1. Chapter 8 The Nervous System

2. ORGANS AND DIVISIONS OF THE NERVOUS SYSTEM (Figure 8-1)
Central nervous system (CNS)—brain and spinal cord
Peripheral nervous system (PNS)—all nerves
Autonomic nervous system (ANS)

4. CELLS OF THE NERVOUS SYSTEM
Neurons
Consist of three main parts—dendrites; cell body of neuron; axon (Figure 8-2)
Dendrites—branching projections that conduct impulses to cell body of neuron
Axon—elongated projection that conducts impulses away from cell body of neuron
Neurons classified according to function
Sensory (afferent) neurons—conduct impulses to the spinal cord and brain
Motor (efferent) neurons—conduct impulses away from brain and spinal cord to muscles and glands
Interneurons—conduct impulses from sensory neurons to motor neurons or among a network of interneurons; also known as central or connecting neurons

6. CELLS OF THE NERVOUS SYSTEM (cont.)
Glia (neuroglia)
Support cells, bringing the cells of nervous tissue together structurally and functionally
Three main types of glial cells of the CNS (Figure 8-3)
Astrocytes—star-shaped cells that anchor small blood vessels to neurons
Microglia—small cells that move in inflamed brain tissue carrying on phagocytosis
Oligodendrocytes—form myelin sheaths on axons in the CNS
Schwann cells form myelin sheaths on axons of the PNS (Figure 8-2)

8. NERVES AND TRACTS (Figure 8-4)
Nerve—bundle of peripheral axons
Tract—bundle of central axons
White matter—tissue composed primarily of myelinated axons (nerves or tracts)
Gray matter—tissue composed primarily of cell bodies and unmyelinated fibers
Nerve coverings—fibrous connective tissue
Endoneurium—surrounds individual fibers within a nerve
Perineurium—surrounds a group (fascicle) of nerve fibers
Epineurium—surrounds the entire nervea

10. REFLEX ARCS
Nerve impulses are conducted from receptors to effectors over neuron pathways or reflex arcs; conduction by a reflex arc results in a reflex (that is, contraction by a muscle or secretion by a gland)
The simplest reflex arcs are two-neuron arcs— consisting of sensory neurons synapsing in the spinal cord with motor neurons
Three-neuron arcs consist of sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons (Figure 8-5)

12. NERVE IMPULSES
Definition—self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane (also called action potential)
Mechanism
At rest, the neuron's membrane is slightly positive on the outside—polarized—from a slight excess of Na+ on the outside
A stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron
Inward movement of Na+ depolarizes the membrane by making the inside more positive than the outside at the stimulated point; this depolarization is a nerve impulse (action potential)
The stimulated section of membrane immediately repolarizes, but by that time, the depolarization has already triggered the next section of membrane to depolarize, thus propagating a wave of electrical disturbances (depolarizations) all the way down the membrane

13. THE SYNAPSE
Definition—the place where impulses are transmitted from one neuron to another (the postsynaptic neuron) (Figure 8-7)
Synapse made of three structures—synaptic knob, synaptic cleft, and plasma membrane
Neurotransmitters bind to specific receptor molecules in the membrane of a postsynaptic neuron, opening ion channels and thereby stimulating impulse conduction by the membrane
Names of neurotransmitters—acetylcholine, catecholamines (norepinephrine, dopamine, and serotonin), endorphins, enkephalins, nitric oxide (NO), and other compounds

15. CENTRAL NERVOUS SYSTEM
Divisions of the brain (Figure 8-9 and Table 8-1)
Brainstem
Consists of three parts of brain; named in ascending order: the medulla oblongata, pons, and midbrain
Structure—white matter with bits of gray matter scattered through it
Functions
All three parts of brainstem conduct impulses to the higher parts of the brain
Sensory tracts in the brainstem conduct impulses to the higher parts of the brain
Motor tracts conduct from the higher parts of the brain to the spinal cord

18. CENTRAL NERVOUS SYSTEM (cont.)
Divisions of the brain (cont.)
Cerebellum
Structure
Second largest part of the human brain
Gray matter outer layer is thin but highly folded, forming a large surface area for processing information
Arbor vitae—internal, treelike network of white matter tracts
Function
Helps control muscle contractions to produce coordinated movements for maintaining balance, moving smoothly, and sustaining normal postures
Variety of additional coordinating effects, assisting the cerebrum and other regions of the brain

19. CENTRAL NERVOUS SYSTEM (cont.)
Divisions of the brain (cont.)
Diencephalon
Hypothalamus
Consists mainly of the posterior pituitary gland, pituitary stalk, and gray matter
Acts as the major center for controlling the ANS; therefore, it helps control the functioning of most internal organs
Controls hormone secretion by anterior and posterior pituitary glands; therefore, it indirectly helps control hormone secretion by most other endocrine glands
Contains centers for controlling body temperature, appetite, wakefulness, and pleasure

20. CENTRAL NERVOUS SYSTEM (cont.)
Divisions of the brain (cont.)
Diencephalon (cont.)
Thalamus
Dumbbell-shaped mass of gray matter extending toward each cerebral hemisphere
Relays sensory impulses to cerebral cortex sensory areas
In some way produces the emotions of pleasantness or unpleasantness associated with sensations
Pineal gland (pineal body)
Small body resembling a pine nut behind the thalamus
Adjusts output of “time-keeping hormone” melatonin in response to changing levels of external light (sunlight and moonlight)

21. CENTRAL NERVOUS SYSTEM (cont.)
Divisions of the brain (cont.)
Cerebrum
Largest part of the human brain
Outer layers of gray matter are the cerebral cortex; made up of lobes; composed mainly of dendrites and cell bodies of neurons
Interior of the cerebrum composed mainly of white matter
Tracts—nerve fibers arranged in bundles
Basal nuclei—islands of gray matter regulate automatic movements and posture
Functions of the cerebrum—mental processes of all types including sensations, consciousness, memory, and voluntary control of movements

22. CENTRAL NERVOUS SYSTEM (cont.)
Spinal cord (Figure 8-11)
Columns of white matter, composed of bundles of myelinated nerve fibers, form the outer portion of the H-shaped core of the spinal cord; bundles of axons called tracts
Interior composed of gray matter made up mainly of neuron dendrites and cell bodies
Spinal cord tracts provide two-way conduction paths—ascending and descending
Spinal cord functions as the primary center for all spinal cord reflexes; sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain

24. CENTRAL NERVOUS SYSTEM (cont.)
Coverings and fluid spaces of the brain and spinal cord
Coverings (Figure 8-13)
Cranial bones and vertebrae
Cerebral and spinal meninges—the dura mater, the pia mater, and the arachnoid mater
Fluid spaces
Subarachnoid spaces of meninges
Central canal inside cord
Ventricles in brain

26. PERIPHERAL NERVOUS SYSTEM
Cranial nerves (Figure 8-16 and Table 8-2)
Twelve pairs—attached to undersurface of the brain
Connect brain with the neck and structures in the thorax and abdomen
Spinal nerves
Thirty-one pairs—contain dendrites of sensory neurons and axons of motor neurons
Conduct impulses necessary for sensations and voluntary movements
Dermatome—skin surface area supplied by a single cranial or spinal nerve

29. AUTONOMIC NERVOUS SYSTEM
Functional anatomy
Autonomic nervous system—motor neurons that conduct impulses from the central nervous system to cardiac muscle, smooth muscle, and glandular epithelial tissue; regulates the body’s automatic or involuntary functions (Figure 8-18)
Autonomic neurons—preganglionic autonomic neurons conduct impulses from spinal cord or brainstem to an autonomic ganglion; postganglionic neurons conduct from autonomic ganglia to cardiac muscle, smooth muscle, and glandular epithelial tissue
Autonomic or visceral effectors—tissues to which autonomic neurons conduct impulses (i.e., cardiac and smooth muscle and glandular epithelial tissue)
Composed of two divisions—the sympathetic system and the parasympathetic system

31. AUTONOMIC NERVOUS SYSTEM (cont.)
Autonomic conduction paths
Consist of two-neuron relays (that is, preganglionic neurons from the central nervous system to autonomic ganglia, synapses, postganglionic neurons from ganglia to visceral effectors)
In contrast, somatic motor neurons conduct all the way from the CNS to somatic effectors with no intervening synapses

32. AUTONOMIC NERVOUS SYSTEM (cont.)
Sympathetic nervous system
Dendrites and cell bodies of sympathetic preganglionic neurons are located in the gray matter of the thoracic and upper lumbar segments of the spinal cord
Axons leave the spinal cord in the anterior roots of spinal nerves, extend to sympathetic or collateral ganglia, and synapse with several postganglionic neurons whose axons extend to spinal or autonomic nerves to terminate in visceral effectors
A chain of sympathetic ganglia is in front of and at each side of the spinal column
Functions of the sympathetic nervous system
Serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and when strong emotions (anger, fear, hate, or anxiety) are triggered
Group of changes induced by sympathetic control is called the fight-or-flight response

33. AUTONOMIC NERVOUS SYSTEM (cont.)
Parasympathetic nervous system
Structure
Parasympathetic preganglionic neurons have dendrites and cell bodies in the gray matter of the brainstem and the sacral segments of the spinal cord
Parasympathetic preganglionic neurons terminate in parasympathetic ganglia located in the head and the thoracic and abdominal cavities close to visceral effectors
Each parasympathetic preganglionic neuron synapses with postganglionic neurons to only one effector
Function—dominates control of many visceral effectors under normal, everyday conditions, counterbalances sympathetic function

34. AUTONOMIC NERVOUS SYSTEM (cont.)
Autonomic neurotransmitters
Cholinergic fibers—preganglionic axons of parasympathetic and sympathetic systems and parasympathetic postganglionic axons release acetylcholine
Adrenergic fibers—axons of sympathetic postganglionic neurons release norepinephrine (noradrenaline)
Autonomic nervous system as a whole
Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis
Many visceral effectors are doubly innervated (that is, they receive fibers from parasympathetic and sympathetic divisions and are influenced in opposite ways by the two divisions)