Motor System Flashcards
What are the inputs to the motor cortex and the outputs from the motor cortex? How is it organised? When is it active?
- The inputs are from the PMA, SMA and primary somatosensory cortex
- The outputs of the motor cortex are to the pyramidal tracts (corticospinal and corticobulbar)
Organised topographically into the homunculus - different body parts have disproportionate respresentation reflecting precision of control.
Active 100ms before movement.
What are the outputs of the premotor cortex? What is it’s function
Outputs are to the pyramidal tract
It codes the motor plan (visually guided movement) and the body set (changing the centre of gravity and postural muscles).
Is it active 800ms before onset of movement
What are the outputs of the supplementary motor area? What is its function. When is it active?
Outputs to premotor and motor cortex
Involved in planning of movement - codes the motor plan, especially complex bilateral movements.
Active 800ms before movement.
Describe the structure of the cerebellum.
The cerebellum is a highly folded region of the brain consisting of a grey matter cortex and a white matter core. It has three peduncles (superior, middle and inferior) which carry output fibres to and from the brainstem. In the core are three pairs of deep nuclei which generate projections to the brainstem.
What are the three zones of the cerebellum and what are their function?
- Vestibulocerebellum (archicerebellum) – receives input from the vestibular system and coordinates balance and ocular reflexes
- Spinocerebellum (paleocerebellum) – compares a “map” of motor output with a “map” of sensory feedback and does error correction on your movement
- Cerebrocerebellum (neocerebellum) – involved in movement planning and motor learning, especially visually guided movements and coordination of muscle activation
What is the function of the cerebellum and what are the inputs and outputs?
The cerebellum is important in the coordination and planning of movement, particularly learnt patterns of movement. Its outputs include the brainstem and thalamus, and it receives feedback from the brainstem.
What nuclei are found in the brainstem?
The brainstem contains both nuclei that are involved in the extrapyramidal tracts, such as the red nuclei and the olivary nuclei, and the cranial nerve nuclei that are involved in controlling the movement of the face.
What are alpha motor neurones? Where are the cell bodies? What is their conduction velocity, diameter, myelination?
Aα Motor neurons have their cell body in the ventral horn of the spinal cord – lamina IX (or the brainstem (Cranial nerve motor nuclei). Upper motor neurones control the activity of lower motor neurones, which act directly on muscle. They have a large diameter, and are myelinated. Their conduction velocity is 60ms-1.
Describe how uncontrolled movements are prevented and voluntary movements occur?
The system is controlled by the brain via descending inhibitory signals shutting them down, and is always under this inhibition from the upper motor neurones. This means there is no chance of uncontrolled limb movements. The cortex gives permission for movements to occur by removing this inhibition and it is this removal that results in voluntary movement.
How is greater control over fine movements allowed for?
Reflexes can be disynaptic, trisynaptic, and quadsynaptic by the set up of additional interneurones between afferent and efferent neurones.
These reflexes provide greater control to the finer movements of the body and are also inhibited by the descending tracts.
What can damage to descending neurones lead to?
Clonus - muscular spasm involving repeated, rhythmic contractions as signals are no longer inhibited and pass round and round interneurones.
What does an alpha motor neurone innervate?
An alpha motor neuron will innervate a group of extrafusal muscle fibres (normal skeletal muscle fibre), distributed evenly throughout the muscle to ensure distribution of the force of contraction. They are myelinated with large cell bodies and have a-α conduction velocity.
The alpha motor neuron and the group of fibres it innervates is called a motor unit.
How do number of motor units vary with muscles?
The number of motor units, and fibres per motor unit, depends on the function of the muscle. Muscles that require very precise control – such as the muscles that control eye movement – have many small motor units. Muscles that provide force but don’t need precise control such as gastrocnemius have fewer, much larger motor units.
What do gamma motor neurones innervate? What is their function? Where do they receive input? Structure?
Gamma motor neurones innervate the muscle spindles (intrafusal fibres), this keeps them taught during contraction so they can continue to detect stretch. They receive input from the reticular formation.
They have small diameter cell bodies and have a-γ conduction velocity. Activity in the gamma motor neuron causes contraction of the poles of the intrafusal fibre, which stretches the central zone and activates the peripheral process of the afferent neurone.
Describe a muscle spindle and their function
Muscle spindles are present in skeletal muscles, being more numerous in muscles that control fine movements. Each spindle consists of a connective tissue capsule in which there are 8-10 intrafusal fibres located in a connective tissue bag, known as fusical. Efferent innervation is provided to the polar ends of the intrafusal fibres, innervated by gamma motor neurons; consequently, muscle spindles detect changes in length of the muscle.
