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Chapter 6 Motor Function
主讲：黄文英
Chapter 6 Motor Function
1
Basic Structure and Function of
Neurons
of the Nerve
2 The Function Activities
System
3
The Nerve Regulation of Motor
2
This chapter describes summary
The purpose of this chapter is to
本章主要介绍神经系统的
give an overview of the basic
structure and mechanisms of the
基本结构及其工作机制，并讲
nervous system and to provide an
述感觉信息与意志是如何通过
idea of how sensory information
神经系统被整合从而产生运动。
and volition are integrated to create
adequate movements.
The CNS receives information
中枢系统通过外感受器
concerning the outside world via
如光，声音，触觉、温度和
exteroceptors as they react to light,
化学物质，与内感受器反应
sound, touch, temperature,or
产生刺激从而使体内发生变
chemical agents and via
interoceptors that are stimulated by
change within the body
化。
Section 1 Basic Structure and Function of Neurons
(1) the cell body, which is the
(2) the dendrites, a set of short,
fine, arborizing processes
“heart” of the nerve cell
adiating from the cell body
1 The consist of nerve
There are approximately 1000 different type of nerves
cells, or neurons. Each has a diameter from5to100 um and
consists of four morphological regions
(3)the axon, from less
than1to20 um in diameter and
from 1mm to1m long and
(4)the axon terminals (figure6.2).
Each region has distinctive
function, which are described in
more detail later
2 Nerve function
Generally, the dendrites and the cell
body receive signals, while the axon
hillock-the initial segment of the axoncombines and integrates them and
“decides” whether or not a signal will
be sent through the axon to the
terminals. The axon may branch off
near its beginning, but more often the
branching takes place close cells is
based on chemical signals released
from the nerve terminal that act on the
target cell in a synapse. In the context
of this book, al synaptic transmission
can be considered chemical.
3 Nerve transmission
The concept of
“transmission” of impulse from
one cell to another becomes
untenable, or at least too easily
misunderstood, when we realize
that some synapses are
inhibitory. Instead of promoting
the creation of a new impulse,
such synapses serve to prevent
this from happening in the
postsynaptic cell
Among the thousands of
synapses acting on a nerve cell,
some are excitatory while others
are inhibitory. For all practical
purposes it can be stated that
individual synapses nerve
change from one kind it the
other.
4 The conduct of nerve impulse
5 Axon structure
the entire nerve cell with all its
processes is covered by a cell membrane.
The basic composition of this membrane
is that of cell membranes in general, as
outlined in chapter2. Of special interest for
the function of nerve cells are the protein
molecules that are embedded in the cell
membrane, forming, among other things,
Axon terminals of other nerves end
at the surface of the soma and the
dendrites.
Actually,
a
large
percentage of the surface of the
soma, the dendrites, and even the
axon hillock can be covered by
synaptic endings from thousands
of other nerve cells.
various kind of ion channels and ion
transporters. In addition to mitochondria,
the cytoplasm of the nerve cell body is
characterized by large amounts of
granular endoplasmic reticulum and tree
ribosomes, serving the requirements of
the entire cell, incuding its processes, for
protein synthesis.
The space between the membranes
of the synaptic ending and the
contacted nerve cell is called the
synaptic cleft. It is about 20to30 nm
wide, and there is no cytoplasmic
continuity between the two cells.
Section 2 The Function Activities of the Nerve
System
1 Neurotransmitters and Neuromodulators
The exact type of ion and the nature of the synaptic potential
depend on the type of transmitter used by the actual synapse and the
nature of the ion channel affected by it.
In excitatory synapses, the transmitter molecules bind to Na+
channels, allowing a flux of Na+ down its steep electrochemical gradient
into the postsynaptic cell and giving rise to an EPSP. In general, the
depolarization of one EPSP is insufficient to reach the firing threshold.
In addition to this classic fast synaptic response, a slower,
metabotropic signal molecules affect the ion channels indirectly, through
intracellular pathways, and modulate the effect of the classical
neurotransmitters.
2 Motor Nerve and Nerve Impulses
(1) Resting Membrane Potential
Nongated ion channels allow K+ to leak out from the
cell, giving the cell interior a charge of -60to-70 mV in
relation to the outside. At this resting membrane potential,
the outward leak of K+ is balanced by the internal negative
charge pulling K+ back into the cell. For Na+ the situation is
different.
Both the concentration gradient and the charge
difference tend to drive Na+ into the cell, eventually
reducing the closed, however, only a minor inward leak of
Na+ occurs, and this is electrically balanced by a
compensatory outward leak of K+. The resulting ionic
perturbation is taken care of by the sodium-potassium
pump.
(2) Excitation and Inhibition
When a nerve impulse arrives at a synaptic terminal,
voltage-gated Ca+ channels in the terminal open, allowing
Ca+ from the outside into the terminal. This initiates a
series of events culminating with a number of synaptic
vesicles emptying their contents into the synaptic cleft.
In excitatory synapses, the transmitter molecules
bind to specific binding sites on ligand-gated Na+ channels
and open the channels, allowing Na+ into the cell. This
makes the local area under the postsynaptic membrane
less negative. Such local changes in the membrane
potential are called synaptic potentials or postsynaptic
potentials in the case of excitatory synapses, they are more
specifically called excitatory postsynaptic(figure6.6).
(3) Temporal and Spatial Summation
Summation of synaptic potentials is necessary to evoke
an action potential. This is one of the reasons why single
impulses are usually not regarded as proper signals in the
nervous system.
To result in an additive effect, synapse potentials must
occur sufficiently close in time for the effect of the ionic
currents to be combined. If the second synapse potential
occurs after the transient ionic perturbation underlying the
first synapse potential has faded away, no summation is
possible.
If the time lag between two or more excitatory potentials
is gradually reduced, the combined synaptic potential is
increased correspondingly, eventually reaching the firing
threshold and resulting in an action potential.
(4) Motor Units, Effectors of the Motor System
An action potential arriving at a motor endplate always
results in a synaptic potential, or more correctly an endplate
potential, well above the firing threshold. This is due to the
anatomy of the motor endplate. Since action potentials in a
rested, normal motoneuron always propagate town every
branch of its axon, all muscle fibers of one particular motor
unit must be active at the same time and to the same
degree.
Each motoneuron supplies from fewer than ten up to
possibly several thousand muscle fibers, referred to as
small and large motor units, respectively. The fiber in a
motor unit are scattered and intermingled with fibers of other
units, and they can be spread over an approximately
circular region with an average diameter of 5mm. In any
case ,
(5) The Motor Endplate
The motor endplate, the synapse between the motoneuron and its
“slave” the skeletal muscle fiber, is the most intensively studied type of
synaptic connection, and acetylcholine (ACh) has long since been
identified as its transmitter substance.
Ach works as a neuromodulator in the CNS. In the autonomic
nervous system, Ach is the transmitter in both parasympathetic and
sympathetic preganglionic neurons and in parasympathetic
postganglionic neurons.
Recall that within the CNS, the algebraic sum of the
electrochemical effects of impulses arriving at excitatory and inhibitory
synaptic terminals in contact with a particular neuron determines
whether of not it will fire an action potential.
(6) Regulation of Contractile Force
The muscle action potential propagates along the membrane, at the
same time penetrating into the interior of the muscle fiber by means of
the T-tubules, and initiates a series of events culminating in the
interaction between myosin and action depends on the number of motor
units activated and on the number of motor units activated and on the
frequency with which each of them is stimulated.
Both in reflex and voluntary contractions, small, slow-twitch motor
units are recruited first in activities with low frequency. With increasing
demand for force, these low-threshold units increasing their discharge
rate, and in addition, new motor units are recruited.
The relatively stereotyped recruitment order of motor units applies
primarily to monofunctional muscles. In muscle serving more than one
joint or a joint with more than one degree of freedom, the recruitment
order may depend on the direction and type of movement.
(7) The Role of Sensory Systems and Reflexes
in Motor Function
Some human behavior is innate and follows a stereotypic pattern,
basically in all individuals. Examples of such behavior pattern are
swallowing when taken by surprise. Centrals programs in the nervous
system can coordinate the motoneurons do not require additional
incoming sensory feedback for the continuation of their essential
pattern, even if a dozen or more muscle groups are involved.
During childhood, we learn new movements, and an onging
process seeks to modify behavior as a result of experience. For an
understanding of some basic principles underlying the function of the
CNS, it is useful to examine how relatively simple reflexes are brought
about and how they are modified, for the reflex is an elementary model
of behavior. Before we do so, however, we take a brief look at the basic
properties of sensory receptors.
Section 3 The Nerve Regulation of Motor
1 The CNS can enhance or suppress sensory information
If the skin touched by an object, touch and eventually
pain receptors are stimulated. The most strongly stimulated
receptors may, through collaterals from the afferent fibers,
stimulate inhibitory interneurons.
Motor function depends heavily no sensory information.
Part of this information reaches consciousness and may serve
as a basis for voluntary movements, but most of it takes part in
reflexes.
All sensory receptors adapt to constant stimulation by
gradually decreasing their response. Such adaption is either
rapid or slow, depending on the type of receptor involved, and
can be regarded as a strategy to reduce the flow of information
to the CNS.
2 The Function of Reflex
Activities and Proprioceptors
(1) General properties of reflexes afferent
In reflexes relevant for motor function, there are at least
two neurons in the reflex chain: the afferent (receptor) neuron
and efferent (effector) neuron. The cell body (perikaryon) of the
afferent neuron is in a dorsal root ganglion or an equivalent
ganglion of a cranial nerve, and it conveys cutaneous,
muscular or special sense information.
To a large extent, neural control of skeletal muscles is
reflexive in nature. The membrane potential of the
motoneurons is increased or decreased, depending on the
sum of the excitatory and inhibitory activity in the synaptic
terminals acting on the motoneuron.
(2) The Muscle Spindle and the Gamma Motor System
From the skeletal muscle, afferent nerves report to the
CNS about the muscle’ tension, length, and position and about
changes in these parameters. These nerves are activated by
special receptors, one of which is the muscle spindle.
The intrafusal muscle fibers are of two major types but
share one important feature. Except in the middle part, both
types show cross striations because of their content of
contractile, myofibrillar material. It is these striated parts that
are innervated by the r motoneutron. The central, unstriated
part, on the other hand, is the main sensory region, innervated
by thick myelinated fibers belonging to group ⅠandⅡafferent
fibers.
(2) The Muscle Spindle and the Gamma Motor System
(3) Golgi Tendon Organs
The golgi tendon organs, a few millimeters in size, are
connected in series with extrafusal muscle fibers and inserted
between the muscles and their tendon(see figure6.12).Each
Golgi tendon organ is responsive to contraction of about 10 to
20 single muscle fibers, each belong to a separate motor unit.
When stimulated, the afferent nerve fibers from. Golgi
tendon organs have been found to cause an inhibition of their
corresponding muscle, elicited via interneurons. This led to the
belief that the function of the tendon organ was to prevent the
development of dangerously high tension in the muscle, a
belief that is now largely abandoned. First, the effect of afferent
impulses from the tendon organ is not always inhibition but
may be excitation of homonymous a motoneurons, depending
on the type and phase of the actual movement
(4) Renshaw Cells and Recurrent Inhibition
Motoneurons give off collateral branches on their way
to a ventral root .They form excitatory synaptic contacts with
interneurons located in the ventromedial region of the
ventral horn . The axons of these Renshaw cells establish
inhibitory synaptic contacts with the same and interneurons
in an overlapping and diffuse fashion.
Since the Renshaw cells project back to the same
motoneurons, which excite them, this is called recurrent
inhibition. Renshaw cells provide a feed-back ,and a single
volley in the axon of the motoneuron can evoke a repetitive
discharge of the Renshaw cell with the consequent tendency
to dampen and stabilize levels adapts the spinal circuitry to
the motor task at hand . Through the Renshaw
cells’inhibition of agonistic motoneurons and simultaneous
disinhibition of antagonistic motoneurons, Renshaw cells
may contribute to the generation of rhythmic movements.
(5) Joint Receptors
Motoneurons give off collateral branches on their way
to a ventral root .They form excitatory synaptic contacts with
interneurons located in the ventromedial region of the
ventral horn . The axons of these Renshaw cells establish
inhibitory synaptic contacts with the same and interneurons
in an overlapping and diffuse fashion.
Since the Renshaw cells project back to the same
motoneurons, which excite them, this is called recurrent
inhibition. Renshaw cells provide a feed-back ,and a single
volley in the axon of the motoneuron can evoke a repetitive
discharge of the Renshaw cell with the consequent tendency
to dampen and stabilize levels adapts the spinal circuitry to
the motor task at hand . Through the Renshaw
cells’inhibition of agonistic motoneurons and simultaneous
disinhibition of antagonistic motoneurons, Renshaw cells
may contribute to the generation of rhythmic movements.
(6) Functional Organization of the Spinal Cord
In a cross section of the spinal cord, the gray
matter occupies a butterfly-like zone in the center,
surrounded by white matter. The gray matter consists
mainly of nerve cell bodies and their dendrites,
including the nerve cells of the local spinal circuitry,
the axons of which are more or less confined to the
gray matter as well.
In the while matter, axons connecting different
segments of the spinal cord, the propriospinal fibers,
are found in a zone closely surrounding the gray
matter.This is as may be expected from the general
and orderly arrangement !of axons in the CNS.
(7) Supraspinal Control of Motoneurons
When dealing with motor control it is customary to
speak about spinal and supraspinal levels of the CNS.
The suprasinal control of the motoneurons takes place
through descending path ways from the cerebral cortex
and the brain stem .In addition to direct connections to the
spinal cord, the cerebral cortex also ahs connections to
the nuclei in the brain stem ,which in turn give rise to
descending axons to the spinal cord .
The term supraspinal alludes to hierarchical
organization of motor control and comprises all areas of
CNS that contribute to motor control but have to do it
through their influence on motoneurons only. Supraspinal
motor areas of the CNS include motor areas of the
cerebral cortex, the cerebellum, and various nuclei in the
brain and brain stem.
3 Cerebral Cortex and Cerebellum
(1) Motor Areas in the Cerebral Cortex
The term supraspinal alludes to hierarchical organization of motor
control and comprises all areas of CNS that contribute to motor
control but have to do it through their influence on motoneurons only.
Supraspinal motor areas of the CNS include motor areas of the
cerebral cortex, the cerebellum, and various nuclei in the brain and
brain stem.
The area of the motor cortex just anterior to M1 is usually
subdivided into the premotor area (PMA) and supplementary motor
area. Both of these also give rise to descending axons to motor areas
in the brain stem and spinal cord, but their main influence on motor
control is due to their connections to M1. This is why they are often
placed above M1in a hierarchical organization of motor control. The
SMA is situated most medially, close to the interhemispheric fissure,
and seems to be important for the planning of complex movements
while the execution of the movement is take care of by M1.
(2) Cerebellum
The cerebellum has a key function in the smooth and efficient
control of movement, It integrates and organizes information arriving
from peripheral proprioceptive receptors and other somatosensory
receptors as well as from other parts of the CNS.
The afferents to the cerebellum are of two kinds ,mossy fibers
and climbing fibers .Both are excitatory, albeit very different in their
behavior ,as we shall see later.the efferent fibers from the cerebellar
cortex , on the other hand, are all axons of Purkinje cells ,It came as
a major surprise ,therefore ,when it was discovered that the Purkinje
cells are inhibitory; Most Purkinje cells ptoject to the cerebellar
nuclei ,which in turn give rise to the major part of the efferent fibers
from the cerebellum. Collaterals from mossy fibers and climbing
fibers provide excitatory drive to the cerebellar nuclei.
(3) Various Nuclei Involved in Movement
Two large nuclear complexes in the brain, the thalamus and the
basal ganglia, server central roles in motor control without sending
any fibers to the spinal cord. The thalamus is a large, ovoid mass of
gray matter, constituting the major part of the diencephalon. It
consists several distinct nuclei and is an important integrating relay
station, handling both sensory information from the spinal cord and
information related to motor control from the cerebellum and the
basal ganglia.
Both the thalamus and the basal ganglia are involved in
supraspinal motor circuits. One such circuit involves the cerebral
cortex, the pontine nuclei, the cerebellum, and the thalamus, which in
turn projects back to the cortex. Another circuit goes from the cerebral
cortex via the basal ganglia and thalamus back to the cortex.
The vestibulospinal tract originates from the ves-tibular nuclei,
agroup of nuclei in the lareral part of the brain stem at the level of the
pons and medulla oblongata .
4 Motor and Nerve Activities
(1)Integration of neuronal Activity in Movement
The principle of reciprocal inhibition is one example of such
integration. Their collaterals, releasing the same transmitter ,at the
same time excite inhibitory interneurons, which in turn inhibit the
motoneurons to antagonistic muscles. It is certainly functionally
economic that a synergistic team of muscles ,when activated ,is not
faced with the resistance of their antagonists.
The traditional point of view has been that the cerebral cortex
reigns at the highest level in the brain’s hierarchical organization for
the motor function .However ,Evarts (1973) argued that the cerebral
motor cortex is at a rather low level of the motor control system ,close
to the muscular apparatus itself .The cerebellum and basal ganglia
are at a higher functional level in the neural chain of command that
initiates and controls movement ,but there that they initiate the
command .
(2) The Role of Reflex Activity in Motor Control
Central pattern generators control stereotypic loco motor
movements such as walking and running. The neural networks are to
a significant degree located within the spinal cord. Surpspinal regions
can activate the relevant spinal programs as well as control and
modify these programs Similarly, powerful signals front peripherl
receptors can control the central pattern generators on the spinal
level or less directly via loops passing through higher levels of the
CNS.
The central program does not require afferent feedback for its
essential pattern or maintain but becomes functionally more efficient
and adaptable to unexpected events when fed afferent signals.
Faster movements are less dependent on external cues than shower
ones .Both spinal and long loop stretch reflexes can be active during
normal movements, but their level of activity is adjusted to suit actual
purpose of the movement.
(3) Motor Learning
In view of the sharply defined projection of impulses within and
between areas of the brain, it is difficult to understand how a general
movement pattern may function. The organization does not appear to
favor a transfer effect；that is learning by practicing a certain
movement pattern does not in itself enhance the performance of
another movement pattern, not even one that is relatively similar.
However, the technique of learning new tasks can be improved.
One can learn and memorize specific activities that can then
be woven together in different combinations. The pianist, having
practiced many hours at the keyboard, has the potential to learn new
pieces of music quickly. Whether it should be called a transfer effect
is a matter of definition; however, the pianist can learn to play a piece
of music slowly and softly, but once he or she has learned it, the
pianist can play it fast and loud just as well.
(4) Coordination
In fast, ballistic movements, an initial spurt of activity in the
agonist produces momentum and kinetic energy in the segment, and
then the muscle relaxes as the limb proceeds by its own momentum.
By reciprocal inhibition the antagonist relaxes completely, except
perhaps at the end of a movement or when the movement is stopped
by the limits of the joint or an external force. In slow movements in
some activities, discrete bursts of neural activity are observed in
agonists and antagonists to accelerate and decelerate the segment
Movements of the hand toward an object that last less than 250
ms are programmed in advance .Whit movements of longer
duration ,the first phase is programmed ,but then there is also visual
control as well as guidance from peripheral proprioceptors .Such
feedback is essential for accuracy and the learning of new exercises .
(5) posture
For the maintenance of a balanced body position in standing,
in locomotion ,and in any kind of exercise, an integrated coordination
of the proprioceptive, visual and vestibular systems is essential. The
proprion ceptive and visual systems can jointly manage most of the
neuromuscular interactions necessary to secure an optimal situation.
The vestibular sys tem seems to function more as a reference
system, controlling which adaptive modifications should be performed
in the corrections elicited by the proprioceptive and visual
systems .Stretch reflexes are important, but there also seem to be
central pattern generators for posture.