Motor Control Flashcards
motor unit
lower motor neurons are located in the ventral spinal cord
in most cases, a single motor neuron innervates a large number of muscle fibers. a motor unit is composed of the motor neuron and all the muscle cells (fibers) that it innervates
motor neuron pool
the population of motor neurons that innervate a single muscle is called the motor neuron pool for that muscle
the cell bodies of neurons in each pool are aggregated into clusters running lengthwise within the spinal cord
muscle force
muscle force is increased by (A) a recruitment of more motor neurons and (B) an increase in activity of motor neurons
phasic and tonic response
motor neurons have phasic and tonic responses
tonic constant activity to keep muscle in a certain position
phasic bursting activity to move the muscle fast
muscle spindles
extrafusal fibers are normal contractile fibers
alpha motor neuron activates the extrafusal muscle fibers
extrafusal muscle fibers are supplied with nerves by alpha motor neurons
gamma motor neurons activate the intrafusal fibers
tonically active 1a afferent sensory neurons and send information to the CNS
intrafusal muscle fibers are innervated by gamma motor neurons
1a spindle afferents send information about muscle stretch to the spinal cord
golgi tendon organs
golgi tendon organ receptor attatched to 1b afferent sensory neuron
receives information from the tendon, that senses tension. how much tension the muscle is exerting
comparison of muscle spindles and golgi tendon organs
muscle spindle and golgi tendon organ afferents increase their discharge rate in response to passively stretching the muscle
when the extrafusal muscle fibers contract by activation of their motor neurons, however, the spindles unload and decrease their discharge rate, whereas the rate of golgi tendon organ firing increases
the role purpose of alpha-gamma-activation
if only alpha motor neurons were activated, only the extrafusal muscle fibers would contract. the muscle spindles would become slack and no action potentials would be generated. the muscle spindle would be unable to signal further length change
normally, alpha-gamma-coactivation occurs. both extra- and intrafusal fibers contract. tension is maintained and muscle spindles can still signal changes in length
stretch reflex
- load added to muscle
- muscle and muscle spindle stretch as arm extends. muscle spindle afferents fire more frequently
- reflex contraction initiated by muscle stretch restores arm position and prevents damage from overstretching
patellar tendon reflex
- tapping patellar tendon stretches quadriceps femoris (extensor muscle)
- muscle spindle in quadriceps femoris stretches, activating 1a afferent to fire action potentials
- 1a afferent directly synapes (monosynaptic) on alpha motor neuron to quadriceps femoris - muscle contracts and lower leg swings forward
- collateral from the 1a afferent also excites an inhibitory interneuron in the spinal cord
- inhibitory interneuron inhibits alpha motor neuron to antagonistic (hamstring) muscle. the hamstring is a flexor muscle
- antagonistic muscle relaxes (reciprocal innervation inhibition) so leg can extend and swing out
organization of spinal cord
the medial parts of the spinal cord gray matter are involved primarily in the control of posture. the lateral parts are involved in the fine control of the distal extremities
organization of primary motor cortex
the motor map shows the same disproportions observed in the somatotopic map in the primary somatosensory cortex of the postcentral gyrus
musculature used in a task requiring fine motor control occupy a greater amount of space than does the musculature requiring less precise motor control
corticospinal tract
the corticospinal pathway (pyramidal tract) is the primary pathway that leaves the motor cortex to innervate motor neurons in the spinal cord
note that the left side of the body is primarily being controlled by the right side of the motor cortex and the right side of the body is primarily by the left side of the motor cortex
axons cross over either in the brain stem or the spinal cord
population coding in motor cortex
cortical motor neurons are directionally tuned. although individual motor neurons cannot precisely specify the direction of an arm movement because they are tuned too broadly, movement direction can be decoded by a larger population of neurons
brain machine interfaces (BMIs)
the goal of BMIs is the restoration of motor functions for patients with spinal cord damage or locked-in patients
invasive BMIs record neural activity with implanted microelectrodes and use population decoding algorithms to control prosthetic devices, computers or even the patient’s own muscles
movements require multiple brain regions
planning movement
initiating movement
executing movement
cerebellum
function: evaluates differences between intended action and actual action while a movement is in progress
receives input from the cerebral cortex, brain stem and spinal cord and sends projections back to them, forming a vast loop
synaptic transmission within this loop can be modified, which is crucial for motor adaption and learning
cerebellar lesions
typical defects of cerebellar diseases include dysmetria and decomposition of movements
cerebellum: input and output
projections to the cerebellum:
cerebral cortex: synapse in the pontine nuclei of the pons first
direct sensory inputs: vestibular, muscle spindles, other mechanoreceptors
other brain stem inputs: inferior olive and the locus coeruleus-sen modulatory inputs for learning and memory
output from the cerebellum:
cerebellar cortex to deep cerebellar nuclei and onto motor cortex (via the thalamus)
cerebellar circuit
there are two types of input neurons:
mossy fibers: synapse on granule cells. Axons of granule cells form parallel fibers which synapse and excite Purkinje cells
climbing fibers: synapse directly on Purkinje cells to excite it
Purkinje cells provide the output of the cerebellum through inhibitory synapses
one Purkinje cell can receive input from over a million granule cells (convergence), but only 1 climbing fiber
basal ganglia
function: involved in regulating and planning movements.
receives inputs from the cortex and projects back to the cortex through the thalamus
this circuit will influence brain stem outputs and ultimately neurons of the spinal cord
organization of basal ganglia
the basal ganglia consists of four major interconnected nuclei found in the white matter of the cerebrum
- striatum (caudate and putamen)
- globus pallidus
- substantia nigra (pars compacta and pars reticulata)
- subthalamic nucleus
inputs to basal ganglia
the caudate nucleus and the putamen are the input nuclei of the basal ganglia. almost all cortical areas project to the caudate nucleus or the putamen, except the primary visual cortex and the primary auditory cortex
the caudate and putamen also receive dopaminergic inputs from the substantia nigra pars compacta in the midbrain
output of the basal ganglia
the caudate and putamen send output through the globus pallidus and the substantia nigra pars reticulata
the globus palldis can be further subdivided into an external segment and internal segment, which are crucial for motor control
output is sent on to the superior colliculus, subthalamic nucleus, the thalamus, and back to the cerebral cortex
activity in the basal ganglia
excitatory and inhibitory basal ganglia activity
parkinson’s disease
parkinson’s is a progressive, neurodegenerative disease
usual onset is between 50-70 years
progresses slowly usually, leads to death 10-20 years later
100,000 people in Canada have the disease
parkinson’s disease is caused by a loss of the dopaminergic neurons of the substantia nigra pars compacta
excitation and inhibition of parkinson’s disease activity
