Motor Neurons Flashcards
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 that may extend over one to four segments within the ventral spinal cord.
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 and for diagnosing pathology.
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 by varying the number of activated motor units.
The force exerted by a muscle depends on which four factors?
The number of motor units activated during a contraction.
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?
The mechanical response to a series of action potentials that produce overlapping twitches.
How is the sequence of motor neuron recruitment determined?
By the properties of the spinal neurons and not by supraspinal regions of the nervous system, so the brain cannot selectively activate specific motor units.
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?
The magnitude of the depolarisation generated by excitatory inputs and the intrinsic membrane properties of the motor neurons in the spinal cord.
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 norepinephrine induce a huge increase in conductance by activating L-type calcium channels that are located on the dendrites of the motor neurons.
What happens when serotonin and norepinephrine activate L-type calcium channels?
It results in inward calcium currents that can enhance synaptic currents by three- to five-fold.
What is self-sustained firing?
In an active motor neuron, augmented current can sustain an elevated discharge rate after a brief depolarising input has ended.
How can self-sustained firing be terminated?
By a brief inhibitory input, e.g. from a spinal reflex pathway.
Where are corticomotoneuronal (CM) cells located?
Only in the most caudal part of M1 that lies within the anterior bank of the central sulcus.
When no load is applied during wrist movements, when does the neuron fire?
Before and during flexion.
When a load opposing wrist flexion is applied, how does this impact flexor muscle and neuron activity?
The activity of the flexor muscles and the neuron increases.
When a load assisting wrist flexion is applied, how does this impact flexor muscle and neuron activity?
The flexor muscles and neuron fall silent.
Why does neural activity change when the wrist displacement is the same but the load conditions are different?
The loads and compensatory muscle activity change, which impacts neural activity.
Activity in many M1 neurons correlates with what?
The level of force and direction of force exerted in an isometric action.
