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Flashcards in Descending Motor Systems Deck (23):

What is the myotactic stretch reflex? Be sure u can draw it.

When a heavy object is unknowingly placed on a tray someone is holding, the increased weight stretches the biceps muscle, which results in the activation of the muscle spindle’s Ia afferents. The Ia afferents have their cell bodies in the dorsal root ganglia of the spinal cord, send projections into the spinal cord, and make synapses directly on alpha motor neurons that innervate the same (homonymous) muscle. Thus, activation of the Ia afferent causes a monosynaptic activation of the alpha motor neuron that causes the muscle to contract. As a result, the stretch of the muscle is quickly counteracted, and the person is able to maintain the tray at the same position.


What is a stretch spindle in the myotactic reflex?

Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons.


What is meant by a monosynaptic junction in a myotactic reflex?

In the knee jerk example- when the knee is tapped, afferents are sent to the spinal cord which directly synapse on the efferent motor neuron that send a signal to the thigh muscle that causes lower leg extension.


What is reciprocal inhibition in the myotactic reflex?

when a muscle spindle is stretched and the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the homonymous muscle. This inhibition is accomplished by an inhibitory interneuron in the spinal cord. The Ia afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the behavioral reflex. The other branch innervates the Ia inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.

I know that was over the top but I think it was a good explanation


What is the lower motor neuron?

The lower motor neuron innervates striated muscle and it's the last neuron in a chain of neurons.


What kind of motor neurons are included in the lower motor neuron and what kind of fibers do they contain?

Alpha motor neuron- extrafusal muscle fibers.
Gamma motor neuron- Intrafusal muscle fibers


What are the signs and symptoms of a lower motor neuron lesion?

Antonia- loss of muscle tone
Areflexia- loss of myotactic reflex
Flaccid paralysis
Fasciculations- spontaneous muscle contractions
Atrophy- loss of muscle tissue


What is a neuromuscular spindle?

It is a extrafusal muscle fiber that effects limb movement. It functions to provide muscle tone, it is the basis of the myotactic reflex and aids in proprioception.


What is the upper motor neuron?

Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the lower motor neurons.


What are the signs and symptoms of an upper motor neuron lesion?

Spastic paralysis
Hypertonus: flexors of arm, extensors of leg
Negative plantar reflex
Atrophy of disuse


What is spastic paralysis?

A chronic pathological condition in which the muscles are affected by persistent spasms and exaggerated tendon reflexes because of damage to the central nervous system.


What is the flexor reflex?

The flexor reflex is a spinal reflex intended to protect the body from damaging stimuli.It is polysynaptic, causing stimulation of sensory, association, and motor neurons.

Say one steps on a nail. A cutaneous receptor senses this and sends the signal to the spinal cord. This afferent signal synapses on an interneuron which synapses on an alpha motor neuron. This motor neuron sends the signal to the target muscle to pull the foot from the nail. ( this pathway has 2 synapses which is different than the myotactic reflex)


What is the significance of the internal capsule?

It contains all of the pathways that allow information to be transferred between the cerebral cortex and the spinal cord, brainstem, and subcortical structures (ie: thalamus, basal ganglia).


What is the significance of the cerebral peduncle?

The cerebral penduncle (correctly spelled cerebral "peduncle") is made of nerve fibers, and there is one on each side of the brain. They help transport nerve impulses from the higher part of the brain (cortex) and the brain stem, or lower part of the brain. The main function of the cerebral peduncle impulses is to control body movement.


What is the significance of pyramids? ( lol that sounds funny. Not like Egyptian pyramids, but those of the brain)

The pyramids are two elongated swellings on the ventral aspect of the medulla. Each pyramid contains approximately 1,000,000 CORTICOSPINAL AXONS. As the name suggests, these axons arise from the cerebral cortex and descend to terminate within the spinal cord. As corticospinal axons descend from the cortex, they course through the internal capsule, the cerebral peduncle of the midbrain and the ventral pons and onto the ventral surface of the medulla as the pyramids.


Name the path of the corticospinal tract.

This pathway originates in about 60-80% from the motor cortex.

The tract courses through the internal capsule through the cerebral peduncle in the midbrain. It then enters the pyramid in the medulla where it dicussates and finally terminates in the spinal cord gray.


What is the origin, termination and function of the corticobulbar tract?

Origin: cortex.
Termination: brainstem.
Function: the descending fibers of the corticobulbar tracts serve as upper motor neurons to the cranial nerve motor nuclei onto which they make synaptic contact. These upper motor neurons thus serve as the anatomical substrate for voluntary control of the muscles of facial expression, eye movements, jaw opening and closing, and movements of the tongue.


What is the origin, termination and function of the corticopontine tract?

Origin: cortex
Termination: basilar pons
Function: CORTICOPONTINE fibers convey information from motor related areas of cortex (i.e., the cells of origin) to neurons in the IPSILATERAL pontine grey (pontine grey neurons). More specifically, corticopontine axons convey to the pontine grey neurons information that is used in the planning and initiation of movements.


What is the effect on leg and arm of activation of the corticospinal tract?

Facilitates the extensor muscles in the upper extremity and facilitates flexor muscles in the lower extremity.

Primary effect- control of hand musculature.


What does the rubrospinal tract control?

Control of proximal arm and leg musculature.


What does the reticulospinal tract control?

Controls axial musculature- walking.


What does the vestibulospinal tract control?

Control of axial musculature- balance.


What does the raphe spinal tract control?

Control of incoming pain signals?