Flashcards in Motor control Deck (94):
The activation of muscle fibres is ___ or ____.
The activation of muscle fibres is all or none.
A skeletal muscle is attached to the bone by a ______.
A skeletal muscle is attached to the bone by a tendon.
A skeletal muscle comprises of several what?
Each muscle fasiculus comprises of several what?
Each muscle fibre comprises of several what?
Name the protein filaments contained within myofibrils.
Actin and myosin myofilaments.
What is a motor unit?
An alpha motor neurone and all the extrafusal skeletal muscle fibres it innervates.
What is an alpha motor neurone?
A lower motor neurone.
The muscle fibres in a single motor unit are located in the...
same muscle and are distributed throughout the muscle.
What determines the variation of movement of a muscle?
The number of muscle fibres a single motor neurone innervates.
How does the number of muscle fibres a single motor neurone innervates affect variation of movement?
The less fibres innervated by the motor neurone, the greater the variation of movement.
Give examples of areas with a great variation of movement.
Finger tips and tongue.
What is the effect of the activation of an alpha motor neurone?
Depolarises and causes the contraction of all the fibres in that unit.
What determines the power of a motor unit?
The number of muscle fibres contracting, which is determined by the frequency a motor neurone fires.
Where in the spinal cord are alpha motor neurones controlling the distal muscles located?
Laterally in the spinal cord.
Where in the spinal cord are alpha motor neurones controlling the proximal muscles located?
Medially in the spinal cord.
What is a neuromuscular junction?
The junction between a motor neurone and a muscle fibre.
What does the axon terminal of a motor neurone contain?
Vesicles containing neurotransmitter.
Which neurotransmitter is used at a neuromuscular junction?
The postsynaptic membrane of the neuromuscular junction is called what?
Motor end plate.
How does the arrival of an action potential in a motor neurone effect the plasma membrane?
The action potential depolarises the plasma membrane.
Describe the effect of depolarisation of the plasma membrane at a neuromuscular junction.
Voltage-gated Ca2+ channels open, allowing Ca2+ to diffuse into the axon terminal from the extracellular fluid.
How does the action of Ca2+ ions lead to the release of ACh from the axon terminal.
Ca2+ ions bind to proteins that enable to membranes of the vesicles containing ACh to fuse with the neuronal plasma membrane, and release ACh.
By which process is ACh released into the extracellular cleft at a neuromuscular junction.
ACh diffuses from the axon terminal to the motor end plate, where it binds to which type of receptor?
Cholinergic nicotinic receptor.
What effect does the binding of ACh to cholinergic nicotinic receptors have?
The ion channel within the receptor protein is opened.
Describe how opening the ion channels on the motor end plate produces a local depolarisation.
Na+ and K+ pass down their electrochemical gradients. More Na+ moves in than K+ out - becomes relatively more positive.
What is the name of the local depolarisation at the motor end-plate called?
The end-plate potential (EPP).
Other than acetylcholine, the neuromuscular junction contains what?
What is the magnitude of an EPP relative to that of an EPSP?
Why is the magnitude of an EPP much larger than that of an EPSP?
The neurotransmitter is released over a much larger surface area - neurotransmitter binds to more receptors opening more ion channels.
What is the significance of a large magnitude EPP at a neuromuscular junction?
One EPP is normally sufficient to depolarise the muscle plasma membrane, adjacent to the end-plate membrane, to its threshold potential - thereby initiating an action potential.
Stretch receptors monitor...
muscle length and the rate of change of muscle length.
Describe the structure of a muscle spindle.
Stretch receptors consist of peripheral nerve endings of afferent nerve fibres, which are wrapped are intrafusal muscle fibres and enclosed in a capsule.
Name the neurones that innervate intrafusal muscle fibres at both ends.
Gamma motor neurones.
What is the function of gamma motor neurones in the muscle spindle?
Keep the intrafusal fibres set at a length that optimises muscle stretch detection.
Describe the property of the two ends of the muscle spindle compared to the central portion.
The ends of the muscle spindle are contractile while the central portion is non-contractile.
Name the two types of stretch receptor in a muscle spindle.
Nuclear chain fibres and nuclear bag fibres.
What do nuclear chain fibres respond best to?
How much a muscle is being stretched.
What do nuclear bag fibres respond best to?
The magnitude and speed of a stretch.
How are muscle spindles attached to the extrafusal fibres that make up the bulk of muscle contraction?
By connective tissue.
Describe how action potentials along a motor neurone effect the amount of sensory information received from that muscle.
Action potentials cause contraction of extrafusal fibres, shortening the muscle and removing tension on the spindle, slowing the rate of firing in the stretch receptor and reducing the amount of sensory information received.
Name the process that prevents the loss of sensory information during muscle contraction.
What is the function of the contractile ends of the intrafusal fibres?
To maintain tension and stretch in the central receptor region of the intrafusal fibre.
How does alpha-gamma co-activation prevent the loss of sensory information?
It prevents the central region from going slack during the shortening of muscle contraction.
What is the effect of activating gamma motor neurones alone?
Increase in sensitivity of the muscle to stretch.
Tension depends on which three factors?
Muscle length, load on the muscles and degree of muscle fatigue.
Name the receptor type that specifically monitors how much tension contracting motor units are exerting.
The Golgi tendon organ.
How do Golgi tendon organs monitor tension?
By measuring the force developed by the muscles and any resultant change in length.
Other than the Golgi tendon organs, how else is tension perceived?
Vision, as well as afferent input from the skin, muscle and joint receptors.
Describe the structure of a Golgi tendon organ.
Endings of afferent fibres that wrap around collagen bundles in the tendons near their junction with the muscles.
Describe the collagen fibres in the Golgi tendon organ in their resting state.
The collagen fibres are slightly bowed in their resting state.
Afferent fibres run from the Golgi tendon organ to...
the anterior horn of the spinal cord.
What type are the afferent fibres that run between the Golgi tendon organ and the anterior horn of the spinal cord?
How does tension activate the Golgi tendon organ receptor endings?
The tension straightens the collagen bundles and distorts the Golgi tendon receptor endings.
What is the inverse stretch (myotatic) reflex?
Branches of 1b afferent neurones cause the inhibition of alpha motor neurones of the contracting muscle and its synergists via interneurons.
What is the function of the inverse stretch (myotatic) reflex?
To regulate muscle tension at a normal range and protect it from overload.
Other than the inverse stretch function, what else do Golgi tendon organs do?
Stimulate the motor neurones of antagonistic muscles.
The stretch/myotatic reflex is crucially important for what?
The control of skeletal muscle tone.
What happens to the afferent fibres from the muscle spindle once they enter the CNS?
The afferent fibres divide into branches that take different paths.
Name an example of the stretch reflex.
Knee jerk reflex.
Describe how stretch receptors are activated in the knee jerk reflex.
The patellar tendon is tapped and as it is pushed in, the thigh muscles it is attached to are stretched and the stretch receptors within these muscles are activated.
What is the effect of activation of stretch receptors in the knee jerk reflex?
Action potentials in afferent nerve fibres form stretch receptors activate excitatory synapses on motor neurones that control the same muscles. Stimulation of motor units of the thigh muscles, they contract, extension of lower leg.
What does the presence of a knee jerk indicate?
All of mechanism functioning normally.
What is a monosynaptic reflex?
The afferent fibres synapse directly with the motor neurones without any interneurones interneurones.
What is the name of reflex arcs that have at least one interneurone?
How are antagonistic muscle affected in the stretch reflex?
Branches of afferent neurones from stretch receptors end on inhibitory interneurones, which when activated inhibit the motor neurones controlling antagonistic muscles.
Why are antagonistic muscles inhibited in the stretch reflex?
So as to not interfere with the reflex response.
What term describes the activation of neurones to one muscle and simultaneous inhibition of neurones to its antagonist muscle?
What name is given to the muscles, other than the muscles of the thigh, whose motor neurones are also activated in the knee jerk?
What is the function of synergistic muscles?
Contract to assist the intended motion.
Information from the stretch receptors in the knee jerk are transmitted to.... as well as for the activation/inhibition of motor neurones.
Information is transmitted to higher centres.
What information in the knee jerk is transmitted to higher centres, and why?
Information about changes in muscle length is important in slow, controlled movements, such as the performance of unfamiliar actions. The information also contributes to proprioception.
Give an important function of stretch reflexes.
Control of body posture.
The withdrawal reflex is important for...
moving away from a harmful stimulus.
What is the result of painful stimulation as part of the withdrawal reflex?
If stepping on a pin, flexor muscles are activated and extensor muscle are inhibited on the ipsilateral side. Opposite response on the contralateral side to support the body's weight.
What is a muscle tone?
Muscle tone is the degree of contraction of a muscle or the portion of motor units that are active at any one time.
What are the properties of a muscle with high tone?
The muscle feels firm, or rigid and resists passive stretch.
What are the properties of a muscle with low tone?
The muscles feels soft, or flaccid and offers little resistance to passive stretch.
How does being very relaxed determine the effect of alpha neurone activity?
Alpha neurone activity does not make a significant contribution to the resistance of stretch.
What is hypertonia?
Abnormally high muscle tone.
What are the signs of hypertonia?
Increased resistance which keeps the muscle contracted despite attempts to relax it.
Hypertonia usually occurs with disorders of what?
The descending pathways.
Why do disorders of the descending pathways lead to hypertonia?
The descending pathways usually inhibit motor neurones.
What are upper motor neurones?
The descending pathways and neurones of the motor cortex.
Hypertonia is an example of what group of disorders?
Upper motor neurone disorders.
What is spasticity?
A form of hypertonia. The muscles do not develop increased tone until they are stretched a little. After brief increase in tone, contraction subsides for short time.
Name the phenomenon that occurs after a period of resistance that leads to 'give;
What causes the clasp-knife reflex?
1b afferent fibres from the Golgi tendon organs inhibit alpha motor neurones once the Golgi tendon organs detect tension.
The clasp knife reflex is an example of which other reflex?
Inverse stretch (myotatic) reflex.
The clasp knife reflex is a characteristic of what type of lesion?
Upper motor neurone lesion.
What is rigidity?
A form of hypertonia. Increased muscle contraction is continual and the resistance to passive stretch is constant.
What is hypotonia?
A condition of abnormally low muscle tone, accompanied by weakness, atrophy and decreased reflex responses.