Lower Motor Neurons

Question 1

Lower motor neurons

Answer 1

• coming from the motor cortex, neurons project through the corticospinal tract and synapse within the spinal cord and project the secondary neuron out of the spinal cord
• synapses in the grey area of the spinal cord (cell body neurons)
• lower motor neurons can have their cell bodies in different levels of the nervous system (coming out form the spinal cord, or coming out from the brain stem)

Question 2

Nervous control of movement

Answer 2

• electrical stimulation of muscles, and the nerves that supply muscles causes muscular contraction
• this demonstrates the electrical basis for control of movement by nervous system
• external electrical stimulation can be used as a therapeutic tool to manage conditions such as drop foot

Question 3

drop foot

Answer 3

condition that happens after strokes, where people will be paralysed on one side of the body

people relearn to walk, but a complication is that you cannot stimulate the muscles in the front of your leg

electrical stimulating can be used to overcome footdrop

pads are placed on the front of the anterior muscle of the leg, than the electrical stimulation passed through those causes your foot to rise up when you walk

Question 4

brain computer inferface implanted in the brain

Answer 4

• closed loop control over movement

- the person thinks about moving their arm, and the thought turns into a signal that causes their arm to contract

Question 5

nervous control of movement

Answer 5

• electrical stimulation of muscles, and the nerves that supply muscles causes muscular contraction
• demonstrates the electrical basis for control of movement by nervous system

Question 6

Neuromuscular junction

Answer 6

Action potential is able to propagate along neurons

Gates along the myelin sheath allow for the electrical signal to flow along the inside

Axon and the terminal part of the axon –> the neuromuscular junction

In the end of the cell - vesicles contain a neurotransmitter (acetylcholine)

as the action potential propagates along the axon, it reaches the neuromuscular junction, the axon terminal depolarises, and this allows for calcium from the outside to go in –> this allows the vesicles to bind to the membrane, allowing the acetylcholine to diffuse across the membrane and binds to the receptors on the other side, allowing for depolarisation of the muscle fibre on the other side

Question 7

what encourages the contraction of the muscle?

Answer 7

depolarisation of the sarcolemma encourages calcium to flow in and initiate contraction

Question 8

the process of contraction

Answer 8

calcium attaching to the troponin-tropomyosin complex

actin filament
myosin filament: myosin bridges across the opposing actin filaments, and the binding of the myosin and flexing of the myosin is what causes the actin filaments to pull together

calcium fluxing in allows for the movement of the dark blue helix, exposes the binding side for the myosin to adhere to the actin filament

the sarcolemma reduces its length, muscle contraction

Question 9

What is the neuromuscular junction? What is the neurotransmitter?

Answer 9

Junction: the synapse formed between an alpha motor neuron axon and a muscle fibre

ACh is the neuromuscular junction transmitter
Release of ACh produces a large endplate potential (which opens Ca channels)

Calcium entry triggers myosin-actin interaction, which shortens muscle fibre

Question 10

Development of muscle tension

Answer 10

• SINGLE MUSCLE TWITCHES: at relatively slow stimulation freq, the force drops right back to resting state
• TEMPORAL SUMMATION: if we start to increase the rate at which we stimulate the muscle, we see that the muscle is unable to fully relax and go back to resting state, therefore summation of force
• UNFUSED TETANUS: increase it even more, we get sustained contraction, where there is almost no drop off of force before the next simulation comes
• FUSED TETANUS: rapid build up of force and the force remains constant and smooth

Question 11

Process of muscle tension development

Answer 11

Myoplasmic Ca concentration creates this

Apply a single electrical stimulation, you get a single twitch and an influx of calcium which is rapidly removed and pumped into cytoplasmic reticulum

At higher forces, the mechanisms in the cell that reduce the calcium concentration stays high over the entire duration of stimulus

Question 12

The motor unit

Answer 12

Any alpha motor neuron combined with all of those muscle fibres it supplies

Question 13

Dealing with different force demands

Answer 13

• different sized motor units reflect a tradeoff between force and precise control
• there is a further division of motor unit types based on their relative fatigability

Question 14

Types of motor units

Answer 14

Each axon can form synapses with several muscle fibres (forming a motor unit)
The precision of the muscle is related to motor unit size.

Small: precise movements of hte hand (e.g. fingers 1:<10)
Large: movements of the leg (e.g. 1:>300)

3 types of motor unit
Fast fatigable
Fast fatigue resistent
Slow

Question 15

Types of motor units, force and fatigue

Answer 15

Rate of ofrce development for fast fatigable is very high, whereas the fast fatigue resistant and slow units produce a much smaller force in the same amount of time.

In terms of time, slow motor units can be much more sustained.

Question 16

Medial gastrocnemius muscle under different behavioural conditions

Answer 16

muscles inclined for postural tasks have higher concentrations of slow fibres, whereas muscles that produce power and explosive actions have higher concentrations of fast fatigable fibres

recruitment: the first 25% of the motor neuron recruitment for any given task are slow fibres - while they reflect 25% of motor pool, they produce only 5% of the force

Question 17

motor unit plasticity

Answer 17

motor units can change over time through training

Question 18

Turning on muscle controls

Answer 18

The impetus for contraction comes from different places:

• Can come from voluntary commands descending form upper motor neurons
• local circuits that control stereotypical movement patterns are able to sustain themselves (CPGs)
• low level activations - afferent inputs that synapse directly or indirectly with interneurons onto alpha motor neurons (example being the knee jerk reflex)

Question 19

Stretch reflex circuitry

Answer 19

• a monosynaptic reflex that provides negative feedback for changes in muscle length
• is mediated by teh muscle spindle which is a stretch receptor
• both rapid changes in length and sustained stretch are transduced by the muscle spindle
• ## 1a afferents relay stretch info back to spinal cord

Question 20

gamma motor neurons

Answer 20

• gain in the stretch reflex is controlled by tension in the extrafusal muscle fibre
• tension in the intrafusal muscle fibre is controlled by gamma motor neuron activity

Question 21

golgi tendon organs

Answer 21

• transduce tension within the musculotendinous unit
• provides negative feedback, acting to reduce tension by inhibiting alpha motor neuron activity
• important for reducing risk of damage due to high load

Muscle spindle is only sensitive to changes in length or stretch, not to changes in force

negative feedback loop

Question 22

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Answer 22

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