Neuromuscular and spinal cord Flashcards

1
Q

How may the membrane potential of the postsynaptic neurone be altered by inputs?

A

It can be made less negative, i.e. be brought closer to threshold for firing- this is an excitatory postsynaptic potential (EPSP).

It can be made more negative, i.e. be brought further away from threshold for firing- this is an inhibitory postsynaptic potential (IPSP).

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2
Q

What is an excitatory postsynaptic potential (EPSP)?

A

Membrane potential of the postsynaptic neurone can be made less negative, i.e. be brought closer to threshold for firing.

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3
Q

What is an inhibitory postsynaptic potential (IPSP)?

A

Membrane potential of the postsynaptic neurone can be made more negative, i.e. be brought further away from threshold for firing.

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4
Q

What does the degree of summation of EPSPs and IPSPs determine?

A

How readily a neuron can reach threshold to produce an action potential.

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5
Q

What is a neuromuscular junction?

A

A specialised synapse between the lower motor neuron and the motor end plate (highly folded muscle fibre membrane).

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6
Q

How is a neuromuscular junction activated?

A

When an action potential arrives at the NMJ, Ca2+ influx causes ACh release from vesicles.
ACh binds to receptors on the motor end plate.
Ion channel opens- Na+ influx causes action potential in muscle fibre.

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7
Q

What is a miniature end plate potential (mEPP)?

A

At rest, individual vesicles release ACh at a very low rate.

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8
Q

What is an alpha motor neurone?

A

Lower motor neurones of the brainstem and spinal cord.
Innervate the extrafusal muscle fibres of the skeletal muscles.
Their activation causes muscle contraction.
Motor neurone pool contains all alpha motor neurones innervating a single muscle.

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9
Q

Where are alpha motor neurones found?

A

Ventral horn of spinal cord.

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10
Q

What is a motor unit?

A

A single motor neurone and all of the muscle fibres it innervates.
The smallest functional unit that causes contraction/ produces force.
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit.

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11
Q

Approximately how many motor neurones do humans have?

A

420,000

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12
Q

Approximately how many skeletal muscle fibres do humans have?

A

250 million

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13
Q

On average how many muscle fibres does each motor neurone supply?

A

About 600

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14
Q

What are the different types of motor unit?

A

Slow (S, type I).
Fast, fatigue resistant (FR, type IIA).
Fast, fatiguable (FF, type IIB).

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15
Q

What are the features of a slow (S, type I) motor unit?

A
Smallest diameter cell bodies. 
Small dendritic trees. 
Thinnest axons. 
Slowest conduction velocity. 
Slow twitch. 
Low tension. 
Fatigue resistant.
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16
Q

What are the features of a fast, fatigue resistant (FR, type IIA) motor unit?

A
Larger diameter cell bodies. 
Larger dendritic trees. 
Thicker axons. 
Faster conduction velocity. 
Fast twitch. 
Moderate tension. 
Fatigue resistant.
17
Q

What are the features of a fast, fatiguable (FF, type IIB) motor unit?

A
Larger diameter cell bodies. 
Larger dendritic trees. 
Thicker axons. 
Faster conduction velocity. 
Fast twitch. 
High tension. 
High fatigue.
18
Q

How are the 3 different motor unit types classified?

A

Amount of tension generated.
Speed of contraction.
Fatiguability of the motor unit.

19
Q

What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?

A

Recruitment.

Rate coding.

20
Q

What is recruitment?

A

Motor units are not randomly recruited- there is an order.
Governed by the ‘size principle’.
Smaller units are recruited first (generally the slow twitch units).
As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.

21
Q

What is rate coding?

A

A motor unit can fire at a range of frequencies.
Slow units fire at a lower frequency.
As the firing rate increases, the force produced by the unit increases.
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.

22
Q

What are neurotrophic factors?

A

Growth factors.
Prevent neuronal death.
Promote growth of neurons after injury.

23
Q

What are the effects of neurotrophic factors?

A

Motor unit and fibre characteristics are dependent on the nerve which innervates them.
If a fast twitch muscle and a slow muscle are cross innervated, slow becomes fast and fast becomes slow.
The motor neuron has some effect on the properties of the muscle fibres it innervates.

24
Q

Discuss the plasticity of motor units/ muscle fibres.

A

Fibre types can change properties under many different conditions.
Type IIB to IIA most common following training.
Type I to II possible in cases of severe reconditioning or spinal cord injury. Microgravity during spaceflight results in shift from slow to fast muscle fibre types.
Ageing associated with loss of type I and II fibres but also preferential loss of type II. This results in a larger proportion of type I fibres in aged muscles (slower contraction times).

25
Q

What are the pyramidal tracts?

A

Lateral corticospinal tract.

Anterior corticospinal tract.

26
Q

What are the extrapyramidal tracts?

A

Rubrospinal tract.
Reticulospinal tract.
Vestibulospinal tract.
Olivospinal tract.

27
Q

What is a reflex?

A

An automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector without reaching the level of consciousness.
An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.

28
Q

What are the components of a reflex arc?

A

Sensory receptor- responds to a stimulus by producing a generator or receptor potential.
Sensory neuron- axon conducts impulses from receptor to integrating centre.
Integrating centre- one or more regions within the CNS that relay impulses from sensory to motor neurons.
Motor neuron- axon conducts impulses from integrating centre to effector.
Effector- muscle or gland that responds to motor nerve impulses.

29
Q

What is the Hoffman (H-) reflex?

A

Electrically stimulate the nerve instead of stretching muscle mechanically to produce reflex response.
More reliable than using a tendon hammer, exactly the same stimulus every time and in the same place, etc.
Used clinically to assess excitability of reflex pathways, and which nerves may be affected by a disorder.

30
Q

What is the Jendrassik maneouvre?

A

Traditionally we think of reflexes as being automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors. Can they be influenced?
The Jendrassik manoeuvre- clench teeth or make a fist when having patellar tendon tapped, reflex is more pronounced due to voluntary command to other muscles.

31
Q

Describe the supraspinal control of reflexes.

A

Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex.
Inhibitory control dominates in normal conditions.
Decerebration reveals the excitatory control from supraspinal areas.
Rigidity and spasticity can result from brain damage giving over-active or tonic stretch reflex.

32
Q

How do higher centres influence reflexes?

A
Activating alpha motor neurons. 
Activating inhibitory interneurons. 
Activating propriospinal neurons. 
Activating gamma motor neurons. 
Activating terminals of afferent fibres.
33
Q

What are the higher centres and pathways involved in the supraspinal control of reflexes?

A

Cortex- corticospinal (fine control of limb movements, body adjustments).
Red nucleus- rubrospinal (automatic movements of arm in response to posture/balance changes).
Vestibular nuclei- vestibulospinal (altering posture to maintain balance).
Tectum- tectospinal (head movements in response to visual information).

34
Q

What is the gamma reflex loop?

A

If the knee is extended and the muscle goes slack, the spindle is shortened to maintain its sensitivity.

35
Q

What can hyperreflexia result from?

A

e.g. a stroke.

Loss of descending inhibition.

36
Q

What is hyporeflexia?

A

Below normal or absent reflexes.

Mostly associated with lower motor neuron diseases.