Motor Physiology Flashcards
What is the hierarchy of the motor system?
Higher center - motor cortex, premotor cortex
Middle center - brainstem, cerebellum, BN
Lower center - spinal cord
Higher centers control more voluntary behavior; lower centers control more reflexive involuntary behavior
What do the various levels of the motor system have in common?
Each motor area contains a map of the body = somatotopic organization
Each level receives information from the periphery - especially proprioception
Levels are highly interconnected
What are the parts of a spinal reflex arc?
Receptor - to afferent sensory neuron - to control center (spinal cord) - to efferent motor neuron - to effectors
Where is the spinal reflex arc found?
ventral anterior horn of the spinal cord
What are muscle spindles and Golgi tendon organs and where are they found?
Two types of muscle receptors
Golgi is found in the tendons; in series with 15-20 extrafusal fibers
Muscle spindle receptors are found within the belly of the muscle interspersed among the extrafusal fibers; they are in parallel with the muscle extrafusal fibers.
What kinds of stimuli excite muscle spindles and Golgi tendons?
Golgi - stretch is a weak stimulus; strong contractions will excite these receptors
Muscle spindle - excited by stretch of the muscle. Stimulation of gamma motor neurons causes contraction of intrafusal fibers and imposes a stretch on the spindles. IF the spindles are stretched they will depolarize.
Can muscle spindles/Golgi tendons be influenced by CNS?
Muscle spindles are unique in that the CNS can control the sensitivity of the muscle spindles. CNS can set a length for muscle to contract to by stimulating the gamma MN. CNS can continue to stimulate gammas so that the muscle spindle continues to be stretched during a contraction. If not, as muscle contracts spindles are “unloaded” (no longer stretched, no longer sending AP to CNS). As muscle contracts, to prevent the spindles from being unloaded, the alphas and gammas are activated together.
Function of A alpha neuron
innervates extrafusal fibers - skeletal muscle
Function of A gamma neuron
Innervates intrafusal fibers - muscle spindles
What is co-activation in motor physiology
Alphas and gammas are activated together
What is reciprocal innervation?
Permits reciprocal inhibition or the inhibition of the antagonist muscle as the agonist muscle is activated.
The spinal cord is the source of gaiting behavior such the stepping reflex in newborns.
How does reciprocal innervation permit a reflex like the cross extensor reflex?
Enlargement of withdrawal reflex; with a more intense noxious stimulus, more interneurons are activated so not only is the affected body part withdrawn but the opposite side of the body is extended.
What kinds of motor tracts come from the brain stem?
Descending pathways from the brainstem
- Ventromedial pathways - control proximal muscles, balance, posture.
- Dorsolateral pathways - control distal muscles, fine control of fingers and hands
What are the major pathways from the cortex?
Source of discrete voluntary movements
- Corticospinal tract
- Corticobulbar tract
What is the significance of + or - Babinski?
test for integrity of the corticospinal tract
The destruction of motor cortex controlling foot changes the reflex response to a firm touch of the lateral sole of the foot. The negative sign is normal in adults; the toes flex down. The positive sign ins abnormal in adults.; toes extend up and fan out. The positive sign occurs with damage of the corticospinal tract but not collateral tracts. The positive Babinski occurs normally in in ants; disappears as NS develops and they prepare to walk.
What is the dysfunction in cerebral palsy and ALS?
Cerebral palsy - group of motor disorders manifested by irreversible loss of muscle control and coordination. Damage to motor cortex in fetus, neonate or infant.
ALS - progressive degeneration of corticospinal tract; there is a progressive weakness starting with arms or legs with normal sensation. Amyotrophy means neurogenic atrophy of muscle. Lateral sclerosis means hardening and scarring lateral columns of spinal cord.
A possible mechanism is a glutamate receptor disturbance bc glutamate blockers seem to help prevent destruction.
Somatatopic representation of the primary motor cortex
contralateral representation
stimulation of cortex in humans activates contralateral muscles.
The head is represented close to Sylvian fissure with arms, trunk and legs above. Muscle groups used in movements requiring fine control (hands and face) have a large representation in motor cortex.
What are other parts (not somatatopic) of the motor cortex?
Basal nuclei Red nucleus RAS (to cerebellum) Vestibular nuclei (to cerebellum) Pontine nuclei Inferior olive
Result: Each time the corticospinal tract is activated; strong stimuli also go to the basal nuclei, brainstem and cerebellum
What is involved in performing a voluntary movement?
Requires a plan of action which is supplied by the supplementary motor area and the premotor cortex
The motor system has to activate the descending pathway to command the necessary muscles
When the corticospinal pathway is activated alphas and gammas are stimulated together directly. Interneurons that are responsible for reflexes can be activated such that interneurons inhibit antagonistic muscles indirect stimulation of spinal cord via the brain stem.
What are the functions of the cerebellum?
Not a source of motor activity
Coordinates and adjusts motor function from the motor cortex.
Receives information on the intention of the cortical motor system and information from the periphery about what the body is actually doing. the cerebellum compares these two sources of info and makes any necessary adjustments.
Helps coordinate components parts of complicated activity like driving a car; works closely with premotor cortex.
What motor deficits occur with cerebellar lesions?
Dysmetria - overshooting
ataxia - uncoordinated movements
intention tremor
Cerebellar nystagmus - tremor of eyeballs
Balance problems, particularly with changing directions
Hypotonia - loss of tone
Gait problems - wide stance, drunken sailor gain
What is the function of the basal nuclei?
Mostly input (via the thalamus) to and from the cortex
Funcitons in association with the corticospinal tract in controlling patters of movement
Parts:
Caudate nucleus, putamen, Globus pallidus, substantia nigra, subthalamus
What are the two major pathways in the BN?
Putamen Circuit - essential for patterns of movement
Caudate pathway - cognitive control, thinking about appropriate motor actions, timing and scaling of movements
What are the nature of the motor deficits in Parkinson’s and Huntington’s?
Parkinson’s - loss of dopaminergic cells in substantia nigra. symptoms include: rigidity, shuffling gate, resting tremor, depression and dementia
Huntington’s - autosomal dominant genetic disease. Only need one mutated gene to develop disease. Defective protein called huntingtin accumulates in the basal nucelli and kills BAB and ACH producing neurons. Over production of glutamate without ACH and BAB to balance. Symptoms - wild writing movements, behavioral problems, dementia, insanity.
Corticobulbar tract
projects form the motor cortex through the internal capsule and synapses on the bran stem nuclei
innervates both sides of the face except for the lower part of the facial and the hypoglossal. So with a stroke on one side, the opposite lower face and tongue could be paralyzed but the eyes would be okay.
Midbrain nuclei
oculomotor - external eye movements trochlear - eye muscles trigeminal - soft palate, chewing abducen - eye muscles Facial nerve
Medullary nuclei
glossopharyngeal - pharynx muscle
vagus - throat muscles
hypoglossal - tongue muscle
spinal accessory - neck muscles
NTz of basal nuclei
dopamine from substantia nigra to BN - inhibitory pathway for stabilizing motor function
- GABA from caudate nucleus to putamen, Globus pallidus and substantia nigra for negative feedback also stabilizing motor control
- ACH from and cortex to BN
- NE, 5HT and Enkephalin from brainstem to BN and cerebellum
NTz of basal nuclei
dopamine from substantia nigra to BN - inhibitory pathway for stabilizing motor function
- GABA from caudate nucleus to putamen, Globus pallidus and substantia nigra for negative feedback also stabilizing motor control
- ACH from and cortex to BN
- NE, 5HT and Enkephalin from brainstem to BN and cerebellum
Major pathways of cerebellum
- Vestibulocerebellar - coordination of equilibrium and posture; integration of timing and direction form periphery
- Spinocerebellar - feedback from distal limbs; accuracy through comparing intention with progress; rapid movements
- Cerebrocerebellar - coordination of many small movements into one; timing of movements so that progresses smoothly; anticipation and prediction.
