Biomedical Science - motor tracts

motor neurons are needed for

voluntary and involuntary muscle contraction

hierarchy of motor pathways

-Primary motor cortex (in the brain)

-Upper motor neurons - axons from the primary motor cortex descending directly to the brainstem or spinal cord

-Lower motor neurons - receive an impulse from an upper motor neurone (at a synapse) and then exit the CNS to cause contraction of a muscle in the periphery

-Lower motor neurones that exit at the brainstem are the cranial nerves and they innervate the muscles of the head and neck

-Lower motor neurons that exit from the spinal cord are the spinal nerves that will innervate the muscles of the lower body

-Lower motor neurons that synapse in the spinal cord exit the spinal cord from the ventral (anterior) horn of the grey matter

contraction and motor neurons

motor neurone must release ACh from its synaptic vesicles into synaptic cleft to initiate voluntary muscle contraction

3 areas of the frontal lobe involved in motor movement

1. The primary motor cortex (M1)occupying the precentral gyrus

2. The premotor cortex (PM) - (lateral, anterior to M1): receives sensory input from parietal lobe and basal ganglia; helps plan and prepare movement

3. The supplemental motor area (SMA) -(medial, anterior to M1) (coordinates complex fine movements and posture)

primary motor cortex

The primary motor cortex is located in the precentral gyrus in each hemisphere

It is arranged somatopically - the body is represented in the correct sequence but upside down

medullary pyramids

-Corticospinal axons form a ridge on either side of the anterior medulla (the pyramids) as they descend from the motor cortex - these are known as the medullary pyramids

-The right ridge mainly contains the corticospinal axons from the right motor cortex The left ridge mainly contains the corticospinal axons from the left motor cortex

-Most of the axons decussate (cross to the opposite side) low down at the pyramidal decussation just before the medulla exits the skull to become the spinal cord

corticobulbar tract contains

motor neurons to move the eyes, facial muscles, tongue, larynx & pharynx

corticospinal tract

contains motor neurons for tine movement of the lower limbs (arms and legs)

corticobulbar and corticospinal tract BOTH

-receive a range of inputs from: Primary motor cortex, Premotor cortex, Supplementary motor area

-share a common pathway through the cerebral hemispheres: passing through the internal capsule in a somatotopic organisation

then descending to the anterior part of the brainstem at the midbrain

corticobulbar tract pathway

-Originates in the primary motor cortex

-Follows a pathway down toward the medullary pyramids (in the anterior medulla)

-However, several branches come off at different levels to terminate on the motor nuclei in the midbrain, pons and medulla

-Here, they synapse with lower motor neurons, which are the different cranial motor nerves

-These carry the motor signals to the muscles of the face and neck

-They control voluntary control of facial expression, mastication (chewing), swallowing, and vocalisation and speech

lateral corticospinal tract

- starts in the cortex (rather than the brainstem)

-Decussates in the medulla

-It is the larger one of the two corticospinal tracts

- skilled and fine motor control in the upper and lower distal areas of limbs such as the fingers and toes - e.g writing, playing musical instruments

ventral (anterior) corticospinal tract

This is a tract that starts in the cortex (rather than the brainstem)

It is a branch of the main corticospinal tract

It carries on ipsilaterally through the medulla and only decussates at the level of the spinal cord, then synapses with a lower motor neuron

Its function is to maintain posture, balance, and purposeful (voluntary) movements especially of the trunk, neck and shoulder regions (upper limbs, closer to the body)

what is the internal capsule?

a white matter structure in the brain that serves as a major pathway for communication between different regions of the cerebral cortex and the subcortical structures, including the thalamus, basal ganglia, and brainstem

The x and y make up parts of the descending tracts

corticospinal and corticobulbar tracts

where does the internal capsule receive its blood supply

through a series of small perforating arteries entering from the base of the brain

vestibulospinal tract

Starts at the vestibular nuclei which span the pons and medulla

The vestibular nuclei receive information about balance from the semi-circular canals in the inner ear and also information back from the vestibulocerebellum to modulate and refine motor output

vestibulospinal tract controls balance and posture by

Innervating the 'anti-gravity' muscles in the trunk and spine to maintain upright posture

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Responding to changes in head position and movement - essential for reflexive adjustments to maintain balance and prevent falls

tectospinal tracts

Originate at the superior colliculus in the midbrain

The neurons then decussate, and descend to enter the spinal cord

They terminate at the cervical levels of the spinal cord

They are thought to control reflex postural movements in the head and neck mainly in response to sounds and visual stimuli