Chapter 6 Motor Function

主讲：黄文英

2

Basic Structure and Function of

Neurons 1

The Function Activities of the Nerve System

2

The Nerve Regulation of Motor 3

Chapter 6 Motor Function

This chapter describes summary

The purpose of this chapter is to

give an overview of the basic

structure and mechanisms of the

nervous system and to provide an

idea of how sensory information

and volition are integrated to create

adequate movements.

The CNS receives information c

oncerning the outside world via ext

eroceptors as they react to light, so

und, touch, temperature,or chemical

agents and via interoceptors that ar

e stimulated by change within the b

ody

本章主要介绍神经系统的基本结构及其工作机制，并讲述感觉信息与意志是如何通过神经系统被整合从而产生运动。 中枢系统通过外感受器如光，声音，触觉、温度和化学物质，与内感受器反应产生刺激从而使体内发生变化。

(1) the cell body, which is the

“heart” of the nerve cell

(2) the dendrites, a set of short,

fine, arborizing processes adi

ating from the cell body

(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

Section 1 Basic Structure and Function of Neurons

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

2 Nerve function

Generally, the dendrites and the cell

body receive signals, while the axon

hillock-the initial segment of the axon-

combines 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

the entire nerve cell with all its proce

sses is covered by a cell membrane. The b

asic composition of this membrane is that

of cell membranes in general, as outlined i

n chapter2. Of special interest for the func

tion of nerve cells are the protein molecul

es that are embedded in the cell membran

e, forming, among other things, various ki

nd of ion channels and ion transporters. In

addition to mitochondria, the cytoplasm of

the nerve cell body is characterized by lar

ge amounts of granular endoplasmic retic

ulum and tree ribosomes, serving the requ

irements of the entire cell, incuding its pro

cesses, for protein synthesis.

5 Axon structure

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.

The space between the membranes of the synaptic ending and the contacted nerve cell is called the synapt

ic cleft. It is about 20to30 nm wide, and there is no cytoplasmic continu

ity 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 differ

ence 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 p

ossibly 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 nerv

ous 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 electrochemica

l 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 un

its 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 pri

marily 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, b

asically 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 proces

s 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 and afferent fibers.Ⅰ Ⅱ

(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 orde

rly arrangement !of axons in the CNS.

(7) Supraspinal Control of Motoneurons

When dealing with motor control it is customary to sp

eak 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.