13. The Nervous System

Learning Objectives Describe the structures and functions of the neurons and neuroglia of the cerebrum, the cerebellum, the diencephalon, and the brain stem. Differentiate between white matter and gray matter. Describe the functions of afferent and efferent nerves. List the components of the central nervous system and the peripheral nervous system. Differentiate between the autonomic and somatic nervous systems. Describe the process of depolarization and repolarization of neurons. List the excitatory and inhibitory neurotransmitters and describe their role in conduction of nerve impulses. Describe the connective tissue layers surrounding the brain and spinal cord. Explain the function of the cerebrospinal fluid. List the cranial nerves and describe their functions. Differentiate between the sympathetic and parasympathetic nervous systems and between autonomic and somatic reflexes. Describe the components of a reflex arc and explain the role of each. Describe the stretch reflex, withdrawal reflex, crossed extensor reflex, palpebral reflex, and pupillary light reflex.

Neurons Basic functional units of the nervous system High requirement for oxygen Cannot reproduce but can regenerate cell processes if the cell body remains intact Neuroglia (glial cells) - provide structural and functional support and protection to neurons

Neuron Structure Central cell body (soma or perikaryon) Cell processes Dendrites – receive stimuli Axons - conducts nerve impulses away

Dendrites Receive stimuli from other neurons and conduct the stimuli to the cell body May serve as sensory receptors for heat, cold, touch, pressure, stretch, or other physical changes from inside or outside the body Short, numerous, multibranched

Axons Conduct nerve impulse away from cell toward another neuron or an effector cell Single, long process; may be covered with myelin White matter: tissue containing myelinated axons

Axons Myelinated axons conduct impulses faster than unmyelinated ones Myelin sheath: cell membrane of glial cells tightly wrapped around the axon Oligodendrocytes in the brain and spinal cord Schwann cells in the nerves outside the brain and spinal cord

Axons Multiple Schwann cells or oligodendrocytes cover the entire length of the axon Nodes of Ranvier: gaps between adjacent glial cells

Organization of Nervous System Anatomical Central nervous system (CNS) Brain and spinal cord Peripheral nervous system (PNS) Extends outward from the central axis toward the periphery of the body Cranial nerves originate directly from the brain Spinal nerves emerge from the spinal cord

Organization of Nervous System Direction of Impulses Afferent nerves - conduct impulses toward CNS Also called sensory nerves - conduct sensations from sensory receptors in the skin and other locations in the body to the CNS Efferent nerves - conduct impulses away from CNS Also called motor nerves - cause skeletal muscle contraction and movement Cranial and spinal nerves in the PNS and nerve tracts (bundles of axons) in the CNS may carry nerve fibers that are sensory, motor, or both

Organization of Nervous System Function: Autonomic vs. Somatic Somatic nervous system - actions under conscious, or voluntary, control Autonomic nervous system - controls and coordinates automatic functions Example: slowing of the heart rate in response to an increased blood pressure

Neuron Function Resting state - when a neuron is not being stimulated Resting membrane potential - difference in electrical charge across neuronal membrane Results from differences in distribution of positive and negative charges from sodium, potassium, proteins, and other charged ions on either side of the neuronal membrane

Neuron Function Sodium-potassium pump: specialized molecule that helps maintain cell resting state Pumps (Na + ) from inside of neuron to the outside Pump (K + ) from outside of neuron to the inside

Depolarization Neuron receives external stimulus Sodium channel opens on neuron cell membrane Sodium ions flow into cell by passive diffusion

Depolarization Action potential During depolarization, inside of the neuron goes from a negatively charged resting membrane potential to a net positive charge due to inflow of sodium ions Creates large change in electrical charge from negative to positive

Repolarization Sodium channels close K + channels open K + diffuses out of the cell Resting state restored

Repolarization As repolarization ends, sodium-potassium pump moves sodium and potassium ions back to their original sides Resting state restored

Threshold Stimulus Stimulus must be sufficient to make the neuron respond and cause complete depolarization “ All-or-nothing principle” - neuron depolarizes to its maximum strength or not at all Conduction of the action potential - spreading wave of opening sodium channels in sufficient numbers to allow sodium influx and depolarization Wave of depolarization or nerve impulse

Refractory Period Time period during which a neuron is insensitive to additional stimuli Cell is still in depolarization/early repolarization Absolute refractory period - during sodium influx and early potassium outflow Relative refractory period - during end of the repolarization period May be possible to stimulate another depolarization if stimulus is very large

Saltatory Conduction Rapid means of conducting an action potential Depolarization in myelinated axons can only take place at the nodes of Ranvier

Synaptic Transmission Synapse - junction between two neurons or a neuron and a target cell Synaptic cleft - gap between adjacent neurons Presynaptic neuron - neuron bringing the depolarization wave to the synapse Releases neurotransmitter Postsynaptic neuron - contains receptors for the neurotransmitter

Synaptic Transmission Telodendron - branched structure on presynaptic neuron Terminal bouton - slightly enlarged bulb on each end of telodendron (synaptic end bulb, synaptic knob) Vesicles in the knob contain the neurotransmitter When depolarization wave reaches synaptic knob, vesicles fuse with the knob's cellular membrane and dump neurotransmitter into the synaptic cleft

Synaptic Transmission Neurotransmitters diffuse across the synaptic cleft toward the postsynaptic membrane Receptors on the postsynaptic membrane bind the neurotransmitter

Types of Neurotransmitters Excitatory neurotransmitters Usually cause an influx of sodium so that the postsynaptic membrane moves toward threshold Inhibitory neurotransmitters Move the charge within the postsynaptic cell farther away from threshold

Types of Neurotransmitters Acetylcholine Can be either excitatory or inhibitory depending on its location in the body Catecholamines Norepinephrine and epinephrine - associated with "fight or flight" reactions of the sympathetic nervous system Dopamine - involved with autonomic functions and muscle control Gamma-aminobutyric acid (GABA) and glycine - inhibitory

Recycling the Neurotransmitter Acetylcholinesterase - found on postsynaptic membrane; breaks down acetylcholine Monoamine oxidase (MAO) - breaks down norepinephrine Catechol-O-methyl transferase (COMT) - breaks down norepinephrine that is not reabsorbed

Central Nervous System Brain Cerebrum Cerebellum Diencephalon Brain stem Spinal Cord

Cerebrum Gray matter - cerebral cortex; outer layer of the brain White matter - fibers beneath the cortex and corpus callosum (fibers that connect the two halves of the cerebral cortex) Area of the brain responsible for higher-order behaviors (learning, intelligence, awareness, etc.)

Cerebrum Gyri (gyrus): folds in cerebral hemispheres Fissures: deep grooves separating the gyri

Cerebrum Sulci (sulcus): shallow grooves separating the gyri Divides the cerebral hemispheres into lobes Longitudinal fissure: prominent groove that divides the cerebrum into right and left cerebral hemispheres

Cerebellum Located just caudal to the cerebrum Area of the brain responsible for coordinated movement, balance, posture, and complex reflexes

Diencephalon Passageway between brain stem and cerebrum Structures associated with the diencephalon: Thalamus - acts as a relay station for regulating sensory inputs to the cerebrum Hypothalamus - interface between the nervous system and the endocrine system Pituitary - endocrine “master gland”

Brain Stem Connection between the rest of the brain and the spinal cord Composed of the medulla oblongata, the pons, and the midbrain Area of the brain responsible for basic support functions of the body Many of the cranial nerves originate from this area of the brain

Meninges Connective tissue layers that surround brain and spinal cord Contain blood vessels, fluid, and fat Supply nutrients and oxygen to the superficial tissues of the brain and spinal cord Provide some cushioning and distribution of nutrients for the CNS

Meninges Three layers: Dura mater - tough, fibrous Arachnoid - delicate, spiderweb-like Pia mater - very thin; lies directly on surface of brain and spinal cord

Cerebrospinal Fluid Fluid between layers of the meninges and in canals and ventricles inside the brain and central canal of spinal cord Provides cushioning function May play role in regulation of autonomic functions such as respiration and vomiting

Blood-brain Barrier Separates the capillaries in the brain from the nervous tissue Capillary walls in the brain have no fenestrations; covered by cell membranes of glial cells Prevents many drugs, proteins, ions, and other molecules from readily passing from the blood into the brain

Cranial Nerves 12 nerve pairs in PNS that originate directly from the brain Numbered in Roman numerals from I through XII (1 through 12) Each nerve may contain axons of motor neurons, axons of sensory neurons, or combinations of both

Spinal Cord Medulla: central part of spinal cord Composed of gray matter Central canal – center of medulla

Spinal Cord Cortex: outer part of spinal cord White matter Surrounds the gray matter

Spinal Cord Dorsal and ventral nerve roots emerge from between each pair of adjacent vertebrae Dorsal nerve roots contain sensory fibers Ventral nerve roots contain motor fibers

Spinal Cord Dorsal horns: neurons in gray matter that forward sensory nerve impulses to brain or other parts of spinal cord Ventral horns: neurons in gray matter that forward motor (efferent) nerve impulses to the spinal nerves

Autonomic Nervous System Controls automatic functions at the subconscious level Sympathetic nervous system - nerves emerge from thoracic and lumbar vertebral regions (thoracolumbar system) Parasympathetic nervous system - nerves emerge from the brain and the sacral vertebral regions (cranial-sacral)

Sympathetic/Parasympathetic Systems Efferent motor nerves are composed of a sequence of two neurons Preganglionic neuron - cell body in the brain or spinal cord and axon extended out to an autonomic ganglion Synapses with one or more additional neurons (postganglionic neuron) that are then connected to the target organ Sympathetic ganglion chain - series of autonomic ganglia outside thoracolumbar area of spinal column

Sympathetic System Sympathetic preganglionic neuron may: Synapse with a neuron within the ganglion chain Pass through the ganglionic chain and synapse with a neuron located beyond the sympathetic chain Usually synapses with many postganglionic neurons in the sympathetic chain or ganglions outside the sympathetic chain Sympathetic postganglionic neuron is much longer than its corresponding preganglionic neuron

Parasympathetic System Parasympathetic preganglionic neuron travels directly from the CNS to its target organ Synapses with a short postganglionic neuron in the target organ Parasympathetic preganglionic neuron is relatively long compared with the very short postganglionic neuron

Table 13-3. Effects of Sympathetic and Parasympathetic Nervous Systems Sympathetic System Parasympathetic System Effect Effect Heart rate Increases Decreases Force of heart contraction Increases No significant effect Diameter of bronchioles Increases (dilates) Decreases (constricts) Diameter of pupil Increases (dilates) Decreases (constricts) Gastrointestinal motility, secretions, and blood flow Decreases Increases Diameter of skin blood vessels Decreases No significant effect Diameter of muscle blood vessels Increases No significant effect Diameter of blood vessels to kidney Decreases No significant effect

Neurotransmitters and Receptors Primary neurotransmitter of sympathetic nervous system is norepinephrine Adrenergic neurons - neurons that release norepinephrine Epinephrine and norepinephrine also released from adrenal medulla

Neurotransmitters and Receptors Blood vessels in skin, GI tract, and skeletal muscle have adrenergic (catecholamine) receptors to epinephrine or norepinephrine Alpha 1 - adrenergic receptors - cause vasoconstriction of the skin, GI tract, and kidney Beta 1 -adrenergic receptors - increase heart rate and force of contraction Beta 2 -adrenergic receptors - cause bronchodilation

Neurotransmitters and Receptors Primary neurotransmitter of parasympathetic nervous system is acetylcholine Cholinergic neurons - neurons that release acetylcholine Nicotinic acetylcholine receptors - on postganglionic neurons of sympathetic and parasympathetic systems and between motor neurons and muscle Muscarinic acetylcholine receptors - on target organs and tissues supplied by the postganglionic neuron of the parasympathetic nervous system

Reflexes Somatic reflexes - involve contraction of skeletal muscles Autonomic reflexes - regulate smooth muscle, cardiac muscle, and endocrine glands Contralateral reflex - starts on one side of body and travels to opposite side Ipsilateral reflex - stimulus and response are on the same side of the body

Reflex Arc Sensory receptor sends an action potential along the sensory neuron to the gray matter of the spinal cord or brain stem Sensory neuron synapses with other neurons; incoming sensory impulse integrated with other impulses from other sensory neurons Integrated response of the reflex is sent out by the motor neuron, which ends at the target organ

Stretch Reflex Monosynaptic reflex arc Involves a sensory neuron and a motor neuron Only one synapse between them without any interneurons

Withdrawal Reflex Also called flexor reflex Several interneuron synapses Results in contraction or flexing of muscles

Crossed Extensor Reflex Contralateral reflex If withdrawal reflex initiated, afferent sensory neuron synapse with interneurons Causes contraction of opposite extensor muscles

CNS Moderation of Reflexes Upper CNS normally produces an inhibitory effect on the reflex arcs With injury, intact reflex arcs caudal to the spinal cord trauma become hyperreflexive Trauma where reflex arc enters or leaves the spinal cord, or damage to the sensory nerve or motor nerve of the reflex, results in either hyporeflexive or absent reflex arcs

Other Clinically Significant Reflexes Palpebral reflex arc: light tap on the medial canthus of the eye produces a blink of the eyelids. Pupillary light reflex (PLR): normal response to shining light in the eye of an animal is for the iris in both eyes to constrict Shining the light in one eye causes a constriction in both eyes

13. The Nervous System

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13. The Nervous System

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