week 4 - motor control, neurons and pathways

where is the primary motor cortex located?

precentral gyrus of the frontal lobe, also called area 4 or m1.

what do upper motor neurons in pmc do?

execute commands to lower motor neurons to produce movement.

what happenes when you stimulate pmc and how is it oraganised?

• simple movements of single joints are produced.

• somatotopically as the motor homunculus, mapping body parts to cortical areas.

what does the premotor cortex do?

plans movement based on sensory input and guides actions.

what is the role of the supplementary motor cortex?

sequences complex movements and is active during mental rehearsal.

what are reflexes?

involuntary, rapid, stereotyped movements triggered by stimuli.

what are rhythmic motor patterns?

repetitive movements combining voluntary and reflexive components, like walking.

what are voluntary movements?

complex, purposeful, goal-directed actions initiated at the cerebral cortex.

what are the three steps to generate movement?

planning (strategy), programming (constructing subparts), execution (sending commands to muscles).

what are cpgs - central pattern generator

• neuronal circuits generating rhythmic patterns without sensory input.

walking with alternating flexor and extensor muscle contraction.

stepping pattern generators (spgs) + what modifies output

activate lower motor neurons for hip/knee flexion and extension during walking.

sensory feedback, environment, walking cycle stage, and brain signals.

proprioception

feedback from muscles, tendons, and joints about limb position and weight.

vision and vestibular system

• guides actions and corrects movements toward objects.

• provides information about head position and balance.

hierarchial model

top-down control where higher centers regulate lower reflex centers.

limitation: cannot explain reflex dominance in normal adults.

what is the dynamical systems theory?

body as a mechanical system; variability is needed for flexible movement.

• limitation: may underemphasize the nervous system’s role.

what is ecological theory?

movement emerges from interaction with environment using perception/action system.

where are umns located + pathways

primary motor cortex, premotor, supplementary motor areas, and brainstem.

• corticospinal, corticobrainstem, vestibulospinal, reticulospinal, rubrospinal, tectospinal.

corticospinal

origin: primary motor cortex, premotor, supplementary, and somatosensory areas.

decussate: at the pyramids of the medulla (~85% lateral, ~10% anterior).

terminate: on lower motor neurons (alpha) in anterior (ventral) HORN of spinal cord

FUNCTION

• voluntary control of precise movements involving distal muscles of limbs (lateral CST)

function of lateral and medial corticospinal

lateral - precise control of distal limb muscles.

medial - control of trunk and proximal muscles for posture.

corticobrainstem

what muscle it controls: face, jaw, tongue, pharynx, larynx, trapezius, and sternomastoid via cranial nerves.

• controls cranial nerve motor nuclei; most nuclei receive bilateral input except lower facial and hypoglossal nuclei.

origin: lateral aspect of primary motor cortex (homunculus area representing face and head)

movement of the neck, head, facial expressions and tounge

course:

• descend via internal capsule (medial to corticospinal tract)

• most cranial nerve nuclei receive bilateral UMN innervation except VII (7) (only lower half of face) and XII (12)

• contralateral fibres decussate at the level of brainstem where cranial cell bodies are

corticobrainstem (2)

terminate:

cranial nerve motor nuclei in brainstem which include:

• V – trigeminal: muscles of mastication

• VII – facial: muscles of the face

• IX – glossopharyngeal: stylopharyngeal muscle

• X – vagus: soft palate, larynx, oesophagus

• XI – accessory: sternomastoid and trapezius

• XII – hypoglossal: tongue

cranial nerves - motor output = lower motor neurosn which then talk to muscles

function:

• serves as UMNs to all motor cranial nerves

• facilitates voluntary control of all the aforementioned cranial nerves (LMNs)

other brainstem tracts

• reticulospinal track - reflexive head movement respond to visual or audiotory input. recticular formae: grey matter in CNS

• vestibulospinal: arises from vestibular nucleus to help control the neck and upper back muscles and aid in balance. verstibulo appartus - in the ear

• rubrospinal: arised from red nucleus in the midbrain but had minimal contrubution to upper limb extensor muscles

lmns

transmit signals directly to skeletal muscles to trigger contraction.

the two types: alpha (extrafusal fibers, force) and gamma (intrafusal fibers, spindle sensitivity).

motor unit

one alpha lmns and all the muscle fibers it innervates.

• the strength is determined by the number of size of motor units activated

• inverse relationship with the motor homunculus

• larger cortical tissue (more UMN cell bodies) = small motor unit

• smaller cortical tissue (less UMN cell bodies) = large motor unit

umn lesions signs

spasticity, hyperreflexia, paralysis/paresis, loss of fractionation, involuntary contractions.

lmn lesion signs

flaccidity, hypotonia, hyporeflexia/areflexia, atrophy, fasciculations.

babinski’s sign + adbormal reflexes meaning

stroke lateral sole heel → toes;

big toe extension = umn/cst damage,

areflexia = lmn lesion,

hyperreflexia = umn lesion due to loss of corticospinal inhibition causing excessive muscle response.

main type of reflexes

• phasic stretch reflex (quick contraction to stretch e.g. knee jerk),

• cutaneous reflex (withdrawal from painful stimulus before conscious awareness)

• gag reflex (protective airway reflex via cn ix sensory and cn x motor), swallowing reflex (automatic coordinated swallowing response).