Physiology III
Neuroscience
Motor Control & Spinal Reflexes

Reading: Haines, Ch. 9, pp 136-139, Ch. 23, pp. 336-341

Keywords:

Hierarchical control: The main axis starts in the cortex and goes to the cord. The lowest level is the cord then the brainstem then primary motor cortex and then the parietal and supplemental cortex. Each level connects to the next and the higher levels control the lower ones.

Final common path: Ventral horn motor neurons represent the only direct link btw the nervous system and skeletal muscle. This path plays a central role in production of movement.

Flexor-extensor rule:

Upper motor neuron: Brainstem and cortical neurons. This usually refers to corticospinal cell bodies and their axons. They have no direct synaptic link with the muscles.

Lower motor neuron: Spinal ventral horn matter neurons whose axons innervate skeletal muscle. These cells stimulate muscle to produce characteristic movements of a body part. They are arranged topographically according to muscle groups they innervate.

Fractionation: This is the precise control that the cortex has over the muscles of the body. It can be related to the homunculus and to the size of each represented body part. The larger area devoted to each part, the more precise the control over it.

Muscle spindle: Is a long, thin encapsulated structure that typically contains about seven striated intrafusal muscle fibers. The capsule of the spindle is attached to and oriented in parallel with the extrafusal fibers that comprise the bulk of the muscle. The output of these spindles signals a change in muscle length and the rate of change in muscle length. Activation of these leads to excitation of alpha motor neurons innervating the muscle associated with the activated spindle.

Ia afferent: Is heavily myelinated, has a conduction velocity of 80 to 120m/sec, and is typically associated with the nuclear bag fibers. The distal end of the fiber is wrapped around the central (noncontractile) region of an intrafusal fiber. The endings are mechanoreceptors and when stretched will mechanically open ion channels in its membrane. The firing frequency is directly proportional to the degree to which the spindle is stretched.

Golgi tendon organ: These are located in tendons near their junctions with muscle fibers and consist of networks of thin nerve fibers intertwined with the collagen fibers of the tendon. They are mechanoreceptors. When force is applied to the tendon, the sensory fibers are stretched, which opens ion channels in the nerve fiber membrane. The fibers that lead from the tendon organs to the cord are type Ib fibers which are large in diameter and heavily myelinated. They enter the cord and traverse the intermediate zone to reach the ventral horn where they form excitatory synapses with interneurons. There interneurons in turn inhibit alpha motor neurons that innervate the muscle associated with the activated Golgi tendon organ. They react opposite of the muscle spindles.

Ib afferent: These are fibers from the Golgi tendon organs and are also Aa fibers and are myelinated.

Intrafusal fiber: These are the fibers on the inside of the muscle spindle.

Extrafusal fiber: These are the fibers of the working skeletal muscle on the outside of the spindle.

Nuclear bag fiber: One of the two types of intrafusal fibers. These have nuclei that are clustered and the equatorial region is somewhat swollen.

Nuclear chain fiber: These are the 2^nd type of intrafusal fibers. These have nuclei that are arranged in a single row and the equatorial region is not obviously expanded.

Primary endings: In the very center of the receptor area, a large sensory fiber encircles the central portion of each intrafusal fiber, forming the so-called primary ending or annulospiral ending. This nerve fiber is a type Ia fiber averaging 17 micrometers in diameter and has a rapid velocity of conduction.

Secondary endings: The type II fiber is principally associated with nuclear chain fibers. Its connection with the equatorial region of the target intrafusal fiber has the form of a cluster of thin radiating branches and is called a secondary ending or flower-spray ending. It also encircles the intrafusal fibers like the Ia fiber does. This sensory fiber is also activated by mechanical stretch, but it only codes the event, not the rate of the stretch.

Gamma motor neuron: These innervate special type of striated muscle fiber – the intrafusal fibers found only in the muscle spindle. These are located in the cord anterior horns along with the alpha motor neurons. They transmit impulses through type Ag fibers to the intrafusal fibers.

Alpha motor neurons: These innervate ordinary working fibers of skeletal muscles called extrafusal fibers. They give rise to the large type Aa nerve fibers that innervate the large skeletal muscle fibers. Stimulation of a single nerve fiber excites from as few as three to as many as several hundred skeletal muscle fibers, which are collectively called the motor unit.

Static and dynamic responses: The static response is when the receptor portion of the muscle spindle is stretched slowly, the # of impulses transmitted from both the primary and the secondary endings increases almost directly in proportion to the degree of stretching and the endings continue to transmit these impulses for several minutes. This static response means that both the primary and secondary endings continue to transmit their signals for as long as the receptor itself remains stretched. The dynamic response is when the length of the spindle receptor increases suddenly, the primary ending (but not the secondary ending) is stimulated especially powerfully, much more so than the stimulus caused by the static response. It means that the primary ending response extremely actively to a rapid rate of change in spindle length.

Alpha-gamma coactivation: This is a phenomenon of the muscle fiber. As the muscle shortens, the intrafusal fibers also shorten. The relaxation of the equatorial regions that would result from extrafusal muscle fiber contraction is offset by equatorial stretching because of intrafusal fiber contraction, and the equatorial regions remain under nearly constant tension.

Stretch reflex: aka myotactic, patellar tendon, and deep tendon reflex. Many type Ia spindle afferents form monosynaptic excitatory connections with alpha motor neurons that innervate the muscle from which the afferents originated. To tap on a tendon imposes a stretch on a muscle which causes the afferent fibers to fire. The connection is to the Ia fibers which are heavily myelinated and large diameter fibers. They enter the dorsal root and enter the cord, go to the ventral horn and synapse on an alpha motor neuron (excitatory with glutamate non-NMDA receptor). This causes the motor neuron to fire and cause the muscle to contract.

Reciprocal innervation: At the same time as a stretch reflex is occurring this reciprocal innervation is also going on. The Ia fibers activate Ia interneurons that inhibit motor neurons innervating antagonist muscles.

Divergence: Often it is important for signals entering a neuronal pool to excite far greater #’s of nerve fibers leaving the pool. This is divergence and there are two types. Amplifying means that an input signal spreads to an increasing # of neurons as it passes through successive orders of neurons in its path. This is characteristic of the corticospinal pathway in its control of skeletal muscles, with a single large pyramidal cell in the motor cortex capable of exciting many fibers. The 2^nd type is divergence into multiple tracts. Here the signal is transmitted in two directions form the pool. For instance, info transmitted in the dorsal columns of the cord takes two courses in the lower part of the brain – into the cerebellum and through the lower regions of the brain to the thalamus and cerebral cortex.

Convergence: This means signals from multiple inputs converging to excite a single neuron. You can have convergence from a single source as multiple terminals from an incoming fiber tract terminate on the same neuron. You can also have convergence from multiple sources such as the interneurons of the cord receive converging signals from the peripheral nerve fibers entering the cord, propriospinal fibers passing from one segment of the cord to another, corticospinal fibers from the cerebral cortex and several other long pathways descending from the brain into the cord. Then the signals from the interneurons converge on the anterior motor neurons to control muscle function.

Reverberating circuits: or the oscillatory circuit. This circuit is caused by positive feedback within the neuronal circuit that feeds back to re-excite the input of the same circuit. Consequently, once stimulated, the circuit discharges repetitively for a long time. Most reverberating circuits are constituted of many parallel fibers and at each cell station, the terminal fibrils diffuse widely. In such a system, the total reverberating signal can be either weak or strong, depending on how many parallel nerve fibers are momentarily involved in the reverberation.

Local sign: the basic response pattern organized to get the body away from the painful stimulus.

Flexion reflex: aka flexion, nociceptive, withdrawal reflex. If you have pain or nociceptive input via the Ad and C fibers and in certain conditions Ab fibers. The activated fibers go to the cord and synapse with interneurons via an excitatory synapse, which then excites the motor neuron to a flexor muscle to pull the extremity up and away from the pain. The flexion reflex uses polysynaptic since more than one synapse occurs. If the stimulus is strong enough you will get crossed extension on the contralateral side. The duration and latency of this reflex is slower and longer than the stretch reflex. It is also not confined to the distal end of an extremity as a hip in pain will also push away by extending the leg or an appendix would cause you to double over.

Flexor reflex afferents: the Ad and C fibers and in certain conditions Ab fibers

Withdrawal reflex: same as the flexion reflex

Mesencephalic locomotor region: somewhere in the midbrain!

L-dopa: part of a drug given to patients with Parkinson’s disease. L-dopa or L-3,4-hydroxyphenylalanine will cross the blood-brain barrier and helps to replace the dopamine lost in Parkinson’s disease.

NMDA receptor: one of the receptors located in the nervous system that opens calcium ion channels.

Distinguish between voluntary, reflexive and rhythmic movements: Voluntary movements are modulated in the cerebral cortex in the motor cortex in Brodmann’s area 4 and in the premotor and supplementary motor areas. Reflexive movements occur according to the reflex arc they utilize. Voluntary movements often utilize the arc to aid the movement. Rhythmic movements are modulated in the cerebellum via the vestibular nuclei and the locomotion areas to coordinate the activity and make the movements smooth.

Compare and contrast lateral vs. medial descending control systems: The medial tracts go to the medial parts of the spinal gray which tends to control things like balance and posture while the lateral tracts go to the lateral parts of the spinal gray and control locomotion.

Describe the response of the primary spindle afferent to stretch: Muscle spindle is a long, thin encapsulated structure that typically contains about seven striated intrafusal muscle fibers. The capsule of the spindle is attached to and oriented in parallel with the extrafusal fibers that comprise the bulk of the muscle. The output of these spindles signals a change in muscle length and the rate of change in muscle length. Activation of these leads to excitation of alpha motor neurons innervating the muscle associated with the activated spindle

Describe the responses of Golgi tendon organs to tension: These are located in tendons near their junctions with muscle fibers and consist of networks of thin nerve fibers intertwined with the collagen fibers of the tendon. They are mechanoreceptors. When force is applied to the tendon, the sensory fibers are stretched, which opens ion channels in the nerve fiber membrane. The fibers that lead from the tendon organs to the cord are type Ib fibers which are large in diameter and heavily myelinated. They enter the cord and traverse the intermediate zone to reach the ventral horn where they form excitatory synapses with interneurons. There interneurons in turn inhibit alpha motor neurons that innervate the muscle associated with the activated Golgi tendon organ. They react opposite of the muscle spindles. The Golgi tendon organs have a dynamic and static response. When the Golgi tendon organs of a muscle are stimulated by increased muscle tension, signals are transmitted into the cord to cause reflex effects in the respective muscle. This reflex is entirely inhibitory. Thus, this reflex provides a negative feedback mechanism that prevents the development of too much tension on the muscle.

Describe the influence of gamma motor neurons on spindle afferent responses, and the significance of the mechanical properties of nuclear bag and nuclear chain fibers: the nuclear bag fibers is one of the two types of intrafusal fibers. These have nuclei that are clustered in the central portion of the receptor area. The nuclear chain fibers are the 2^nd type of intrafusal fibers. These have nuclei that are arranged in a single row or chain throughout the receptor area. The primary nerve ending innervates both the nuclear bag and chain fibers. On the other hand, the secondary endings usually innervate only the nuclear chain fibers. The gamma dynamic motor nerve excites the nuclear bag fibers and the gamma static excites the nuclear chain fibers. When the gamma-d fibers excite the nuclear bag fibers, the dynamic response of the muscle spindle becomes tremendously enhanced, whereas the static response is hardly affected. On the other hand, stimulation of the gamma-s fibers, which excite the nuclear chain fibers, enhances the static response while having little influence on the dynamic response.

Describe the neural circuit for the stretch reflex: The basic circuit is a type Ia nerve fiber originating in a muscle spindle and enters the dorsal root of the cord. In contrast to most other nerve fibers entering the cord, one branch of it passes directly to the anterior horn of the cord gray matter and synapses directly with anterior motor neurons that send nerve fibers mainly back to the same muscle from whence the muscle spindle fiber originated. Thus, this is a monosynaptic pathway that allows a reflex signal to return with the shortest possible delay back to the muscle after excitation of the spindle.

Explain the significance of alpha-gamma coactivation: This is a phenomenon of the muscle fiber. As the muscle shortens, the intrafusal fibers also shorten. The relaxation of the equatorial regions that would result from extrafusal muscle fiber contraction is offset by equatorial stretching because of intrafusal fiber contraction, and the equatorial regions remain under nearly constant tension.

Describe the neural circuit for alternating activity for extensors and flexors for a joint: The gamma efferents modulate the descending control to the motor neurons – sends the same signal to alpha and gamma neurons. The goal is to contract the antagonist while you extend the agonist and vice versa.

Describe the circuit for Renshaw cell inhibition: Located in the ventral horns of the spinal cord in close association with the motor neurons are a large # of small interneurons called Renshaw cells. Almost immediately after the axon leaves the body of the anterior motor neuron, collateral branches from the axon pass to the adjacent Renshaw cells. These in turn are inhibitory cells that transmit inhibitory signals back to the nearby motor neurons. Thus, stimulation of each motor neuron tends to inhibit the surrounding motor neurons; an effect called recurrent inhibition.

Describe the Golgi tendon organ circuit: This is stimulated by the tension produced by the small bundle of muscle fibers. It provides the CNS with instantaneous info on the degree of tension in each small segment of each muscle. Signals from the tendon organ are transmitted through the Ib fibers, which transmit signals both in local areas of the cord and through long fiber pathways such as the spinocerebellar tracts into the cerebellum and through still other tracts to the cerebral cortex. The local cord signal excites a single inhibitory interneuron that in turn inhibits the anterior motor neuron. This local circuit directly inhibits the individual muscles without affecting adjacent muscles.

Compare and contrast the flexion reflex and stretch reflex anatomically, physiologically and functionally:

Discuss the significance of reciprocal inhibition and reciprocal innervation in rhythmic limb movements: Excitation of one group of muscles is usually associated with inhibition of another group. When a stretch reflex excites one muscle, it often, at the same time, inhibits the antagonist muscles. This is reciprocal inhibition and the neuronal circuit that causes this reciprocal relation is called reciprocal innervation. Likewise, reciprocal relations often exist btw the two sides of the cord.

Describe the mass reflex: In a spinal animal or human being sometimes the spinal cord suddenly becomes excessively active, causing massive discharge of large portions of the cord. The usual stimulus that causes this is a strong nociceptive stimulus to the skin or excessive filling of a viscus, such as overdistention of the bladder or the gut. Regardless of the type of stimulus, the resulting reflex, called the mass reflex, involves large portions or even all of the cord, and its pattern of reaction is the same. The effects are a major portion of the body goes into strong flexor spasm, the colon and bladder are likely to evacuate, the arterial pressure often rises to maximal levels, and large areas of the body break out into profuse sweating.

Describe spinal shock and the typical course of recovery from spinal shock: When the spinal cord is suddenly transected in the upper neck, essentially all cord functions, including the cord reflexes, immediately become depressed to the point of total silence, a reaction called spinal shock. The reason for this is that normal activity of the cord neurons depends to a great extent on continual tonic excitation by discharges of nerve fibers entering the cord from higher centers, particularly discharges transmitted through the reticulospinal tracts, vestibulospinal tracts, and corticospinal tracts. After a few hours to a few weeks, the spinal neurons gradually regain their excitability. This seems to be a natural characteristic of neurons everywhere in the CNS. Typically some of the spinal functions specifically are affected during or after spinal shock. The arterial BP falls drastically, thus demonstrating that sympathetic activity becomes blocked. All skeletal muscle reflexes integrated in the cord are blocked initially but in humans can return after 2 wks to several months. Some reflexes can become hyperexcitable. The first reflexes to return are the stretch reflexes then the flexor, postural antigravity, and remnants of the stepping. The sacral reflexes for control of bladder and colon evacuation are suppressed in humans for the first few weeks after cord dissection but they eventually return.

Last Updated 09/06/01 08:53:33 PM
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