Motor Neurons Flashcards
(32 cards)
What are the components of a motor unit?
Motor neuron and muscle fibre.
Motor neurons innervating one muscle are usually clustered into what?
An elongated motor nucleus.
The axons from a motor nucleus exit the spinal cord where?
In several ventral roots and peripheral nerves.
Which neurotransmitter is released at the NMJ?
Acetylcholine.
The action potentials in all the muscle fibres of a motor unit occur at approximately the same time. What are the implications of this?
They contribute to extracellular currents that sum to generate a field potential near the active muscle fibres.
What are compound action potentials?
The activation of many motor units during muscle contraction produce currents that sum to produce signals that can be detected by EMG.
The timing and amplitude of EMG activity reflects what?
The activation of muscle fibres by motor neurons.
What is EMG used to study?
The neural control of movement.
What is the innervation number?
The number of muscle fibres innervated by one motor neuron.
What does differences in innervation number determine?
The differences in increments in force produced by activation of different motor units in the same muscle.
What is the relationship between innervation number and motor control?
The smaller the innervation number, the finer the control achieved.
The force exerted by a muscle depends on which four factors?
The number of motor units activated.
Contraction speed of motor units.
Maximal force of motor units.
Fatigability of motor units.
What is a twitch contraction?
The mechanical response to a single action potential.
What is contraction time?
The time it takes a twitch contraction to reach its peak force.
What is a tetanic contraction?
A series of action potentials produce overlapping twitches.
How is contraction force increased?
By recruiting the smallest motor neuron first and the largest motor neuron last.
Why are motor units activated in order of increasing strength?
Axons arising from small motor neurons are thinner than those associated with large motor neurons and innervate fewer muscle fibres.
Why are the earliest recruited motor units the weakest ones?
Because the number of muscle fibres innervated by a motor neuron is a key determinant of motor unit force.
What influences the discharge rate of motor neurons?
Magnitude of depolarisation.
Intrinsic membrane properties of the motor neurons.
What can modify the intrinsic membrane properties of the motor neurons in the spinal cord?
Input from monoaminergic neurons in the brainstem.
In the absence of monoaminergic neuronal input, what are the input-output properties of motor neurons?
The dendrites of motor neurons passively transmit synaptic current to the cell body, resulting in a modest depolarisation that immediately ceases when the input stops.
What is the relationship between input current and discharge rate?
Linear over a wide range.
When does the input-output relationship become nonlinear?
When serotonin and noradrenaline induce a huge increase in conductance by activating L-type calcium channels on the dendrites of the motor neurons.
What happens when serotonin and noradrenaline activate L-type calcium channels?
Inward calcium currents that can enhance synaptic currents by three- to five-fold.
