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Nervous System (11) and
Cranial Nerves (13a)
Human Anatomy & Physiology, Sixth Edition
Elaine N. Marieb
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11
13a
Nervous System
The master controlling and communicating system of the body
Functions
 Sensory input – monitoring stimuli occurring inside and outside the
body
 Integration – interpretation of sensory input
 Motor output – response to stimuli by activating effector organs
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Nervous System
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Figure 11.1
Organization of the Nervous System
Central nervous system (CNS)
 Brain and spinal cord
 Integration and command center
Peripheral nervous system (PNS)
 Paired spinal and cranial nerves
 Carries messages to and from the spinal cord and brain
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Peripheral Nervous System (PNS): Two
Functional Divisions
Sensory (afferent) division
 Somatic afferent fibers – carry impulses from skin,
skeletal muscles, and joints to the brain
 Visceral afferent fibers – transmit impulses from visceral
organs to the brain
Motor (efferent) division
 Transmits impulses from the CNS to effector organs
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Motor Division: Two Main Parts
Somatic motor nervous system
 Conscious control of skeletal muscles
Autonomic motor nervous system (ANS)
 Regulates smooth muscle, cardiac muscle, and glands
 Divisions – sympathetic and parasympathetic
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Histology of Nerve Tissue
The two principal cell types of the nervous system are:
 Neurons – excitable cells that transmit electrical signals
 Supporting cells – cells that surround and wrap neurons
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Supporting Cells: Neuroglia
The supporting cells (neuroglia or glial cells):
 Provide a supportive scaffolding for neurons
 Segregate and insulate neurons
 Promote health and growth
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Astrocytes
Most abundant, versatile, and highly branched glial
cells
They cling to neurons and their synaptic endings, and
cover capillaries
Functionally they:
 Support and brace neurons
 Anchor neurons to their nutrient supplies
 Guide migration of young neurons
 Control the chemical environment
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Astrocytes
Figure 11.3a
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Microglia and Ependymal Cells
Microglia – small, ovoid cells with spiny processes
 Phagocytes that monitor the health of neurons
Ependymal cells – range in shape from squamous to columnar
 They line the central cavities of the brain and spinal column
 Help to circulate cerebrospinal fluid with cilia
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Microglia and Ependymal Cells
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Figure 11.3b, c
Oligodendrocytes, Schwann Cells, and Satellite
Cells
 oligodendrocytes – branched cells that wrap around nerve
fibers (axons) of neurons in the CNS to form a myelin sheath
 Schwann cells – wrap around nerve fibers (axons) of
neurons in the PNS to form a myelin sheath
 satellite cells - surround neuron cell bodies in the PNS
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Oligodendrocytes and Schwann Cells
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Figure 11.3d, e
Neurons (Nerve Cells)
Structural units of the nervous system
 Composed of a body, axon, and dendrites
 Long-lived, amitotic, and have a high metabolic rate
Their plasma membrane functions in:
 Electrical signaling
 Cell-to-cell signaling during development
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Neurons (Nerve Cells)
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Figure 11.4b
Nerve Cell Body (Perikaryon or Soma)
 Contains the nucleus and a nucleolus
 Is the major biosynthetic center
 Is the focal point for the outgrowth of neuronal processes
 Has well-developed Nissl bodies (rough ER)
 Contains an axon hillock – cone-shaped area from which
axons arise
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Processes
 Armlike extensions from the soma
 Called tracts in the CNS and nerves in the PNS
 There are two types: axons and dendrites
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Dendrites of Motor Neurons
 Short, tapering, and diffusely branched processes
 They are the receptive, or input, regions of the neuron
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Axons: Structure
 Slender processes of uniform diameter arising from the
hillock
 Long axons are called nerve fibers
 Usually there is only one unbranched axon per neuron
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Axons: Function
 Generate and transmit action potentials
 Secrete neurotransmitters from the axonal terminals
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Myelin Sheath
Whitish, fatty (protein-lipoid), segmented sheath around most
long axons
It functions to:
 Protect the axon
 Electrically insulate fibers from one another
 Increase the speed of nerve impulse transmission
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Myelin Sheath and Neurilemma: Formation
 Formed by Schwann cells in the PNS and oligodendrocytes in
the CNS
 A Schwann cell:
 Envelopes an axon in a trough
 Encloses the axon with its plasma membrane
 Has concentric layers of membrane that make up the myelin
sheath
 Neurilemma – remaining nucleus and cytoplasm of a Schwann
cell
 Nodes of Ranvier: Gaps in the myelin sheath between adjacent
Schwann cells
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Myelin Sheath and Neurilemma: Formation
Figure 11.5a-c
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Axons of the CNS
 Both myelinated and unmyelinated fibers are present
 Myelin sheaths are formed by oligodendrocytes
 Nodes of Ranvier are widely spaced
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Regions of the Brain and Spinal Cord
 White matter – dense collections of myelinated fibers
 Gray matter – mostly nerve cell bodies (soma) and
unmyelinated fibers
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Neuron Classification
Structural:
 Multipolar — three or more processes
 Bipolar — two processes (axon and dendrite)
 Unipolar — single, short process
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Comparison of Structural Classes of Neurons
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Table 11.1.1
Comparison of Structural Classes of Neurons
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Table 11.1.2
Comparison of Structural Classes of Neurons
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Table 11.1.3
Neuron Classification
Functional:
 Sensory (afferent) — transmit impulses toward the CNS
 Motor (efferent) — carry impulses away from the CNS
 Interneurons (association neurons) — shuttle signals
through CNS pathways
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Structure of a Nerve
Nerve – cordlike organ of the PNS consisting of peripheral
axons enclosed by connective tissue
Connective tissue coverings include:
 Endoneurium – loose connective tissue that surrounds
axons
 Perineurium – coarse connective tissue that bundles
fibers into fascicles
 Epineurium – tough fibrous sheath around a nerve
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Structure of a Nerve
Figure 13.3b
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Classification of Nerves
Sensory and motor divisions
 Sensory (afferent) only – carry impulse to the CNS
 Motor (efferent) only – carry impulses from CNS
 Mixed – sensory and motor fibers carry impulses to and
from CNS; most common type of nerve
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Peripheral Nerves
 Mixed nerves – carry somatic and autonomic (visceral)
impulses
 The four types of mixed nerves are:
 Somatic afferent and somatic efferent
 Visceral afferent and visceral efferent
 Peripheral nerves originate from the brain or spinal column
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The 12 Cranial Nerves
 Twelve pairs of cranial nerves arise from the brain
 They may have sensory, motor, or both sensory and motor
functions
 Each nerve is identified by a number (I through XII) and a
name
 Four cranial nerves carry parasympathetic fibers that serve
muscles and glands
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Cranial Nerves
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Figure 13.5a
Summary of Function of Cranial Nerves
Figure 13.5b
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Make up your own memory device!
I. O______________ VII. F_______________
II. O______________ VIII. V______________
III. O_____________ IX. G_______________
IV. T_____________
X. V________________
V. T______________ XI. A________________
VI. A_____________ XII. H_______________
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Cranial Nerve I: Olfactory
 Functions solely by carrying afferent impulses for
the sense of smell
 Arises from the olfactory epithelium
 Passes through the cribriform plate of the ethmoid bone
 Fibers run through the olfactory bulb and terminate in the
primary olfactory cortex
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Cranial Nerve I: Olfactory
Figure I from Table 13.2
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Cranial Nerve II: Optic
 Functions solely by carrying afferent impulses for vision
 Arises from the retina of the eye
 Optic nerves pass through the optic canals and converge at
the optic chiasm
 They continue to the thalamus where they synapse
 From there, the optic radiation fibers run to the visual cortex
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Cranial Nerve II: Optic
Figure II Table 13.2
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Cranial Nerve III: Oculomotor
 Functions in raising the eyelid, directing the eyeball,
constricting the iris, and controlling lens shape
 Fibers extend from the ventral midbrain, pass through the
superior orbital fissure, and go to the extrinsic eye muscles
 Parasympathetic cell bodies are in the ciliary ganglia
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Cranial Nerve III: Oculomotor
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Figure III from Table 13.2
Cranial Nerve IV: Trochlear
 Functions primarily as a motor nerve that directs the eyeball
 Fibers emerge from the dorsal midbrain and enter the orbits
via the superior orbital fissures; innervate the superior
oblique muscle
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Cranial Nerve IV: Trochlear
Figure IV from Table 13.2
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Cranial Nerve V: Trigeminal
 Conveys sensory impulses from various areas of the face
(V1) and (V2), and supplies motor fibers (V3) for mastication
 Composed of three divisions: ophthalmic (V1), maxillary
(V2), and mandibular (V3)
 Fibers run from the face to the pons via the superior orbital
fissure (V1), the foramen rotundum (V2), and the foramen
ovale (V3)
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Cranial Nerve V: Trigeminal
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Figure V from Table 13.2
Cranial Nerve VI: Abducens
 Functions primarily as a motor nerve innervating the lateral
rectus muscle
 Fibers leave the inferior pons and enter the orbit via the
superior orbital fissure
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Figure VI from Table 13.2
Cranial Nerve VII: Facial
 Motor functions include facial expression, and the
transmittal of autonomic impulses to lacrimal and
salivary glands; Sensory function is taste from the
anterior two-thirds of the tongue
 Fibers leave the pons, travel through the internal
acoustic meatus, and emerge through the
stylomastoid foramen to the lateral aspect of the face
 Mixed nerve with five major branches
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Cranial Nerve VII: Facial
Figure VII from Table 13.2
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Cranial Nerve VIII: Vestibulocochlear
 Functions are solely sensory – equilibrium and hearing
 Fibers arise from the hearing and equilibrium apparatus of
the inner ear, pass through the internal acoustic meatus, and
enter the brainstem at the pons-medulla border
 Two divisions – cochlear (hearing) and vestibular (balance)
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Cranial Nerve VIII: Vestibulocochlear
Figure VIII from Table 13.2
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Cranial Nerve IX: Glossopharyngeal
 Motor – innervates part of the tongue and pharynx, and
provides motor fibers to the parotid salivary gland; Sensory
– fibers conduct taste and general sensory impulses from the
tongue and pharynx
 Fibers emerge from the medulla, leave the skull via the
jugular foramen, and run to the throat
 Nerve IX is a mixed nerve with motor and sensory functions
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Cranial Nerve IX: Glossopharyngeal
Figure IX from Table 13.2
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Cranial Nerve X: Vagus
 Motor - Most motor fibers are parasympathetic fibers to the
heart, lungs, and visceral organs; Sensory function is in taste
 The only cranial nerve that extends beyond the head and
neck
 Fibers emerge from the medulla via the jugular foramen
 The vagus is a mixed nerve
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Cranial Nerve X: Vagus
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Figure X from Table 13.2
Cranial Nerve XI: Accessory
Primarily a motor nerve:
 Supplies fibers to the larynx, pharynx, and soft palate
 Innervates the trapezius and sternocleidomastoid, which
move the head and neck
 Formed from a cranial root emerging from the medulla and a
spinal root arising from the superior region of the spinal cord
 The spinal root passes upward into the cranium via the
foramen magnum
 The accessory nerve leaves the cranium via the jugular
foramen
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Cranial Nerve XI: Accessory
Figure XI from Table 13.2
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Cranial Nerve XII: Hypoglossal
 Innervates both extrinsic and intrinsic muscles of the tongue,
which contribute to swallowing and speech
 Fibers arise from the medulla and exit the skull via the
hypoglossal canal
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Cranial Nerve XII: Hypoglossal
Figure XII from Table 13.2
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