinal cord or brain stem so centers in the right hemisphere are interpreting or controlling the left side of the body, and vice versa. 70

71 3. Hemisphere Dominance a. Both cerebral hemispheres function in receiving and analyzing sensory input and sending motor impulses to the opposite side of the body, but one side is dominant. b. For most people, the left hemisphere is dominant for the language-related activities of speech, writing, and reading, as well as complex intellectual functions. c. Some individuals have the right hemisphere as dominant and others show equal dominance in both hemispheres. 71

72 Hemisphere dominance, cont. d. The non-dominant hemisphere specializes in nonverbal functions and controls emotions and intuitive thinking. e. The nerve fibers of the corpus callosum connect the two hemispheres. 72

73 4. The basal nuclei are masses of gray matter located deep within the cerebral hemispheres that relay motor impulses from the cerebrum and help to control motor activities by producing the inhibitory neurotransmitter, dopamine. a. Basal nuclei include the caudate nucleus, the putamen, and the globus pallidus. b. The neurons of the basal nuclei interact with the motor cortex, thalamus, and cerebellum c. Altered activity of these nuclei neurons produces the signs of Parkinson disease and Huntington disease. 73

74 D. Ventricles and Cerebrospinal Fluid 1. The ventricles are a series of connected cavities within the cerebral hemispheres and brain stem. 2. Names: 2 Lateral ventricles, third ventricle, and fourth ventricle. The interventricular foramen connects the lateral ventricles to the third ventricle and the cerebral aqueduct connects the third and fourth ventricles. 3. The ventricles are continuous with the central canal of the spinal cord and the subarachnoid space, and are filled with cerebrospinal fluid. 74

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76 4. Choroid plexuses, specialized capillaries from the pia mater, secrete cerebrospinal fluid into the ventricles; most cerebrospinal fluid arises in the lateral ventricles. 5. Cerebrospinal fluid has nutritive as well as protective (cushioning) functions. 6. CSF circulates through the ventricles and connecting passageways into the subarachnoid space where it is reabsorbed back into the blood. 76

77 E. Diencephalon 1. The diencephalon lies between the cerebral hemispheres and above the midbrain. 2. The main parts are the thalamus and hypothalamus. 3. Other portions of the diencephalon are the optic tracts and optic chiasma, the infundibulum (attachment for the pituitary), the posterior pituitary, mammillary bodies, and the pineal gland. 77

78 4. The thalamus functions in sorting and directing sensory information arriving from other parts of the nervous system, performing the services of both messenger and editor. a. Pinpoints the origin of the sensory input. b. Produces general awareness of the sensation. c. Descending fibers from the cerebral cortex also communicate with the thalamus. 78

79 5. The hypothalamus maintains homeostasis by regulating a wide variety of visceral activities and by linking the endocrine system with the nervous system. a. Regulates heart rate and arterial blood pressure b. Regulates body temperature, water and electrolyte balance, hunger and body weight c. Controls movements and secretions of the digestive tract. d. Helps to regulate sleep and wakefulness. e. Stimulates the posterior pituitary gland to secrete stored hormones. 79

80 6. The limbic system, in the area of the diencephalon, controls emotional experience and expression. By generating pleasant or unpleasant feelings about experiences, the limbic system guides behavior that may enhance the chance of survival. 80

81 F. Brain Stem 1. The brain stem, consisting of the midbrain, pons, and medulla oblongata, lies at the base of the cerebrum, and connects the brain to the spinal cord. 81

82 2. Midbrain a. The midbrain, located between the diencephalon and pons, contains bundles of myelinated nerve fibers that convey impulses to and from higher parts of the brain. b. It also contains masses of gray matter that serve as centers for auditory and visual reflexes. 82

83 3. Pons a. The pons, lying between the midbrain and medulla oblongata, transmits impulses between the brain and spinal cord, and contains centers that help regulate the rate and depth of breathing. b. The ventral surface has transverse fibers that communicate with the cerebellum. 83

84 4. Medulla Oblongata a. The medulla oblongata transmits all ascending and descending impulses between the brain and spinal cord. b. Most of these tracts will cross in the pyramids of the medulla oblongata. c. The medulla oblongata also houses nuclei that control visceral functions, including the cardiac center that controls heart rate, the vasomotor center for blood pressure control, and the respiratory center that works, along with the pons, to control the rate and depth of breathing. 84

85 Medulla oblongata, cont. d. Other nuclei in the medulla oblongata are associated with coughing, sneezing, swallowing, and vomiting. 85

86 5. 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. 86

87 G. Cerebellum 1. The cerebellum is located beneath the posterior part of the cerebrum and on top of the brain stem. 2. The cerebellum is made up of two hemispheres connected by the vermis. 3. A thin layer of gray matter called the cerebellar cortex lies outside a core of white matter called the arbor vitae. 87

88 Cerebellum, cont. 4. The cerebellum communicates with other parts of the central nervous system through cerebellar peduncles. 5. The cerebellum functions to integrate sensory information about the position of body parts and coordinates skeletal muscle activity and maintains posture. 88

89 9.15 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. The PNS is made up of the somatic nervous system that oversees voluntary activities, and the autonomic nervous system that controls involuntary activities. 89

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91 C. Cranial Nerves 1. Twelve pairs of cranial nerves arise from the underside of the brain, most of which are mixed nerves. 2. The 12 pairs are designated by number and name and include the olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal nerves (XII). 91

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93 D. Spinal Nerves 1. Thirty-one pairs of mixed nerves make up the spinal nerves. 2. Spinal nerves are grouped according to the level from which they arise and are numbered in sequence, beginning with 8 pairs of cervical nerves, then 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves and 1 pair of coccygeal nerves. 3. Each spinal nerve arises from two roots: a sensory dorsal root, and a motor ventral root. a. The dorsal root has the dorsal root ganglion 4. Roots join and exit through intervertebral foramen. 93

94 5. The main branches of most spinal nerves form networks called plexuses. a. Cervical Plexuses (C1-C5) lie on either side of the neck and supply muscles and skin of the neck, and also includes the phrenic nerves which serve the diaphragm. b. Brachial Plexuses arise from lower cervical and upper thoracic nerves (C6-T1) and lead to the upper limbs including the musculocutaneous, ulnar, median, radial, and axillary nerves. 94

95 Plexuses, cont. c. Lumbosacral Plexuses arise from the lower spinal cord (T12-S5) and lead to the lower abdomen, external genitalia, buttocks, and legs, including the obturator, femoral, and sciatic nerves. 6. Anterior branches of the thoracic spinal nerves do not form plexuses but become the intercostal nerves. 95

96 9.16 Autonomic nervous system A. The autonomic nervous system has the task of maintaining homeostasis of visceral activities without conscious effort. 1. Supplies smooth muscle, cardiac muscle, and glands. 2. Helps maintain homeostasis, responds to emotional stress, and prepares the body for strenuous activity. 96

97 B. General Characteristics 1. Autonomic activities are regulated by visceral reflexes. 2. The autonomic nervous system includes two divisions: the sympathetic and parasympathetic divisions, which exert opposing effects on target organs. a. The parasympathetic division operates under normal, restful conditions. b. The sympathetic division operates under conditions of stress or emergency (fight or flight). 97

98 C. Autonomic Neurons 1. In the autonomic motor system, motor pathways include two fibers: a preganglionic fiber that leaves the CNS, and a postganglionic fiber that innervates the effector. The two fibers communicate through a ganglion. 98

99 2. Sympathetic Division a. Short preganglionic fibers in the sympathetic division arise from the thoracic and lumbar regions of the spinal cord, and synapse in paravertebral ganglia close to the vertebral column. b. Long postganglionic axons lead to an effector organ (most will travel with a spinal nerve). 99

100 3. Parasympathetic Division a. Long, preganglionic fibers in the parasympathetic division arise from the brainstem and sacral region of the spinal cord, and synapse in ganglia close to the effector organ. b. Short, postganglionic fibers continue into the effector organs.

101 D. Autonomic Neurotransmitters 1. Preganglionic fibers of both sympathetic and parasympathetic divisions release acetylcholine and are called cholinergic fibers. 2. Parasympathetic postganglionic fibers are cholinergic fibers and release acetylcholine. 3. Sympathetic postganglionic fibers are adrenergic and release norepinephrine. 4. The effects of these two divisions, based on the effects of releasing different neurotransmitters to the effector, are generally antagonistic. 101

102 Autonomic Neurotransmitters, cont. 5. Most organs receive innervation from both divisions. 6. Some effectors are innervated by only one division; example blood vessels only have sympathetic nerves. 102

103 E. Control of Autonomic Activity 1. The autonomic nervous system is largely controlled by reflex centers in the brain and spinal cord. 2. The limbic system and cerebral cortex alter the reactions of the autonomic nervous system through emotional influence. 103

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