The Somatomotor System Flashcards
What does the somatic motor system consist of?
All of the skeletal muscles and the elements of the nervous system which control them
The neural element of the somatic motor system comprises what two aspects?
Upper and lower motor neurones
Where are the upper motor neurones found?
Within the brain, cerebellum and descending tracts
Where are the lower motor neurones found?
In the anterior horn of the spinal cord
Complete the sentence: lower motor neurones are located in the ? horn of the spinal cord, with axons which project to the musculature in the ?
Anterior / peripheral nerve
Upper motor neurones supply input to where? To do what?
To lower motor neurones, to regulate their activity
As well as from upper motor neurones, where else do lower motor neurones receive input?
Proprioceptors and interneurons
What are the two main types of lower motor neurone which are relevant in the somatic nervous system?
Alpha and gamma motor neurones
‘Transmit information rapidly to muscle fibres to activate muscle contraction via the neuromuscular junction’ is the role of which type of motor neurone?
Alpha motor neurones
‘Adjust the sensitivity of muscle spindles and stretch receptors to regulate muscle tone and power’ is the role of which type of motor neurone?
Gamma motor neurones
Where is the primary motor cortex located?
The precentral gyrus of the frontal lobe
The biceps brachii and brachialis muscles work together as what?
Synergists
The biceps brachii and brachialis muscles oppose the triceps brachii and aconeus muscles. How is this described?
They are antagonists to each other
Axons of the LMNs exit the spinal cord where?
In the central roots, or via cranial nerves
Motor neurones are not distributed equally within the spinal cord. Why?
There is a greater number in the cervical enlargement to supply the upper limb, and a greater number in the lumbar enlargement to supply the lower limb
What is a motor unit?
An alpha motor neurone, and all of the skeletal muscles it innervates
What is the smallest functional component of the motor system?
The motor unit
The cell body of an alpha motor neurone will be found where?
In the anterior horn of the spinal cord
What is a motor neurone pool?
A collection of alpha motor neurones that innervate a single muscle
Communication between motor neurones and muscle fibres occurs where?
The neuromuscular junction
A single action potential in an alpha motor neurone will cause a muscle fibre to do what?
Twitch
If there is an extremely high firing rate of action potentials in an alpha motor neurone, meaning that there is no division between the action potentials, what does this cause the muscle to do?
Form a fused tetanus
Why are small motor units with small cell bodies recruited before large ones?
Because they are more easily excited
Skeletal muscle fibres can be fast or slow. How do these differ?
In how fast myosin ATPTase splits ATP to provide energy for cross bridge formation
What is muscle tone?
The tension in a muscle
What is muscle power?
The strength in a muscle
Both muscle tone and muscle power are controlled by a combination of specialised sensory and motor components. What are some of these?
Golgi tendon organs, muscle spindles and both alpha and gamma neurones
What does muscle power depend on?
The number of motor units which are recruited at one time (the more motor units, the more power)
Complete the sentence: muscle tension is detected by specialised sensory receptors, the ? located at the ?
Golgi tendon organs / muscle-tendon border
Complete the sentence: afferents from the Golgi tendon organ enter the ? and inhibit ?, thereby ? contraction and protecting the muscle from damage
Spinal cord / alpha motor neurones / reducing
A deep tendon (myotatic) reflex is activated by what? This type of reflex has how many synapses?
Stretching the tendon / one synapse (monosynaptic reflex arc)
During a deep tendon reflex, sensory input activates motor neurones directly to produce what?
Muscle contraction
During a deep tendon reflex, the change in length (and the rate of change) of the muscle is registered by a sensory organ within the muscle known as what? This structure contributes to what?
The muscle spindle / non-conscious proprioception
In a monosynaptic reflex arc, what triggers the whole thing starting?
The stretch of a muscle spindle
In a monosynaptic reflex arc, after the muscle spindle has been stretched, what is activated?
The Ia primary afferent
In a monosynaptic reflex arc, there is what type of synaptic transmission in the spinal cord? This is mediated by what?
Excitatory / glutamate
In a monosynaptic reflex arc, once the excitatory synapse has gone ahead, there is activation of what? Which causes what?
The alpha motor neurone / muscle contraction and activation of gamma motor neurones
In a monosynaptic reflex arc, once the gamma motor neurone has been activates, what does this do?
Changes the length of the spindle, allowing it to respond rapidly to additional changes if needed
The activation of muscle spindles causes the muscles to do what? What is the function of this?
Contract / protect from overstretching and tearing
The intrafusal fibres of muscle spindles are innervated by what?
Gamma motor neurones
Stimulation of gamma motor neurones causes what?
The muscle spindle to contract
Complete the sentence: during voluntary movement, alpha and gamma motor neurones are usually ? so that the intrafusal fibres contract in ? with the extrafusal fibres
Co-activated / parallel
What are Golgi tendon organs innervated by?
Group Ib sensory afferents
Where does proprioceptive information arise from?
Muscle spindles, Golgi tendon organs and joint receptors
What activates the reverse myotatic reflex?
Increased muscle tension
What happens to the flexor and extensor muscles in the flexor withdrawal reflex?
The flexor muscle is activated while the extensor muscle is inhibited
What is the function of the flexor reflexes?
To withdraw a limb from a dangerous stimulus
Complete the sentence: in the flexor withdrawal reflex, noxious stimuli cause the limb to ? by contraction of flexor muscles via ? interneurones and relaxation of extensor muscles via ? interneurones
Flex / excitatory / excitatory and inhibitory
Complete the sentence: in the crossed extensor reflex, noxious stimuli will cause the limb to ? by contraction of extensor muscles via ? interneurones and relaxation of flexor muscles via ? interneurones
Extend / excitatory / excitatory and inhibitory
The highest level of motor control is strategy - achieved by thinking, what is the aim of the movement and how can it best be achieved? What parts of the brain are responsible for this?
Neocortical association areas in the brain / basal ganglia
The middle level of motor control is tactics - achieved by thinking, what sequence of muscle contractions and relaxations in time and space will fulfil the strategic aim? What parts of the brain are responsible for this?
The motor cortex and cerebellum
The lowest level of motor control is execution - achieved by commanding the desired movement. What parts of the brain are responsible for this?
Brainstem and spinal cord
Where do descending spinal cord tracts originate from?
The cerebral cortex and brainstem
What are the two main types of descending spinal cord tract?
Lateral and ventromedial pathways
What is the major lateral descending pathway?
Corticospinal tract
What is the minor lateral descending pathway?
Rubrospinal tract
Where do axons in the corticospinal tract cross over?
Most in the medulla to form the lateral tract, some in the spinal cord to form the ventral tract
Lesions of the lateral descending pathways are associated with what problem?
Loss of ‘fractioned’ movements and slowing/impairment of accuracy of voluntary movement
If there is damage to the corticospinal tract, why can this get better over time?
The rubrospinal tract is capable of compensating
If there is damage to the corticospinal tract and the rubrospinal tract has compensated, what deficits may still be present?
Weakness of distal flexors and inability to move fingers
