Chapter 8 The Nervous System

Organs and Divisions of the Nervous System Central nervous system (CNS)—brain and spinal cord Peripheral nervous system (PNS) —all nerves Autonomic nervous system (ANS)

Cells of the Nervous System Neurons Consist of three main parts—1. dendrites, which conduct impulses to cell body of neuron; 2. cell body of neuron; and 3. axon, which conducts impulses away from cell body of neuron, covered by Schwann cells in PNS Classified according to function—1. sensory: conduct impulses to the spinal cord and brain; 2. motor: conduct impulses away from brain and spinal cord to muscles and glands; and 3. interneurons: conduct impulses from sensory neurons to motor neurons (Cont’d…)

Cells of the Nervous System (…Cont’d) Glia (neuroglia) Support cells, bringing the cells of nervous tissue together structurally and functionally (Cont’d…)

Cells of the Nervous System (…Cont’d) Three main types of glial cells of the CNS Astrocytes—star-shaped cells hold small blood vessels close to neurons, forms BBB (blood-brain barrier) Microglia—small phagocytic cells that destroy inflamed brain tissue Oligodendrocytes—hold nerve fibers together and form myelin sheaths on axons in the CNS (brain & spinal cord) NOTE: Schwann cells provide same function as oligodendrocytes, but form myelin sheaths on axons in the PNS

Nerves Nerve coverings—fibrous connective tissue (…Cont’d) Nerve coverings—fibrous connective tissue Endoneurium—surrounds individual fibers within a nerve Perineurium—surrounds a group (fascicle) of nerve fibers Epineurium—surrounds the entire nerve

Nerves Nerve—bundle of peripheral axons Tract--bundle of fibers in CNS White matter—tissue composed primarily of myelinated axons (nerves or tracts) Gray matter—tissue composed primarily of cell bodies and unmyelinated fibers (Cont’d…)

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) (Cont’d…)

Reflex Arcs (…Cont’d) 1.Simplest reflex arcs are two-neuron arcs—consist of sensory neurons synapsing in the spinal cord with motor neurons; “Knee-Jerk” 2. three-neuron arcs consist of sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons “Irritant or Withdrawl reflex”

Nerve Impulses Saltatory Conduction – Impulses that jumps From Definition—self-propagating wave of electrical disturbances that travel along the surface of a neuron membrane (Cont’d…) Saltatory Conduction – Impulses that jumps From Schwann cell to Schwann cell.

Nerve Impulses Mechanism (…Cont’d) Mechanism A stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron Inward movement of positive sodium ions leaves a slight excess of negative ions outside at a stimulated point; marks beginning of a nerve impulse

The Synapse Definition—chemical compounds released from axon terminals (of a presynaptic neuron) into a synaptic cleft Neurotransmitters bind to specific receptor molecules in the membrane of a postsynaptic neuron, opening ion channels and thereby stimulating impulse conduction by the membrane (Cont’d…)

The Synapse Names of neurotransmitters: (…Cont’d) Names of neurotransmitters: Acetylcholine, catecholamines (norepinephrine, dopamine, and serotonin), and other compounds How neurotransmittersare inactivated 1.Enzymes 2.meds

Central Nervous System Divisions of the brain Brainstem Consists of three main parts of brain; medulla oblongata, pons, and midbrain Structure—white matter with bits of gray matter scattered through it Function—gray matter in brainstem functions as reflex centers (for example, heartbeat, respirations, vasodilation/vasoconstriction); sensory tracts in brainstem conduct impulses to the higher parts of the brain; motor tracts conduct from the higher parts of the brain to the spinal cord

Central Nervous System (…Cont’d) Diencephalon Hypothalamus--Structure and function Consists posterior pituitary gland, pituitary stalk, and gray matter Acts as the major center for controlling the ANS; therefore helps control the functioning of most internal organs Controls hormone secretion by anterior and posterior pituitary glands; therefore indirectly helps control hormone secretion by most other endocrine glands Contains centers for controlling appetite, wakefulness, pleasure, etc. (Cont’d…)

Central Nervous System Thalamus--Structure and function Dumbbell-shaped mass of gray matter in each cerebral hemisphere Relays sensory impulses to cerebral cortex sensory areas In some way produces the emotions of pleasantness or unpleasantness associated with sensations (Cont’d…)

Central Nervous System Cerebellum Second largest part of the human brain Helps control muscle contractions to produce coordinated movements so that we can maintain balance, move smoothly, and sustain normal posture (Cont’d…)

Central Nervous System Cerebrum Largest part of the human brain Outer layer of gray matter is 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 (that is nerve fibers arranged in bundles called tracts) Functions of the cerebrum—mental processes of all types, including sensations, consciousness, memory, and voluntary control of movements

Central Nervous System (…Cont’d) Spinal cord Outer part is composed of white matter made up of many bundles of axons called tracts; interior composed of gray matter made up mainly of neuron dendrites and cell bodies Functions as the center for all spinal cord reflexes; sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain (Cont’d…)

Central Nervous System (…Cont’d) Coverings and fluid spaces of the brain and spinal cord Coverings Cranial bones and vertebrae Cerebral and spinal meninges—the dura mater, the pia mater, and the arachnoid mater

Central Nervous System (…Cont’d) Coverings and fluid spaces of the brain and spinal cord Fluid spaces—subarachnoid spaces of meninges, central canal inside cord, and ventricle in brain

Cerebrospinal Fluid “CSF” Forms from fluid filtering out of the blood capillaries called choroid plexus

Peripheral Nervous System Cranial nerves *12 pairs—attached to undersurface of the brain *Connect brain with the neck and structures in the thorax and abdomen

Spinal Nerves 31 pairs 8 Cervical 12 Thoracic 5 Lumbar 5 Sacrospinal 1 Coccygeal These form a “Plexus” or network of braided nerve fibers

Spinal nerves Structure—contain dendrites of sensory neurons and axons of motor neurons Functions—conduct impulses necessary for sensations and voluntary movements

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 autonomic or involuntary functions (Cont’d…)

Autonomic Nervous System (…Cont’d) Autonomic neuron preganglionic autonomic neurons conduct from spinal cord or brainstem to an autonomic ganglion postganglionic neurons conduct from autonomic ganglia to cardiac muscle, smooth muscle, and glandular epithelial tissue (Cont’d…)

Autonomic Nervous System (…Cont’d) Autonomic or visceral effectors—tissues to which autonomic neurons conduct impulses (that is, cardiac and smooth muscle and glandular epithelial tissue) Composed of two divisions—the sympathetic system and the parasympathetic system (Cont’d…)

Autonomic Nervous System (…Cont’d) 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 (Cont’d…)

Autonomic Nervous System (…Cont’d) Sympathetic nervous system Structure 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 (Cont’d…)

Autonomic Nervous System (…Cont’d) 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 (Cont’d…)

Autonomic Nervous System (…Cont’d) Functions Serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and strong emotions (anger, fear, hate, or anxiety) Group of changes induced by sympathetic control is called the fight-or-flight response (Cont’d…)

Autonomic Nervous System (…Cont’d) 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 (Cont’d…)

Autonomic Nervous System (…Cont’d) Parasympathetic preganglionic neurons terminate in parasympathetic ganglia located in the head and the thoracic and abdominal cavities close to visceral effectors Function—dominates control of many visceral effectors under normal, everyday conditions (Cont’d…)

Autonomic Nervous System 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 (…Cont’d) Autonomic nervous system as a whole Regulates the body’s autonomic 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)

The End