1 Structure and Function of the Neurologic System Chapter 14.

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Structure and Function of the Neurologic SystemChapter 14

Mosby items and derived items © 2006 by Mosby, Inc.

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Overview of the Nervous System Central nervous system (CNS)

Brain and spinal cord Peripheral nervous system (PNS)

Cranial nerves Spinal nerves Pathways

Afferent (ascending) Efferent (descending)


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Overview of the Nervous System Peripheral nervous system (PNS)

Somatic nervous system Motor and sensory pathways regulating voluntary

motor control of skeletal muscle Autonomic nervous system

Motor and sensory pathways regulating the body’s internal environment through involuntary control of organ systems Sympathetic Parasympathetic


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Cells of the Nervous System Neuron

Variable size and structure throughout the nervous system

Cellular constituents Microtubules, neurofibrils, Nissl substances, and

ribosomes

Some neurons like olfactory neurons continue to divide


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Neuron Three components

Cell body (soma) Located mainly in the CNS Densely packed cell bodies in the CNS are nuclei Densely packed cell bodies in the PNS are ganglia

Dendrites Receptive portion of the neuron


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Neuron Axons

Carry nerve impulses away from the cell body Axon Hillock

Cone-shaped, Nissl-free area, where the axon leaves the cell body

Initial segment of the axon Area of the axon having the lowest threshold for

stimulation, so the action potentials begin at this point


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Neuron Axons

Myelin Segmented layer of lipid material Insulating substance Formed and maintained by the Schwann cell

Endoneurium Delicate layer of connective tissue around each axon

Neurilemma Thin membrane between the myelin sheath and the

endoneurium


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Neuron Axons

Nodes of Ranvier Regular interruptions of the myelin sheath

Saltatory conduction The flow of ions between segments of myelin rather than along

the entire length of the axon Divergence

Ability of branching axons to influence many neurons Convergence

Branches of numerous neurons converging on one or a few neurons


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Structural Classification of Neurons Based on the number of processes extending

from the cell body Unipolar Pseudounipolar Bipolar Multipolar


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Functional Classification of Neurons Sensory

Transmit impulses from sensory receptors to the CNS

Associational Transmit impulses from neuron to neuron

Motor Transmit impulses from the CNS to an effector

organ


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Neuroglia


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Neuroglia “Nerve glue” Support the neurons of the CNS

Astrocytes Oligodendroglia (oligodendrocytes) Microglia Ependymal cells


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Neuroglia


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Nerve Injury and Regeneration Wallerian degeneration

Occurs distal to the cut Swelling appears Neurofilaments hypertrophy Myelin sheath shrinks and disintegrates Axon portion degenerates and disappears


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Nerve Injury and Regeneration Proximal to the cut

Swelling and dispersal of the Nissl substance Cell increases in metabolic activity, protein synthesis, and

mitochondrial activity New terminal sprouts project from the proximal segment Process limited to myelinated axons

Generally only in the PNS

Depends on location, type of injury, the inflammatory response, and scar tissue formation


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Neuroglia


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Nerve Impulse Neurons generate and conduct electrical and

chemical impulses by selectively changing the electrical portion of their plasma membranes and influencing other nearby neurons by the release of neurotransmitters


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Synapses Region between adjacent neurons is called a synapse Impulses are transmitted across the synapse by

chemical and electrical conduction Presynaptic neurons and postsynaptic neurons

Connections between pre- and postsynaptic neurons Axoaxonic Axosomatic Axodendritic Dendrodentritic


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Synapses


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Synapses Neurotransmitters

>30 substances Excitatory (excitatory postsynaptic potential) Inhibitory (inhibitory postsynaptic potential)

Synaptic boutons Synaptic cleft Summation

Temporal summation Spatial summation


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Central Nervous System Forebrain

Cerebral hemispheres Midbrain

Corpora quadrigemina, tegmentum, and cerebral peduncles

Hindbrain Cerebellum, pons, and medulla


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Central Nervous System Reticular formation

Network of connected nuclei that regulate viral reflexes, such as cardiovascular and respiratory function

Together with the cerebral cortex is referred to as the reticular activating system


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Central Nervous System


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Central Nervous System Forebrain

Telencephalon Cerebrum

Gyri, sulci, and fissures Gray matter and white matter

Cerebral nuclei (basal ganglia)


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Forebrain


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Forebrain


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Central Nervous System Diencephalon

Epithalamus Thalamus Hypothalamus Subthalamus


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Central Nervous System Midbrain

Corpora quadrigemina (tectum) Superior and inferior colliculi

Tegmentum Red nucleus and substantia nigra

Basis pedunculi


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Central Nervous System Hindbrain (metencephalon)

Cerebellum Pons

Myelencephalon Medulla oblongata


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Central Nervous System


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Spinal Cord Lies within the vertebral canal and is

protected by the vertebral column Connects the brain and the body Conducts somatic and autonomic reflexes Provides motor pattern control centers Modulates sensory and motor function


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Spinal Cord


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Spinal Cord


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Spinal Cord


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Reflex Arc Receptor Afferent (sensory) neuron Efferent neuron Effector


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Reflex Arc


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Upper and Lower Motor Neurons Upper motor neurons

Efferent pathways primarily relaying information from the cerebrum to the brain stem or spinal cord

Synapse with interneurons Lower motor neurons

Neurons having direct influence on muscles Cell bodies originate in the gray matter of the

spinal cord, but their axons extend into the PNS


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Neuromuscular Junction


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Motor Pathways Lateral corticospinal Corticobulbar Basal ganglia Vestibulospinal


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Sensory Pathways Anterior spinothalamic Lateral spinothalamic


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Protective Structures Cranium

Eight bones Frontal Occipital Temporal (2) Parietal (2) Sphenoid Ethmoid

Galea aponeurotica


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Protective Structures Meninges

Protective membranes surrounding the brain and spinal cord Dura mater Arachnoid Pia mater


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Meninges


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Protective Structures Cerebrospinal fluid (CSF) and the ventricular

system CSF is a clear, colorless fluid similar to blood

plasma and interstitial fluid 125 to 150 mL Produced by the choroid plexuses in the lateral,

third, and fourth ventricles Reabsorbed through the arachnoid villi


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Protective Structures Vertebral column

33 vertebrae 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 4

fused coccygeal

Intervertebral disks Nucleus pulposus


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Vertebral Column


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Vertebral Column


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Blood Supply to the Brain 800 to 1000 mL per minute CO2 is the primary regulator for CNS blood

flow Internal carotid and vertebral arteries Arterial circle (circle of Willis)


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Blood Supply to the Brain


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Blood Supply to the Spinal Cord Vertebral arteries

Anterior spinal arteries Posterior spinal arteries

Aorta


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Blood Supply to the Spinal Cord


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Peripheral Nervous System 31 pairs of spinal nerves

Names correlate with the vertebral level from which they exit

Mixed nerves Arise from the anterior and posterior horn cells of

the spinal cord 12 pairs of cranial nerves

Sensory, motor, and mixed


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Peripheral Nervous System Fascicles

Bundles of myelinated axons in the PNS Rami Plexuses

Brachial plexus Lumbar plexus Sacral plexus

Dermatomes


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Peripheral Nervous System


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Autonomic Nervous System Located in both the CNS and PNS Coordinates and maintains a steady state among the

visceral (internal) organs Neurons

Preganglionic (myelinated) Postganglionic (unmyelinated)

Two divisions Sympathetic Parasympathetic


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Sympathetic Nervous System Mobilizes energy stores in times of need

“Fight or flight response” Receives innervation from cell bodies located

from the first thoracic through the second lumbar Thoracolumbar division

Sympathetic (paravertebral) ganglia


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Sympathetic Nervous System


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Parasympathetic Nervous System Functions to conserve and restore energy

“Rest or repose response” Receives innervation from cell bodies located

in the cranial nerve nuclei and sacral region of the spinal cord Craniosacral division

Preganglionic neurons travel to ganglia close to the organs they innervate


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Parasympathetic Nervous System


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Neurotransmitters and Neuroreceptors of the ANS Sympathetic preganglionic fibers

Acetylcholine and cholinergic receptors Sympathetic postganglionic fibers

Norepinephrine and adrenergic receptors Parasympathetic pre- and postganglionic

fibers Acetylcholine and cholinergic receptors


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Neurotransmitters and Neuroreceptors of the ANS


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Aging and the Nervous System Decrease in the number of neurons

Decreased brain weight and size Senile plaques Neurofibrillary tangles Slowing of neurologic responses


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Tests of Nervous System Function Skull and spine roentgenograms Computed tomography Magnetic resonance imaging

Magnetic resonance angiography Positron emission tomography scan Brain scan Cerebral angiography


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Tests of Nervous System Function Myelography Echoencephalography (ultrasound) Electroencephalography Evoked potentials Cerebrospinal fluid analysis

1 Structure and Function of the Neurologic System Chapter 14.

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