THE NEUROMUSCULAR JUNCTIONS AND MOTOR UNITS

Question 1

motor unit definition

Answer 1

A single a-motor neuron and all of the corresponding muscle fibres it innervates

Question 2

Motor unit characteristics

Answer 2

• size of motor units vary
• all fibres innervated simultaneously-all or nothing
• all same fibre-type
• The size of the MU dictates level of control

Question 3

Describe the process of neurotransmission across a neuromuscular junction

Answer 3

• action potential comes down nerve terminal
• opens voltage gates ion channels
• Ca2+ enters
• exocytosis of the synaptic vesicles which fuse to the membrane
• acetylcholine released
• acetylcholine crosses synaptic cleft and binds to ligand gated ion channels
• Na+ enters the muscle fibres
• sarcolemma depolarised
• muscle contraction is initiated
• acetylcholine is then broken down by an enzyme, acetylcholinesterase

Question 4

How can neuromuscular transmission be influenced

Answer 4

• can act at the acetylcholine receptor (eg. Curare related drugs)
• Anticholinesterase drugs (prevent hydrolysis of Acch)
• Botulinum toxin

Question 5

Explain excitation-contraction coupling

Answer 5

Transverse tubules in the muscle fibre activate the vesicles to release Ca2+ simultaneously to ensure contraction of the whole muscle at the same time
- The signal for contraction is a sudden rise in Ca2+ around the myofilaments. This is brought about by depolarisation of the t-tubules and diffusion and Ca2+= ions from the sarcoplasmic reticulum

Question 6

What are t-tubules and what are their roles in excitation contraction coupling

Answer 6

Conduct action potentials from the cell surface to the core of the fibre, are in direct contact with the sacroplassmic reticulum

Question 7

What are the ryanodine receptors

Answer 7

Are a type of Ca2+ channel within the sarcoplasmic reticulum

Question 8

What are the different receptors in muscle

Answer 8

Muscle spindle

Golgi tendon organ

Question 9

What is a muscle spindle

Answer 9

Sensitive to stretch
Provide information on muscle length
Modified muscle fibres

Question 10

What is the golgi tendon organ

Answer 10

Found close to or in tendon
Monitor ensign developed in muscle
Prevents damage during excessive forc generation (excessive stimulation results in reflex relaxation of muscle )

Question 11

What is the size of motor units for muscles which require revise control

Answer 11

Small motor units

Question 12

What type of motor units are recruited firs during normal exercise

Answer 12

Slow motor units

Question 13

What can an electromyograph be used to detect

Answer 13

Neuropathy and myopathy

Question 14

What does an EMG detect

Answer 14

Electrical potential generated by muscle cells

Question 15

Wha might abnormal spontaneous activity indicate on an EMG

Answer 15

Nerve or muscle damage

Question 16

What are intramural muscle fibres

Answer 16

Present in muscle spindles
Well innervated with sensory nerves
Inhibits the anatagonistic muscle and stimulates the agonistic muscle