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