1. How does the muscle get the message?

2. Nerve Impulse ! Motor Neuron – Motor Unit–
Nerves that send messages to muscles
Motor Unit–
One (1) motor neuron (branched out) and all the muscle fibers it controls

3. Motor Unit
How many muscle fibers are stimulated by this ONE neuron?

5. Fine or Gross? Fine motor control – Gross motor control –
Few muscle fibers controlled by one motor neuron.
Move few fibers with each stimulus = fine control
Fingers
Gross motor control –
Many muscle fibers controlled by one motor neuron.
Move lots of fibers with each stimulus – gross control
Legs

6. Where does the message go?
Neuromusclular junction:
Site where a motor neuron and muscle come together.
They Do Not Touch
The space between =
Synaptic cleft (“cliff”)

8. Who Has What? Motor Neuron: Muscle Fiber:
Neurotransmitters = chemicals that carry the message.
Acetylcholine – signal that produces movement.
Acetylcholine esterase (erases the message) stops the movement.
Muscle Fiber:
Motor End Plate = specialized part of the sarcolemma with chemical receptors.

9. How Does It All Work Together?
Muscle Contraction
Nerve impulse – stimulates the release of Acetylcholine into the synaptic cleft
Sarcolemma – depolarizes (swaps charged ions) transmits action potential along T-tubules
Calcium released from Sarcoplasmic reticulum

11. Myosin cross bridges attach to actin Actin fibers are pulled together
Shortening the sarcomere
Acytylcholine Esterase released –
Stops impulse (message)
Calcium transported back into SR (ATP is required).
Myosin detaches from Actin (ATP required)
Actin slides back to relaxed state.

12. Animation: Action Potentials and Muscle Contraction

15. All or Nothing!
Once a threshold stimulus is sent All the fibers stimulated will contract fully.
Heavy Load
Lots of fibers stimulated (lots of motor units)
Light Load
Few fibers stimulated (few motor units)