Spinal Reflexes
Reflex inhibition of antagonist muscles leads to reciprocal inhibition of muscles using primary 1a inhibitory interneurons.
Autogenic inhibition of 1b afferents through Golgi Tendon oranges excites inhibitory interneurons leading to inhibition of homonymous and synergistic interneurons.
Crossed Extension Reflex- uses both sides of body to act in response to noxious stimuli; if stimuli is placed on right foot, massive contraction of muscles occurs ipsilaterally which massive inhibition of movement occurs contralaterally (to bear weight)
Renshaw cells are used as a negative feedback loop for alpha motor neurons; regulate firing of motor neurons if they increase too much
Descending Spinal Pathways
Proximal Distal Rule- (anterior horn) motor neurons that are the most lateral innervate the most distal structures (limbs) while the medial part of the anterior horn innervates the more proximal structures.
Flexor Extensor Rule- flexor muscles are innervated by the posterior (dorsal) regions of the horn which extensor muscles are innervated by the anterior (ventral) regions. Displays topographic organization.
3 basic groups of motor neurons:
Medial pathways- innervate medial structures (vestibulospinal, reticulospinal, tectospinal, anterior part of corticospinal)
Lateral pathways- innervate lateral musculature and some proximal structures (rubrospinal, lateral part of corticospinal)
Aminergic- originate in brainstem and branch diffusely…modulatory in nature
Corticospinal Tract
Forebrain… rostral midbrain… rostral pons… caudal pons… rostral medulla… anterior part (10% stays ipsilateral), lateral part decussates… peripheral motor neuron
-control of fine-movement distal muscles (hands)
-serves some axial muscles and is major voluntary pathway between brain stem and spinal motor system
-allows muscle spindles to sense change in muscle length shortening
-lateral part terminate in intermediate zone and motor neurons in CONTRALATERAL limb
-anterior part terminates in interneurons of intermediate zone and BILATERAL motor neuron limbs
-contact both alpha and gamma motor neurons
Rubrospinal Tract
-red nucleus… decussate in rostral midbrain (in ventral tegmental decussation)… rostral pons… caudal pons… rostral medulla… caudal medulla… peripheral motor neuron
-acts as alternative to corticospinal tract
-receives more input from the cerebellum but some from cortex
-encodes VELOCITY movement
-small In humans and important in recovery functions
-courses in dorsolateral funiculus and terminates in intermediate zone
-excites flexors and inhibits extensors
-excites both alpha and gamma neurons
Vestibulospinal Tract
-rostral medulla… caudal medulla… medial part exits at C8 and lateral part exits ipsilaterally at L4
-Lateral portion:
-originates in lateral vestibular nucleus
-IPSILATERAL projections
-descends entire length of cord terminating in intermediate zone
-excites both alpha and gamma neurons stimulating EXTENSORS
-functions in POSTURAL CONTROL
-Medial portion:
-originates in medial vestibular nucleus
-BILATERAL projections
-descends only to mid-thoracic levels
-terminates in intermediate zone of dorsal horn
-functions sin postural adjustments of HEAD and NECK, ispilateral fibers excite, contralateral fibers inhibit
Reticulospinal Tract
-major role in modulate of segmental stretch reflexes and muscle tone
-Pontine reticulospinal tract:
-originates in pontine reticular nuclei
-IPSILATERAL projections
-descends full length of the cord
-terminates on intermediate zone of alpha and gamma motor neurons
-functions to facilitate VOLUNTARY and REFLEX responses
-Medullary Reticulospinal Tract:
-originates in medullary reticular formation
-BILATERAL projections
-functions in MONOSYNAPTIC INHIBITION of axial motor neurons and inhibits spinal reflexes
-Reticular formation integrates various types of sensory inputs and is also an alternative to the corticospinal tract
Aminergic Pathways alter the excitability of spinal neurons. Ceruleospinal tract uses epinephrine and raphe spinal tract involved in pain modulation using seratonin.
Gamma motor neurons allow spindle to maintain sensitivity over a wide range of muscle lengths during contraction.
Structures in the brain play a major role in the regulation of tension in muscle spindle by modulating gamma neurons. Programmed command signals modulate fusimotor system which “biases” spindle sensitivity according to needs… gamma bias.
Motor cortex can simultaneously excite alpha and gamma motor neurons called alpha-gamma coactivation…. counteracts slackening of intrafusal muscle fibers
Gamma spasticity-results from interruption of gamma motor pathways characterized by excessive and inappropriate muscle contraction.
Hyptononia… decreased muscle tone due to hypoactivity of gamma neurons
Tags: Aminergic, autogenic, Corticospinal Tract, Flexor Extensor, Gamma spasticity, Hyptononia, Lateral pathways, Medial pathways, reflex response, reflux inhibition, Renshaw cells, Reticulospinal Tract, Rubrospinal Tract, Vestibulospinal Tract
