Spinal Reflexes & Descending Motor Pathways

Lateral pathways control...

both proximal and distal muscles responsible for most voluntary movements of arms and legs

The corticobulbar system controls

brainstem nuclei that innervate cranial muscles

Reticulospinal tracts (2) originate from

brainstem reticular formation, a large, diffusely organized collection of neurons in the pons and medulla

Because the red nucleus receives most of its input from the cerebellum, the rubrospinal tract probably plays a role in

transmitting learned motor commands from the cerebellum to the musculature

Example of descending pathway lowering threshold for producing flexor reflex

touching a dish to check if it is hot can also change gain of the reflex

Medial pathways

vestibulospinal tracts (both lateral and medial) reticulospinal tracts (both pontine and medullary) tectospinal tract anterior corticospinal tract

Recurrent inhibition of motor neurons

Axons of alpha motor neurons bifurcate in the spinal cord and innervate an inhibitory interneuron called the Renshaw cell interneuron innervates and inhibits the same motor neuron that caused it to fire (negative feedback loop) stabilizes the firing rate of motor neurons

Crossed extension reflex

Because the weight of the body is supported by both legs, the flexor reflex must coordinate the activity not only of the leg being withdrawn but also of the opposite leg opposite leg must simultaneously extend

Function of autogenic inhibition reflex

For many years, thought to protect the muscle from excessive amounts of force More recent evidence indicates that the Golgi tendon organ is sensitive to much lower levels of force than previously believed --> autogenic inhibition reflex may be more extensively involved in motor control under normal conditions one possibility = helps to spread the amount of work evenly across the entire muscle, so that all motor units are working efficiently

Reciprocal Inhibition in Flexor Reflex

Group III afferents innervate inhibitory interneurons that in turn innervate the alpha motor neurons controlling the antagonist muscle

Reciprocal inhibition in the stretch reflex

Ia afferent of the muscle spindle bifurcates in the spinal cord one branch producing the behavioral reflex other branch innervates the Ia inhibitory interneuron, which innervates the alpha motor neuron that synapses onto the opposing muscle

Reciprocal Excitation in Autogenic Inhibition Reflex

Ib afferent bifurcates in SC One branch innervates the Ib inhibitory interneuron other branch innervates an excitatory interneuron that, in turn, innervates the alpha motor neuron that controls the antagonist muscle

Descending motor pathways are organized into

Lateral pathways Medial Pathways

Function of tectospinal tract

because of the nature of the visual response properties of neurons in the superior colliculus (the optic tectum), it is presumably involved in the reflexive turning of the head to orient to visual stimuli

Lateral vestibulospinal tract excites

antigravity muscles in order to exert control over postural changes necessary to compensate for tilts and movements of the body

Medial pathways control:

axial muscles responsible for posture, balance, and coarse control of axial and proximal muscles

Knee jerk reflex

a myotatic reflex Because the physician taps the tendon, this reflex is also referred to as the deep tendon reflex

Flexor reflex

activation go Group III afferents of pain receptors --> travel up spinal cord ---> innervates several excitatory interneurons --> multiple muscle groups (hip + thigh, etc.) activated --> coordinated activity of muscle groups to withdraw limb (coordinates activity of multiple joints)

Gamma bias

adjustment of spindle sensitivity by gamma activation

Rubrospinal tract - function

alternative by which voluntary motor commands can be sent to the spinal cord role in movement velocity (rubrospinal lesions cause a temporary slowness in movement) major pathway in many animals, minor in humans

Crossed extension reflex - pathway

branch of the Group III afferent innervates an excitatory interneuron that sends its axon across the midline into the contralateral spinal cord excites the alpha motor neurons that innervate the extensor muscles of the opposite leg

Damage to the reticulospinal tract

can cause harmless stimuli, such as gentle touches, to elicit a flexor reflex

Voluntary movements initiated in the:

cerebral cortex motor commands are transmitted to the musculature through a variety of descending pathways (corticospinal, rubrospinal, and reticulospinal)

Anterior corticospinal tract

continue down the spinal cord in the anterior funiculus When they reach the spinal segment at which they terminate, they cross over to the contralateral side through the anterior white commissure innervate alpha motor neurons or interneurons in the anterior horn

Lateral corticospinal tract

continue to course through the lateral funiculus of the spinal cord synapse either directly onto alpha motor neurons or onto interneurons in the ventral horn

Tectospinal Tract

deep layers of the superior colliculus --> crosses the midline immediately --> courses through the pons and medulla, just anterior to the medial longitudinal fasciculus --> courses through the anterior funiculus of the spinal cord --> majority of the fibers terminate in the upper cervical levels

Modulations of spinal reflexes arise from:

descending pathways from the brainstem and cortex

Lateral corticospinal tract controls:

distal muscles include fine control of fingers

Autogenic inhibition reflex is a _______ reflex

disynaptic

Cell bodies of Group Ia afferents are located in:

dorsal root ganglia

Group Ib afferents have cell bodies in:

dorsal root ganglia

Activation of rubrospinal tract causes:

excitation of flexor muscles and inhibition of extensor muscles

Whenever motor commands are sent by descending pathways to alpha motor neurons, the appropriate compensating commands are sent to

gamma motor neurons (alpha-gamma coactivation)

Damage to higher levels results in deficits in

motor planning, initiation, coordination, etc. but movement is still possible

Spinal reflexes can be modulated by

higher levels of the hierarchy can suppress/facilitate reflexes (facilitatory & inhibitory)

Example of descending pathway inhibiting flexor reflex

holding a hot pan before putting it down on a table

Percentage of axons in the corticospinal tract that innervate alpha motor neurons directly is greater in

humans and nonhuman primates than in other mammals presumably reflecting the increased manual dexterity of primates

A hyperactive or hypoactive stretch reflex is an

important clinical sign to localize neurological damage (higher order processing of reflexes)

What causes the flexor reflex?

initiated by cutaneous receptors and pain receptors removes the limb from the damaging stimulus more quickly than if the pain signal had to travel up to the brain, be brought to conscious awareness, and then trigger a decision to withdraw the limb

Lateral pathways

lateral corticospinal tract rubrospinal tract

Lateral vestibulospinal tract

lateral vestibular nucleus --> courses down through anterior funiculus on ipsilateral side ---> exits ipsilaterally at all levels of SC

Function of Reticulospinal tracts

major alternative to the corticospinal tract, by which cortical neurons can control motor function by their inputs onto reticular neurons regulate the sensitivity of flexor responses to ensure that only noxious stimuli elicit the responses circuitry for many complex actions, such as orienting, stretching, and maintaining a complex posture

Medial vestibulospinal tract

medial vestibular nucleus --> splits immediately & courses bilaterally through brainstem via *medial longitudinal fasciculus* & through anterior funiculus --> exits at or above T6

Function of vestibulospinal tracts

mediate postural adjustments and head movements help the body to maintain balance

Medullary reticulospinal tract

medullary reticular formation --> courses mostly ipsilaterall, some fibers cross midline --> through anterior funiculus ---> exit at all spinal levels

Commands that initiate locomotor circuits in the spinal cord are thought to be transmitted through the

medullary reticulospinal tract

Activation of the Ia afferent causes a

monosynaptic activation of the alpha motor neuron ---> that causes the muscle to contract

Corticospinal tracts

motor cortex --> internal capsule ---> crus cerebri (midbrain) ---> medullary pyramids (aka pyramidal tract) --> splits into 2 tracts at caudal medulla 90% cross in pyramidal decussation to form *lateral corticospinal tract* 10% = *anterior corticospinal tract*

The corticospinal system controls

motor neurons and interneurons in the spinal cord

Proximal-distal rule:

motor neurons that innervate distal muscles are located lateral to motor neurons that innervate proximal muscles

Flexor-extensor rule:

motor neurons that innervate flexor muscles are located posteriorly to motor neurons that innervate extensor muscles

Descending motor pathways arise from

multiple regions of the brain send axons down the spinal cord that innervate alpha motor neurons, gamma motor neurons, and interneurons

Myotatic reflex is initiated by the

muscle spindle

Myotatic Reflex (Stretch Reflex)

muscle stretched --> activates Ia afferent --> synapses on alpha motor neuron that innervates same (homonymous) muscle

The medial vestibulospinal tract innervates

neck muscles in order to stabilize head position as one moves around the world also important for the coordination of head and eye movements

Because of the parallel nature of processing,

paralysis is a relatively rare outcome, produced by damage to the lowest level of the hierarchy important for the ability of undamaged parts of the motor system to compensate (at least partially) for injuries to other parts of the system

Damage to the corticospinal tract results in

permanent loss of the fine control of the extremities Although parallel descending pathways can often recover the function of more coarse movements, these pathways are not capable of generating fine, skilled movement

Pontine reticulospinal tract

pontine reticular formation --> courses ipsilaterally through medial longitudinal fasciculus & anterior funiculus --> exits ipsilaterally @ all levels

Function of corticospinal tracts

primary pathway that carries the motor commands that underlie voluntary movement --- limbs & axial muscles (along with corticobulbar tract)

Anterior corticospinal tract controls:

proximal muscles

Rubrospinal tract

red nucleus (midbrain) --> immediately cross to contralateral side & course through brainstem and lateral funiculus of sipnal cord --> innervates spinal nuerons @ all levels of SC

The flow of information through the motor system has both a

serial organization (communication between levels) & parallel organization (multiple pathways between each level)

The corticospinal tract is the only descending pathway in which

some axons make synaptic contacts directly onto alpha motor neurons direct cortical innervation presumably is necessary to allow the powerful processing networks of the cortex to control the activity of the spinal circuits that direct the exquisite movements of the fingers and hands

When a muscle contracts, the antagonist muscle is

stretched during the movement Alpha-gamma coactivation --> relaxing the contraction of the intrafusal fibers of the antagonist muscle, allowing the muscle to be stretched without triggering the stretch reflex during a voluntary movement

Autogenic inhibition reflex

tension is applied to a muscle ---> Group Ib fibers that innervate the Golgi tendon organ are activated --> synapse onto the Ib inhibitory interneuron --> inhibitory synapse onto the alpha motor neuron that innervates the same muscle ---> muscle ceases contraction

Adaptiveness of spinal reflexes can change depending on the

the behavioral context sometimes the gain (strength) or even the sign (extension vs. flexion) of a reflex must be changed in order to make the resulting movement adaptive descending pathways responsible for controlling these variables

Voluntary movement and some sensory-driven reflex actions are also controlled by

the descending pathways

Motor neurons are topographically organized in the anterior horn of the spinal cord according to two rules:

the flexor- extensor rule the proximal-distal rule

A major role of the myotatic reflex is....

the maintenance of posture higher levels of the motor system are able to send a simple command ("maintain current posture") and then be uninvolved in its implementation

Red nucleus receives some input from the motor cortex, and it is therefore probably an important pathway for

the recovery of some voluntary motor function after damage to the corticospinal tract