Motor

Question 1

What is the difference between lower and upper motor neurons?

Answer 1

Lower: A-alpha neurons in the brainstem/ventral horn with direct monosynaptic innervation of muscle.
Upper: cerebral cortex, reticular formation, and vestibular nuclei, projecting down to synapse on lower motor neurons

Question 2

What is a muscle spindle and how does it respond to changing muscle length?

Answer 2

Muscle spindles are bundles of intrafusal muscle fibers surrounded by CT. They are sensitive to changes in muscle length and convey that information to the spinal cord. Protects the muscle from overstretching.

Question 3

Describe the myotatic reflex.

Answer 3

Keeps a limb where you left it. Muscle spindles detect stretch (=muscle lengthening), afferents go to spinal cord and

• activate same and synergistic muscles (contract to stop lengthening)
• inhibit antagonist muscles
  e. g. patellar tap test

Question 4

How do gamma motor neurons affect the myotatic reflex?

Answer 4

Muscle spindle fibers are contractile.
Gamma motor neurons innervate the muscle end of the muscle spindle and regulate contraction of the spindle itself. This allows the CNS to regulate the sensitivity of the reflex (tautening spindle makes it more sensitive to muscle stretch). Also maintains spindle sensitivity regardless of muscle length.

Question 5

What is a golgi tendon organ and how does it respond to muscle tension?

Answer 5

Located in muscle tendons and not innervated by gamma motor neurons. Relax muscle and flex at joint in response to stretch.
1b afferents innervate interneurons in the spinal cord to:
-inhibit alpha motor neurons to the same muscle
-stimulate alpha motor neurons to antagonist
e.g. flexor withdrawal reflex

Question 6

Describe propriospinal neurons.

Answer 6

Interconnect spinal segments for complex actions and regulating reflexes.
Located right at the border between gray and white matter in spinal cord.
Medial (long fibers, gross movements): axons extend over whole spinal cord, regulate proximal/axial muscles and posture
Lateral (short fibers, fine movements): axons extend a few spinal segments, regulate distal muscles for fine control.

Question 7

What are some general functions of upper motor neurons?

Answer 7

Brainstem pathways maintain posture and balance

Motor cortex pathways regulate fine movements in the extremities.

Question 8

Describe the motor control functions of the vestibulospinal and reticulospinal tracts.

Answer 8

Vestibulospinal: vestibular nuclei relay head movements to activate extensors of lower limb and flexors of upper limb to maintain posture and balance.
Reticulospinal: Regulate muscle tone and sensitivity of flexor responses (talk to gamma neurons), coordinating complex actions (breaking a pattern to start something new)

Question 9

What is the antigravity posture and what produces it?

Answer 9

Legs extended arms flexed

Designed to keep you upright, RF default posture. Governed by medial reticulospinal tract, from pons.

Question 10

What are the three descending tracts from motor cortex?

Answer 10

Corticospinal (to ventral horn): Projects to alpha and gamma motor neurons. 90% decussates to do fine movements. Remaining 10% does posture of neck and trunk.

Corticonuclear (to CN motor nuclei): muscles of face, head, and neck.

Corticoreticular (to pontine and medullary reticular formation): Smooths out gross movements by limiting inhibition among leg extensors (antigravity posture)

Question 11

What is the significance of the internal capsule? The medullary decussation?

Answer 11

The internal capsule is a bundle of axons in the cerebral hemispheres and a common site of strokes.
The medullary decussation forms the lateral and medial corticospinal tracts.

Question 12

How do the anterior and lateral corticospinal tracts differ in structure and function?

Answer 12

Lateral: 90% of the descending corticospinal neurons, cross in medulla and descend in lateral funiculus. Govern fine movements.

Anterior: remaining 10% of efferents, descend in ventral funiculus. Govern posture of head and neck.

Question 13

Compare lower motor neuron and upper motor neuron disease. What are the aspects of spasticity?

Answer 13

Lesion of lower motor neurons leads to flaccid paralysis and muscle atrophy.
Lesion of upper motor neurons produces spasticity: inappropriate muscle contraction. Due to disruption of corticoreticular tract, you lose smoothness of gross movement. Due to loss of cortical control, injured side assumes anti-grav posture.

Question 14

Compare the functions of the primary, supplementary and pre-motor cortices.

Answer 14

Primary: force, direction, extent, and speed of movements.
Supplementary: Ensures correct motor sequences, does mental rehearsal.
Premotor: Intention. Planning and preparation for movement.

Question 15

What is efference copy? How does it play a role in self awareness?

Answer 15

When a motor program is sent down the pathway to the muscles, a copy is sent to somatosensory cortex to predict expected sensation and compare projected with actual results.
I have self-agency because the efference copy matches my intentions.
I have self-ownership because the efference copy matches the actual sensory feedback.

Question 16

What are mirror neurons and where are they located?

Answer 16

Mirror neurons unify action perception and action execution–they light up whether an action is performed or observed.
Areas: inferior premotor cortex, inferior parietal cortex

Question 17

Describe the pathway from cerebellar cortex to motor cortex and back.

Answer 17

Purkinje cells receive inputs from spinocerebellar tracts and inferior olive. Then they project to nuclei deep in cerebellar white matter, which then project to thalamus-cortex. The cortex then projects back to check if the actual experience matched the prediction.

Question 18

Compare the location and functions of the vestibulocerebellum, spinocerebellum, and cerebrocerebellum. How do they differ in their connections with the motor cortex and spinal cord? How do they participate in motor learning?

Answer 18

Vestibulocerebellum: Communicates with vestibular nuclei and oculomotor nuclei. Balance and eye movements, esp VOR.

Spinocerebellum: Vermis and intermediate areas project spinal cord and pontine inputs to fastigial and interposed nuclei, then to thalamus-cortex. Motor execution, feedback control of ongoing movements. Checking for disparities between motor (prediction) and sensory inputs.

Cerebrocerebellum: Cerebellar hemispheres project to dentate nuclei, then to thalamus-cortex. Cerebral areas project back down via pontine nuclei. Motor planning, initiate voluntary movement by senting anticipatory information to cortex. This is the part important for motor learning: movements become more precise with practice.

Question 19

Describe the dampening of reflex oscillations.

Answer 19

Anticipatory signals to cortex from spinocerebellum reduce the sensitivity of stretch reflexes, so you don’t get those big oscillations of reflex and overshoot.

Question 20

Dysmetria

Answer 20

Inability to control range of movement: placement falls short or overshoots.

Question 21

Ataxia

Answer 21

Lack of smoothly coordinated movements. Combination of dysmetria and decomposition of movement.

Question 22

Decomposition of movement

Answer 22

Inability to correctly sequence fine movements

Question 23

Dysarthria

Answer 23

Inability to articulate words correctly

Question 24

Dysdiadochokynesia

Answer 24

Inability to perform rapid alternating movements

Question 25

Intention tremor

Answer 25

Erratic alternating movement of a limb as it approaches a target. Not a true tremor, which is very rhythmic and regular.

Question 26

What roles does the cerebellum play in cognition and visceral functions?

Answer 26

Cerebellum helps smooth out cognitive processes, impacting executive functions. Cerebellar dysfunction can lead to impaired executive functions, impaired spatial cognition, personality change, and linguistic difficulties.
Cerebellar cortex projects to brain stem and hypothalamus, regulates visceral systems:
GI motility
Heart rate/BP/baroreflex
Respiration
Micturition
Immune functions