introduction to the control of movement Flashcards
how is the motor system organised?
the motor system is organised in a hierarchical fashion
how is the motor system segregated?
the motor system has functional segregation:
-it is divided into a number of different areas that control different aspects of movement
what do motor neurons leave via in the spinal cord?
via the ventral root
what are alpha motor neurons?
lower motor neurons that innervate skeletal muscles
-they release acetylcholine at a specialised synapse called a ‘neuromuscular junction’
what innervates the motor function of a single muscle?
-motor neurons clustered into spinal motor nuclei (all of the motor neurons in a nucleus innervate a single muscle)
-each muscle fibre is innervated by only one motor neuron, but a single motor neuron can innervate many fibres
what is a motor unit?
a motor neuron and all the fibres it innervates
how does a muscle twitch occur?
-a motor neuron fires an action potential causing the muscle fibres of the muscle unit to contract and then relax
how is a muscle tetanus formed?
-if the firing rate of action potentials from a motor neuron is high enough, individual muscle twitches can fuse together forming a tetanus (the higher the firing rate, the higher the force)
tetanus= a continuous contraction
how is the force of a muscle twitch increased?
-recruit more motor units (a motor neuron and all the fibres it innervates)
-increase the firing rate of action potentials from the motor neurons
what motor units are required to increase the force of small, fine movements?
-small, fine movements require the activation of only ‘S’ units
‘S’ units usually contain only a small number of muscle fibres
what motor units are required for more force than just small, fine movements?
‘F’ units are needed for more force
-these contain a larger number of muscle fibres
describe S units
slow contraction speed
highly fatigue-resistant
contain type I slow-twitch muscle fibres
motor neurons have small cell bodies and dendritic trees
describe FR units
-fast contraction speed
-intermediate force
-fatigue resistant
-contain type IIa (fast twitch, high fatigue resistant) muscle fibres
-motor neurons have larger cell bodies
describe FF motor units
-fast contraction speed
-low fatigueresistant
-contain type IIb / x (fast twitch, low fatigue-resistant) muscle fibres
-motor neurons have large cell bodies
what order are motor neurons activated in?
-the motor neurons with smaller cell bodies are activated before the motor neurons with the larger cell bodies
-they are activated in order of small to big cell body
why are motor neurons with a smaller cell body activated before motor neurons with a larger cell body
-Synaptic inputs are weighted so that small motor neurons are activated first
-For a given synaptic input, a small diameter neuron will be activated before a larger neuron owing to its small surface area, increased membrane resistance, and hence more rapid depolarisation (V=IR)
explain the monosynpatic pathway (reflex stretch)
- Stretch of muscle spindle
- Activation of Ia spindle afferent
- La spindle 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
what controls the sensitivity of the muscle spindle?
-Gamma motor neurons cause intrafusal fibres to contract and thus control the sensitivty of the muscle spindle1. Stretch of muscle spindle
what is the role of the golgi tendon organ?
the golgi tendon organs signals the force of muscle contraction
explain the golgi tendon reflex (or autogenic inhibition/ inverse stretch reflex)
-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 serves to keep muscle tension within an optimal range and may protect the muscle from excessive contraction
how may the gogli tendon reflex protect you when bashing your leg off something in the course of movement
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.
what do C fibres from the sensory terminals of nociceptor neurons respond to?
-multiple types of noxious stimulation i.e. are polymodal
explain the flexion- withdrawal reflex
-flexion withdrawal occurs following a painful cutaneous stimulus
-nociceptor afferents make polysynaptic excitatory connections with motor neurons innervating a single flexor muscle
-connection 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
where are Meissner’s corpuscles located, what do they respond to and what fibres are involved?
-located superficially
-respond to stroking, flutter
-RA1 fibres
where are pacinian corpuscles located, what do they respond to and what fibres are involved?
-deeper tissue
-respond to vibration
-SA1 fibres
where are Merkel disk receptors located, what do they respond to and what fibres are involved?
-superficially
-respond to pressure and texture
-SA1 fibres
what do Ruffinis endings respond to and what fibres are involved?
-skin stretch
-SA2 fibres
