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Section 9
The Nervous System
LIU Chuan Yong
刘传勇
Institute of Physiology
Medical School of SDU
Tel 88381175 (lab)
88382098 (office)
Email: [email protected]
Website: www.physiology.sdu.edu.cn
2
Reference – Website and
Textbook
Chapter 29. Cells in Nervous
System and Their Function

Neurocyte
 Neuroglia (Glia cell)
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
Part 1 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
Dendrites of Motor Neurons
• Short, tapering, and
diffusely branched
processes
• 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 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
Myelin Sheath
• Whitish, fatty (proteinlipid), 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
• CNS: oligodendroglia
or oligodendrocytes
• PNS: Schwann cells ~
Nodes of Ranvier
• Gaps in the
myelin sheath
between
adjacent
Schwann cells
• The sites where
collaterals can
emerge
• Saltatory
conduction
Myelin Sheath and Neurilemma: Formation
• Formed by Schwann cells in the PNS
• A Schwann cell:
– Encloses the axon with its plasma membrane
– Concentric layers of membrane make up the myelin sheath
• Neurilemma （神经鞘）– 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. Function of Nerve Fibers
•
conducting action potential
• 1) Characteristic

physiological integration (anesthetic and
tetrodotoxin, TTX)

insulation

two direction

no fatigue
.
Conducting Action Potential

2). velocities of AP propagation:
– 0.5~120m/s
– The factors that influence the AP propagation

Diameter

Myelin sheath

Temperature
3). Classification of NF

Efferent nerve
– A, A, A, A; B, C.

Afferent nerve
– I, II, III, IV.
4) Axoplasmic transport

The concept: Various organelles and materials
is moved from the cell body 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.

Axoplasmic transport

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：
– 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