The control of movement Flashcards
motor neurone and motor unit?
Alpha motor neurons (or lower motor neurons) innervate skeletal muscles. They release acetylcholine at a specialised synapse called a ‘neuromuscular junction’.
Motor neurons are clustered into spinal motor nuclei; all of the motor neurons in a nucleus innervate a single muscle.
Each individual muscle fibre is innervated by only one motor neuron, but a single motor neuron can innervate many fibres. A motor neuron and all of the fibres it innervates is known as a ‘motor unit’.
Muscle Twitch and Tetanus?
When a motor neuron fires an action potential, the muscle fibres of the motor unit contract, then relax—a muscle twitch.
If the firing rate is high enough, individual muscle twitches fuse together forming a tetanus—a continuous contraction. The higher the firing rate, the higher the force.
Another way to increase the force of contraction is to —- more ——.
recruit
motor units
what are the different types of motor units?
S (slow) low force: slow contraction speed, highly fatigue-resistant, contain type I slow-twitch muscle fibres, motor neurons have small cell bodies and dendritic trees.
FR (fast fatigue-resistant): fast contraction speed; intermediate force; fatigue resistant; contain type IIa (fast twitch, high fatigue resistant) muscle fibres; motor neurons have larger cell bodies.
FF (fast fatiguable): fast contraction speed, low fatigue-resistant, contain type IIb / x (fast twitch, low fatigue-resistant) muscle fibres; motor neurons have large cell bodies.
The ‘Size Principle’?
The Muscle Spindle?
They are inrtrafusal fibres that are proprioceptors. they tell us abt the positions of the miiuscles and joints.
they pick up the length/stretch and velocity; the degree being stretch and the speed of being stretched.
Muscle spindles contain specialised muscle fibres, and are the principal receptors mediating proprioception.
Muscle stretching (i.e. increasing length) causes stretch-sensitive cation channels to open and depolarize sensory fibres.
The Stretch Reflex?
Muscle stretch causes the activation of 1a spindle afferents.
These afferents make monosynaptic connections with alpha motor neurons innervating the same muscle, causing contraction.
At the same time, inhibitory interneurons inhibit the activation of antagonist muscles
Gamma motor neurons cause the intrafusal fibres to contract and thus control the sensitivity of the muscle spindle.
The Golgi Tendon Organ or autogenic inhibition / inverse stretch reflex?
Located at the transition between muscle and tendon.
Golgi tendon organs signal the force of muscle contraction.
Golgi tendon organs send Ib afferents that contact Ib inhibitory interneurons.
An increase in muscle force thus causes a reflex inhibition of the same muscle.
This may protect the muscle from excessive contraction, but probably also serves to keep muscle tension within an optimal range.
Ib interneurons also receive inputs from cutaneous and joint receptors. For example, cutaneous afferents can trigger a reduction in muscle contraction when a limb contacts an object in the course of a movement.
About Cutaneous nociceptors:
The skin contains the sensory terminals of nociceptor neurons that respond to —- stimuli.
These terminals exist as free nerve endings, without a specialised sensory structure.
Thinly myelinated —– fibres typically respond either to damaging extremes of —— or strong mechanical forces.
Unmyelinated —– often respond to multiple types of noxious stimulation, i.e. are polymodal.
These nociceptor afferents can trigger a withdrawal reflex in response to a painful cutaneous stimulus.
damaging
A-delta
temperature
C-fibres
Flexor and Crossed
Extensor Reflexes?
Flexion-withdrawal occurs following a painful cutaneous stimulus.
Nociceptor afferents make polysynaptic excitatory connections with motor neurons innervating a flexor muscle.
Connections with inhibitory interneurons cause inhibition of the antagonist extensor muscle.
Contralateral pathways cause an opposite pattern of extensor activation and flexor inhibition (the crossed extensor reflex) that provides postural support.
—— detect ‘microslips’ during precision lifting.
This causes a reflex increase in grip force ensuring the object does not fall
Meissner’s corpuscles
Deep tendon reflexes?
Descending control
Ia inhibitory interneurons receive excitatory and inhibitory inputs from corticospinal and other descending pathways.
These inputs allow reflexes to be modulated by higher centres.
