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Chapter 10 Nervous System
The Nervous System
• Components
– Brain, spinal cord, nerves, sensory receptors
• Responsible for
– Sensory perceptions, mental activities,
stimulating muscle movements, secretions of
many glands
• Subdivisions
– Central nervous system (CNS)
– Peripheral nervous system (PNS)
Central Nervous System
• Consists of
– Brain
– Spinal cord
• Brain and spinal
cord
– Continuous with
each other
Peripheral Nervous System
• Two subcategories
– Sensory or afferent
– Motor or efferent
• Divisions
– Somatic nervous
system
– Autonomic nervous
system (ANS)
» Sympathetic
» Parasympathetic
» Enteric
Nervous System Organization
Cells of Nervous System
• Neurons or nerve cells
– Receive stimuli and
transmit action
potentials
– Organization
• Cell body or soma
• Dendrites: Input
• Axons: Output
• Neuroglia or glial cells
– Support and protect
neurons
Section 1 General Function of Neuron and Neuroglia
I Neuron
1. Structure and Function
Typical Neuron has 4 Regions
• Cell Body
• Dendrites
• Axon
• Presynaptic Terminals
Each region is specialized
for its particular function
Information flows in a single
direction
Neuron Cell Body Location
 Most are found in the central nervous
system
 Gray matter – cell bodies and unmylenated
fibers
 Nuclei – clusters of cell bodies within the
white matter of the central nervous system
 Ganglia – collections of cell bodies
outside the central nervous system
Functional Classification of
Neurons
 Sensory (afferent) neurons
 Carry impulses from the sensory receptors
 Cutaneous sense organs
 Proprioceptors – detect stretch or tension
 Motor (efferent) neurons
 Carry impulses from the central nervous
system
Functional Classification of
Neurons
 Interneurons (association neurons)
 Found in neural pathways in the central
nervous system
 Connect sensory and motor neurons
By function (connections)
Sensory
Motor
Interneuron
Neuron Classification
Neuron Anatomy
 Extensions
outside the cell
body
 Dendrites –
conduct
impulses toward
the cell body
 Axons – conduct
impulses away
from the cell
body (only 1!)
Dendrites of Motor Neurons
• Short, tapering, and
diffusely branched
processes
• They are the
receptive, or input,
regions of the
neuron
• Electrical signals
are conveyed as
graded potentials
(not action
potentials)
• Slender processes of
uniform diameter arising
from the hillock
• Long axons are called
nerve fibers
• Usually there is only one
unbranched axon per
neuron
• Rare branches, if present,
are called axon collaterals
• Axonal terminal –
branched terminus of an
axon
Axons: Structure
Axons: Function
• Generate and
transmit action
potentials
• Secrete
neurotransmitters
from the axonal
terminals
• Whitish, fatty
(protein-lipid),
segmented sheath
around most long
axons
• It functions in:
– Protection of the axon
– Electrically insulating
fibers from one
another
– Increasing the speed of
nerve impulse
transmission
Myelin Sheath
Myelin
• CNS: oligodendroglia
or oligodendrocytes
• PNS: Schwann cells ~
• Gaps in the
myelin sheath
between
adjacent
Schwann cells
• They are the
sites where
collaterals can
emerge
• Saltatory
conduction
Nodes of Ranvier
Myelin Sheath and Neurilemma: Formation
• Formed by Schwann cells in the PNS
• A Schwann cell:
– Envelopes an axon in a trough
– Encloses the axon with its plasma membrane
– Concentric layers of membrane make up the myelin
sheath
• Neurilemma – remaining nucleus and cytoplasm
of a Schwann cell
Myelin
Sheath and
Neurilemma:
Formation
Figure 11.5a-d
Unmyelinated Axons
• A Schwann cell surrounds nerve fibers but coiling
does not take place
• Schwann cells partially enclose 15 or more axons
Axons of the CNS
• Both myelinated and unmyelinated fibers are
present
• Myelin sheaths are formed by oligodendrocytes
• Nodes of Ranvier are widely spaced
• There is no neurilemma
2. Classification and Function of Nerve
Fibers
•Function: conducting action potential
1)Characteristic
 physiological integration (anesthetic and
tetrodotoxin, TTX)
 insulation,
 two direction
 no fatigue
2). Conducting velocities of AP propagation:
0.5~120m/s.
The factors that influence the AP propagation:
~The diameter of NF: 0.2 -20 mm, The larger the
diameter is , The faster A.P. propagates.
~ Myelin sheath:
~Temperature:
3). The general classification of NF
 Electrical physiological classification (efferent
nerve): A, A, A, A; B, C.
 Morphological classification (afferent nerve): I, II,
III, IV.
4) Axoplasmic transport
The concept: Various organelles and materials must
be moved from the cell body, where they are made, to
the axon and its terminals in order to maintain the
structure and function of the cell axon.
 The mechanisms: Cytoskeletal filaments in the
axon and cell body, which serve as the rails along
which the transport occurs, are linked by proteins to
the substances and organelles being moved.
 Anterograde axoplasmic transport
~ fast axoplasmic transport: 410mm/day, mitochondria,
vesicles, secretory granule.
~ slow axoplasmic transport:1-12 mm/d, cytoskeletal
elements & soluble proteins
 Retrograde axoplasmic transport：Axon transport
of certain materials are from the axon terminals to the
cell body. 205mm/d, NGF, tetanic toxin, horseradish
peroxidase (HRP).
Fig. Axopasmic transport
Fig. The method of horseraidish peroxidase
5) Trophic action of the nerve to the target
The motor nerve release some substance that has
trophic action on the skeletal muscle
The denervated muscle does not receive nerve signals
and due to this, muscle atrophy begins.
After two months, the muscle fibers degenerate and
denervation atrophy follows.
Fibrous tissue replaces the muscle
II Supporting Cells: Neuroglia
Ependymal
Cell
Microglia
Oligodendrocyte
Astrocyte
Martini, F. Fundamentals of Anatomy and Physiology, Fig 12-6.
Neuroglia of CNS
• Astrocytes
– Regulate extracellular brain fluid composition
– Promote tight junctions to form blood-brain barrier
• Ependymal Cells
– Line brain ventricles and spinal cord central canal
– Help form choroid plexuses that secrete cerebrospinal fluid (CSF)
Neuroglia of CNS
• Microglia
– Specialized macrophages
• Oligodendrocytes
– Form myelin sheaths if surround axon
Neuroglia of PNS
• Schwann cells or neurolemmocytes
– Wrap around portion of only one axon to form myelin sheath
• Satellite cells
– Surround neuron cell bodies in ganglia, provide support and
nutrients