Basic Sciences - Physiology of the Neuromuscular Junction

Question 1

Types of cholinergic receptors

Answer 1

Nicotinic ACh receptors
Muscarinic ACh receptors

Question 2

Nicotinic ACh receptors

Answer 2

Ligand gated ion channel

Transmembrane
Open to allow Na+ ions through

Question 3

Site of nicotinic ACh receptors

Answer 3

All autonomic ganglia
Adrenal medulla
Neuromuscular junction

Question 4

Structure of nicotinic ACh receptor

Answer 4

5 subunits in rosette pattern

2 alpha subunits
1 beta subunit
1 delta subunit
1 epsilon subunit (in foetal muscle gamma subunit instead of epsilon)

Question 5

Acetyl choline binding site on nicotinic ACh receptors

Answer 5

ACh binds to 2 alpha subunits

Both subunits need a ACh to bind to open the channel

Question 6

Muscarinic ACh receptors

Answer 6

G protein coupled receptors

Question 7

Site of muscarinic ACh receptors

Answer 7

Predominantly parasympathetic postganglionic junctions

Question 8

Process of muscle contraction following neuron impulse

Answer 8

Nerve impulse initiated and propagated along axon

Impulse terminates at neuromuscular junction and releases ACh into synaptic cleft

ACh binds to nicotinic receptors and causes Na+ ion influx into sarcolemma

Ca2+ ion release from sarcoplasmic reticulum

Impulse further propagated through muscle and contraction occurs

Question 9

Classification of nerve fibre types

Answer 9

Question 10

Description of motor neurons

Answer 10

Cell body containing nucleus with short dendrites

Single long fibrous axon which extends from cell body to muscle

As approaches muscle, axon branches and ends as axon terminals at the NMJ

Question 11

Name for gaps between myelin sheath along axon

Answer 11

Nodes of Ranvier

Question 12

Myelin description

Answer 12

Protein-lipid complex
Formed from layers of plasma membrane derived from Schwann cells

Question 13

Saltatory conduction definition

Answer 13

Action potential jumps from one node of Ranvier to the next

Question 14

Reason for saltatory conduction

Answer 14

Faster conduction velocity and conserves energy

Question 15

Resting membrane potential of neurones

Answer 15

-70 mV

Question 16

What generates the resting membrane potential of neurons

Answer 16

Na+ K+ pump removes 3 Na+ ions in exchange for 2 K+ ions

Membrane is permeable to K+ ions

Therefore results in relative negative charge inside vs outside neuron

Question 17

Threshold potential definition

Answer 17

Voltage at which an action potential will be initiated if a stimulus reaches that threshold

Question 18

All or none law of action potentials

Answer 18

Each action potential has the same magnitude and duration, regardless of stimulus strength

Question 19

Phases of action potential

Answer 19

Depolarisation
Repolarisation
Refractory period

Question 20

Depolarisation phase

Answer 20

Voltage gated Na+ channels open
Membrane now permeable to Na+ ions
Influx of Na+ down concentration gradient
Polarity across membrane changes from -70mV to +40mV

Question 21

Repolarisation phase

Answer 21

K+ gated channels open
K+ ions move outside down concentration gradient
Returns to positive external to internal from membrane again

Question 22

Refractory period

Answer 22

Recovery time for Na+ K+ pump to re-establish resting membrane potential

Question 23

The motor unit

Answer 23

Motor neuron axon branches and a number of muscle fibres that the motor neuron controls

Question 24

What determines size of motor units

Answer 24

The degree of fine control needed

Muscles designed for fine delicate movements have smaller motor units

Question 25

Example of small motor units

Answer 25

Extrinsic muscles of eye
Can be as few as 6 to 10 muscle fibres supplied by a single motor neuron

Question 26

Example of larger motor units

Answer 26

Gastrocnemius
May contain up to 2000 muscle fibres

Question 27

Where does motor neuron make contact with muscle

Answer 27

Motor end plate

Question 28

Number of ACh molecules contained within each presynaptic vesicle

Answer 28

~ 10,000

Question 29

Site of acetylcholinesterase enzymes

Answer 29

Junctional folds on the post synaptic membrane of the motor end plate

Question 30

Synthesis of acetylcholine

Answer 30

Most choline is recycled but can also be synthesised de novo

Question 31

Name of muscle fibre cell membrane

Answer 31

Sarcolemma

Question 32

Muscle fibre make up

Answer 32

Each myofibril made up of multiple filaments (sarcomere)

Question 33

Sarcomere illustration and landmarks

Answer 33

Question 34

Sarcomere definition

Answer 34

Area between Z lines

Question 35

Proteins in dark A zones of sarcomere

Answer 35

Myosin
Thick filaments

Question 36

Proteins in light I zones of sarcomere

Answer 36

Actin
Tropomyosin
Troponin

Long double helix arrangement

Question 37

Types of troponin

Answer 37

Troponin T
Troponin I
Troponin C

Question 38

Role of troponin T

Answer 38

Binds tropomyosin to actin

Question 39

Role of Troponin I

Answer 39

Inhibits interaction of myosin with actin

Question 40

Role of troponin C

Answer 40

Contains Ca2+ binding sites

Question 41

Process of muscle contraction following neuronal stimulus

Answer 41

Like nerves, skeletal muscle is an excitable tissue.

ACh binds to the receptor sites, end-plate potentials are created over the surface of the muscle fibres. Once a threshold is reached, an action potential is propagated through the muscle fibre.

Ca2+ channels then open in the sarcoplasmic reticulum. This triggers contraction of the fibre through a process called ‘excitation-contraction coupling’.

Ca2+ initiates contraction by binding to Troponin C.

Muscle contraction occurs through cross-linkages between actin and myosin. The thick and thin filaments slide past each other producing shortening.

Ca2+ is then pumped back into the sarcoplasmic reticulum where it is stored until another next action potential arrives.

Question 42

Sarcoplasmic reticulum definition

Answer 42

Membranous sac that envelops the myofibrils.

Question 43

Illustration of propagation of action potential from merve to muscle (pre stimulated)

Answer 43