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Structure and Function of
the Neurologic System
Chapter 14
1
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
3
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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
4
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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




Endoneurium


Segmented layer of lipid material
Insulating substance
Formed and maintained by the Schwann cell
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


Saltatory conduction


The flow of ions between segments of myelin rather than along
the entire length of the axon
Divergence


Regular interruptions of the myelin sheath
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


Associational


Transmit impulses from sensory receptors to the
CNS
Transmit impulses from neuron to neuron
Motor

Transmit impulses from the CNS to an effector
organ
10
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Neuroglia
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Neuroglia


“Nerve glue”
Support the neurons of the CNS




Astrocytes
Oligodendroglia (oligodendrocytes)
Microglia
Ependymal cells
12
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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
14
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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
15
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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
17
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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




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>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


Midbrain


Cerebral hemispheres
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
22
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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

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

Epithalamus
Thalamus
Hypothalamus
Subthalamus
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Central Nervous System

Midbrain

Corpora quadrigemina (tectum)


Tegmentum


Superior and inferior colliculi
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
37
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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


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
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

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
42
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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
44
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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
45
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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)
48
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Blood Supply to the Brain
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Blood Supply to the Brain
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Blood Supply to the Brain
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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

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Rami
Plexuses
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
Bundles of myelinated axons in the PNS
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


Receives innervation from cell bodies located
from the first thoracic through the second
lumbar


“Fight or flight response”
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


Receives innervation from cell bodies located
in the cranial nerve nuclei and sacral region of
the spinal cord


“Rest or repose response”
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


Sympathetic postganglionic fibers


Acetylcholine and cholinergic receptors
Norepinephrine and adrenergic receptors
Parasympathetic pre- and postganglionic
fibers

Acetylcholine and cholinergic receptors
63
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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
65
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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
66
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Tests of Nervous System Function

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
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
Myelography
Echoencephalography (ultrasound)
Electroencephalography
Evoked potentials
Cerebrospinal fluid analysis
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