4.1 Neuromuscular Function

Question 1

Motor unit

Answer 1

A single motoneuron & the muscle fibres which it innervates (supply with nerves)

Question 2

Parts of a motor unit

Answer 2

Dendrite, cell body, nucleus, axon, motor end plate, synapse and muscle

Question 3

Dendrites

Answer 3

• Link neuron to neuron

- allows information to flow between nerves

Question 4

Soma/Cell Body

Answer 4

• Contains the nucleus but does not incorporate the dendrites or axon

Question 5

Nucleus

Answer 5

• Membrane-enclosed organelle
• Has the cell’s DNA
• Controls the cell

Question 6

Axon

Answer 6

• Main component of nerve signal transmission
• Has a cover made of myelin (protein)
  
  • makes sure electrical signal is insulated
  • has Nodes of Ranvier (gaps) - aids in transmission of info

Question 7

Motor End Plate

Answer 7

• Where the neuron meets the muscle fibre

Question 8

Synapse

Answer 8

• Gap between neuron and muscle fibre

- Transmission of electrical nerve signals happens across

Question 9

Muscle

Answer 9

• Transmission of electrical nerve signal stimulates muscle (to contract)

Question 10

Neurotransmitters

Answer 10

Chemicals that are used for communication between a neurone at the synapse and another cell

Question 11

Role of neurotransmitters in stimulating skeletal muscle contraction

Answer 11

• Acetylcholine (ACh) is the primary neurotransmitter for the motor neurons that innervate skeletal muscle
• Muscular contractions start w/ electrical impulse
  
  • generated either voluntarily by brain or by reflex
• Electrical impulses travel along motoneuron through motor end plate
• When a signal reaches the synapse, neurotransmitter ACh is released
• ACh changes electrical state of muscle & causes signal called the action potential to travel along muscle fibres
• When nerve is no longer stimulated, ACh is removed by cholinesterase
• Cholinesterase (enzyme) is present in synaptic cleft + rapidly breaks down ACh into acetate + choline
  
  • necessary to allow a neuron to return to its resting state after activation

Question 12

Contracted state of sarcomere

Answer 12

• Contracted state of sarcomere:
  
  • H-zones disappear
  • I-band narrows
    
    • light bands shorten
  • A-band remains the same
    
    • dark bands remain the same

Question 13

Parts of the muscle (getting smaller and smaller)

Answer 13

SKELETAL MUSCLE ⇒ MUSCLE FIBRES ⇒ MYOFIBRILS ⇒ MYOFILAMENTS

Question 14

Sliding filmanet theory

Answer 14

• Relaxed muscle - regulatory protein tropomyosin binds protein sites on actin
• Motor neuron sends a signal to muscle fibre to contract ⇒ sarcoplasmic reticulum releases calcium ions
• Calcium ions bind to a protein called troponin & causes it to change shape ⇒ tropomyosin moves + unblock actin’s binding sites
• Myosin heads bind to site & swivel towards centre of sarcomere
  
  • moving actin filament a small distance
• ATP causes breaking of cross-bridges
  
  • when ATP attaches to myosin head ⇒ head detaches from binding site
• ATP hydrolyses ⇒ ADP + phosphate
• Hydrolysis provides energy for myosin head to cock
  
  • cock: swivel outwards away from centre of sarcomere
• New cross-bridges are formed by myosin binding to actin’s binding site adjacent to ones previously occupied
  
  • each head binds to a site one position further from centre of sarcomere
• Energy stored in myosin head when cocked causes it to swivel inwards towards centre ⇒ moving actin a small distance

Question 15

Slow twitch muscle fibres (Type I)

Answer 15

• slow nerve transmission speeds
• small muscle forces
• can maintain contractions for a long time ⇒ fatigue resistant

e.g. walking, jogging

Question 16

Fast twitch muscle fibres (general)

Answer 16

• high in glycogen content depending on training status
  
  • enzymes necessary for producing energy without oxygen
  • can be broken down to quickly generate glucose for glycolysis

Question 17

Fast twitch muscle fibres (Type IIa)

Answer 17

• fast neural transmission times
• stronger contraction forces
• fatigue resistant

e.g. swimming, cycling

Question 18

Fast twitch muscle fibres (Type IIb)

Answer 18

• fastest contraction times
• largest forces
• can not maintain contractions for a long time ⇒ fatigue at a high rate

e.g. sprinting, jumping, throwing, weightlifting