U3 Lecture 21

Question 1

Define excitation

Answer 1

The events that transmit an electrical signal from a motor neuron to a muscle fiber

Question 2

What happens during excitation?

Answer 2

Neuromuscular Transmission =

• Skeletal muscle is VOLUNTARY which REQUIRES an action potential (AP) to be sent from the central nervous system (CNS)
• Arrival of the motor neuron action potential (AP) results in the generation of an AP in the skeletal muscle fiber membrane (sarcolemma)

Question 3

Define excitation-contraction (EC) coupling

Answer 3

The events that connect excitation to contraction

Question 4

What happens during excitation-contraction (EC) coupling?

Answer 4

Skeletal muscle AP triggers release Ca2+ from the sarcoplasmic reticulum (SR) into the sarcoplasm of the skeletal muscle fiber

Question 5

Define contraction

Answer 5

the events that cause the sarcomeres of the muscle fibers to shorten

Question 6

What happens during contraction

Answer 6

Ca2+ binds to troponin which enables he contraction cycle to begin

• myosin heads then pull on actin

Question 7

What is troponin

Answer 7

A protein involved in muscle contraction. It occurs with tropomyosin in the thin filaments of muscle tissue

Question 8

Define relaxation

Answer 8

The events that cause the sarcomeres of the muscle fiber to return to resting length

• removing Ca2+ from sarcoplasm ends contraction

Question 9

Make up of Skeletal Muscle

Answer 9

Muscles - covered in epimsium

• > fasicles - covered in perimysium
• > axons of motor neurons and muscle fibers - covered in endomysium (on outside) and sarcolemma (on inside)
• > myofibril
• > myofilaments

Question 10

Make up of Axon of motor neuron

Answer 10

• axon collateral of somatic motor neuron (“alpha-motor neuron”)
• axon terminal
• synaptic end bulbs

Question 11

What is a neurouscular junction?

Answer 11

A synapse between a motor neuron and skeletal muscle

Question 12

Define synapse

Answer 12

A junction between two nerve cells consisting of a small gap where impulses pass by diffusion of a neurotransmitter

Question 13

** What is the critically important fact about neuromuscular junctions?

Answer 13

There is only ONE neuromuscular junction between any skeletal muscle fiber and the alpha-motor neuron that triggers its contractions

Question 14

Structure of a motor neuron

Answer 14

• A neuron sends nerve impulses down the axon

- A nerve impulses will continue down each axon branch to the end of all the axon terminal (where the NMJ’s are)

Question 15

Make up of neuromuscular junction

Answer 15

• axon terminal, voltage-gated Ca2+ channel, synaptic vesicle containing acetylcholine (ACh) synaptic end bulb, synaptic cleft, motor end plate, ACh receptor

Question 16

What are the steps of excitation (neuromuscular transmission)

Answer 16

1. AP arrives @ synaptic end bulb and opens voltage gated Ca2+ channels
2. Synaptic vesicles w ACh undergoes exocytosis
3. ACh is released into synaptic cleft and binds to ACh receptors in sarcolemma (motor end plate)
4. ACh receptors open, Na+ enters and makes AP on sarcolemma
5. ACh is broken down to Acetate and Choline by AChE for recycling - ACh also diffuses away

Question 17

Steps of Excitation-Contraction Coupling

Answer 17

1. AP runs along sarcolemma, continues into T-Tubules
2. Triggers release of Ca2+ from SR
3. Ca2+ diffuses into sarcoplasm and myofibrils
4. Ca2+ binds to troponin on thin filament - myosin binding sites on actin are exposed
5. crossbridge forms -> tension is generated
6. Starts contraction cycle

Question 18

Steps of relaxation cycle

Answer 18

1. When APs stop arriving at the NMJ, the SR stops releasing Ca2+
2. Active Ca2+ transporters in SR pump Ca2+ back into the SR
3. Cytoplasmic (Ca2+) decreases
4. As (Ca2+) falls, Ca2+ comes off troponin and tropomyosin covers myosin binding sites on actin
5. crossbridge stops and tension drops
6. titin brings sarcomere back to resting position