Neuro review

__a large, diffusely organized collection of neurons in the pons and medulla

brainstem reticular formation

Descending motor pathways innervate what neurons?

1. alpha motor neurons 2. gamma motor neurons 3. interneurons

fx of lateral corticospinal tract

1. control of *distal* musculature & 2. fine control of digits of hand

fx of anterior corticospinal tract

1. control of *proximal/ axial* musculature

fx of rubrospinal tract

1. excite flexors 2. inhibit extensors 3. mvmt velocity >from cerebellum > learned motor commands >from motor cortex> recovery of motor fx following corticospinal damage

fx of reticulospinal tract

1. integration of sensory input and guide motor output. 2. regulate SENSITIVITY to flexor responses >> ENSURE only NOXIOUS stimuli elicit the responses 3. orientation, streching, maintenance of COMPLEX posture (axial and limb mm) (VOLUNTARY & REFLEX & muscle tone)

Descending motor pathways are organized into two major groups:

1. lateral pathways 2. medial pathways

movements are INITIATED in the cerebral cortex, and the motor commands are TRANSMITTED to the musculature through a variety of descending pathways, including ****

1. the corticospinal tract, 2. the rubrospinal tract, and 3. reticulospinal tracts.

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

1. the flexor-extensor rule 2. the proximal-distal rule

influences of descending pathways on spinal circuits

1. voluntary mvmts 2. reflex modulation

The lateral corticospinal tract...

>90% of the corticospinal tract fibers cross at the pyramidal decussation to form it >arises from numerous cortical areas >Plays a major role in the fine control of DISTAL musculature >Terminate in the *intermediate zone*

explain significant of parallel & serial processing

>If the motor hierarchy had a strictly serial organization, like a series of links on a chain, then damage to any part of the system would produce severe deficits or paralysis in almost all types of movements. >the *parallel* nature of organization is also important for the ability of undamaged parts of the motor system to *compensate* (at least partially) for injuries to other parts of the system

pick up a dish from the stove top, but you are uncertain whether it is hot or cold. >how is the reflex modulated?

>lower threshold of flexor reflex > makes you more likely to pull away even if NOT hot, maybe even many times >the lower threshold makes it easier for a WEAKER nociceptive input to trigger the flexor reflex; >>ALSO the pathways can *change the gain* of the reflex, making the flexor, withdrawal response GREATER than usual.

How is it possible to ever flex a joint when the stretch reflex of the extensor muscle causes it to extend the joint instead?

Alpha-gamma coactivation solves this problem by *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.

The __ is involved in a spinal reflex known as the autogenic inhibition reflex

Golgi tendon organ

___ 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.

Ia afferents

In reciprocal excitation of the Golgi tendon reflex, stimulation of...

Ib afferent fibers causes excitation of antagonist muscles.

Ia

Innervate muscle spindle

Fx of tectospinal tract

Little is known BUT t is presumably involved in the reflexive turning of the head to orient to visual stimuli.

A waiter is holding an empty tray, when unexpectedly a pitcher of water is placed on the tray. >>muscles were not prepared to support the increased weight, the tray should fall. However, a spinal reflex is automatically initiated to keep the tray relatively stable. >When the heavy pitcher is placed on the tray, the increased weight stretches the biceps muscle, which results in the activation of the *muscle spindle's Ia afferents* >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 waiter is able to maintain the tray at the same position

Myotatic reflex (stretch & knee-jerk reflex)

The *most distinctive function* of the descending motor pathways is

The control of voluntary movement.

fx of lateral vestibulospinal tract

antigravity to control posture and compensate for tilts and mvmts

reciprocal inhibition in the flexor reflex -describe

When the knee joints and the hip joints are flexed, the *antagonist extensor muscles must be inhibited* (just as in the stretch reflex). Thus, the *Group III afferents innervate inhibitory interneurons* that in turn innervate the alpha motor neurons controlling the *antagonist muscle*.

controls brainstem nuclei that innervate cranial muscles

corticobulbar system

controls motor neurons and interneurons in the spinal cord.

corticospinal system

__results in a 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 movements

damage to corticospinal tract

harmless stimuli, such as gentle touches, elicit flexor response

damage to reticulospinal tract

temporary slowness in mvmt

damage to rubrospinal tract

lateral corticospinal tract

decussate at caudal medulla synapse w/ alpha mn & interneurons in VENTRAL horn

anterior corticospinal tract

decussate at spinal segment at which they TERMINATE synapse w/ alpha mn & interneurons in ANTERIOR horn

__ is produced by damage to the HIGHEST levels of hierarchy

deficits in motor planning, initiation, coordination, and so forth, but movement is still possible.

if a reflex contains an interneuron between the sensory afferent and the motor neuron, it is an example of a

disynaptic reflex

alpha motor neurons

extrafusal muscle fibers

An important reflex initiated by cutaneous receptors and pain receptors is the <This quick reflex 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

flexor reflex.

the cordination of alpha-gamma motor commands is called *alpha-gamma coactivation*, >>and the *adjustment of spindle sensitivity* by gamma activation is called

gamma bias.

>the __levels of the motor system are able to send a simple command ("maintain current posture") and then be uninvolved in its implementation and free to perform other tasks such as planning the next sequence of movements.

higher

reflexes can be modulated by __ levels ex. facilitatory modulation of reflex in anticipation of burning iron ex. inhibtory modulation of reflex w/ hot plate, don;t want to drop it!

higher; descending pathways from brainstem & cortex

Ib afferents inhibit the __muscle

homonymous

The corticospinal tract is the only descending pathway in which

in __ some axons make synaptic contacts DIRECTLY onto alpha motor neurons

gamma motor neurons

intrafusal fibers @ end of muscle spindle >maintain taut and sensitive

Golgi tendon reflex affects the __side.

ipsilateral

control both proximal and distal muscles and are responsible for most *voluntary movements of arms and legs*. They include the 1 lateral corticospinal tract 2 rubrospinal tract

lateral pathways

The __levels of the hierarchy implement the command "maintain current posture" with such mechanisms as the *myotatic reflex*,

lower

reflexes are performed at __ levels

lower

control axial muscles and are responsible for *posture, balance, and coarse control of axial and proximal muscles*. They include the 1 vestibulospinal tracts (both lateral and medial) 2 reticulospinal tracts (both pontine and medullary) 3 tectospinal tract 4 anterior corticospinal tract

medial pathways

A major role of the __ is the maintenance of posture. If one is standing upright and starts to sway to the left, muscles in the legs and torso are stretched, activating the __to counteract the sway.

myotatic reflex

fx of medial vestibulospinal tract

neck muscle innervation to stabilize head position and coordinate head and eye mvmts

__ procesing allows for compensation if another part is damaged

parallel

(multiple pathways between each level)

parallel organization

paralysis is rare bc of

parallel processing

__ is produced by damage to the LOWEST level of hierarchy

paralysis

sometimes the gain (strength) or even the sign (extension vs. flexion) of a reflex must be changed in order to ADAPT the mvmt

reflex modulation

major alternative to corticospinal tract

reticulospinal tract

The __tract is an alternative by which voluntary motor commands can be sent to the spinal cord. (minor in humans)

rubrospinal

(communication between levels)

serial organization

myotatic reflex is initiated by

the muscle spindle, (not the Golgi tendon organ)

small mvmts of the body are detected by vestibular sensory neurons and MOTOR commands to counteract these mvmts are sent thru

the vestibulospinal tracts

picking up the dish in order to move it to the table AND ITS HOT! >how is the reflex modulated?

you want to wait to get to the table and not drop it so THE DESCENDING PATHWAYS INHIBIT the FLEXOR response