1. Contraction of skeletal muscles

2. Muscles are stimulated by nerves
The place where where the nerve meets the muscle is called the Neuromuscular junction

3. Neuromuscular Junction
Motor unit – is composed of
Motor neuron – type of neuron that stimulates muscle tissue
Muscle fibers – many fibers attach to one motor neuron
Range from as far as 10 to as many as 2000!

4. Neuromuscular Junction
A closer look at the junction
The actual axon of the motor neuron doesn’t touch the sarcolemma of the muscle fiber.
The gap is called the synaptic cleft
Axon terminal – end of the axon gets close to the sarcolemma
Motor end plate – sarcolemma closest to the axon
Synaptic end bulb – portion of the axon that swells at the end
Synaptic vesicles – tiny sacs within the synaptic end bulbs that contain chemicals called neurotransmitters

5. The Junction

6. Neuromuscular Junction
Physiology
When a nerve impulse reaches the synaptic end bulb Ca2+ triggers the release of Actylcholine (ACh), the neurotransmitter
ACh binds with the ACh receptors on sarcolemma
This produces a muscle action potential that results in a muscle contraction

8. Sliding – filament mechanism of contraction
This deals with the interaction between the actin and myosin myofilaments
For this mechanism to work, there needs to be adequate Ca+ and energy.

9. Sliding

10. Sliding Myosin Sarcomere
Head – also called cross bridges, pulls on thin filaments
This causes the thin filaments to slide towards the center of the sarcomere
Sarcomere
The sliding of the myofilaments brings the z disks closer together
The myosin can pull the thin filament so far inward that they overlap

11. Contraction of the muscle
Energy for contraction
A high energy molecule called creatine phosphate is present in muscle.
Used to make ATP quickly during prolonged exercise.
ATP  ADP + P + energy
When creatine phosphate is depleted, glucose is broken down through processes called glycolysis and cellular respiration and ATP is made.

12. Contraction of the muscle
Stimulus
Threshold stimulus – any stimulus that causes a contraction
All of none principle – states that when ever a threshold is reached, the muscle fiber will contract to the fullest extent.

13. Contraction of the muscle
Physiology
When a muscle fiber is relaxed, there is a low concentration of Ca2+ in the sarcoplasm.
It is held in the sarcoplasmic reticulum (SpR)
When a muscle fiber is contracted the SpR releases Ca2+ in to the sarcoplasm and it binds with the troponin activating the filament sliding mechanism.

14. Contraction of the muscle
Steps of Sliding filament sequence
Myosin head becomes activated as it splits ATP into ADP + phosphate using ATPase.
Ca2+ enters the sarcoplasm from the SpR, binding to troponin moving the tropomyosin.
The activated myosin can attach to exposed actin filament
The myosin swivels and pulls the actin filaments past the thick myosin filament this called the Power stroke of the contraction. As the myosin head swivel, the ADP is released.

15. Contraction of the muscle
Steps of sliding filament sequence continued….
When the power stroke is complete, ATP binds to the myosin head and it detaches from actin.
ATP splits and returns myosin head to original position.
Myosin head is ready to combine further down on the filament.

17. Contraction of the muscle
Relaxation (two major changes)
ACh is broken down by acetylcholinease (AChE), this stops the muscle action potentials
Ca2+ is rapidly removed from the sarcoplasm and returned to the SpR, this stops the myosin head attaching to the actin filament because of the return of tropomyosin.
Rigor mortis – state of rigidity after death

18. Kinds of Contractions Myogram – measurement of a muscle contraction
Latent – when Ca2+ is released
Contraction – when muscle contracts
Relaxation – when muscle is relaxing
After a muscle fiber contracts, there is a short period of time that it loses its excitability.

19. Kinds of Contractions
Twitch – brief contraction of all the muscle fibers in a motor unit
Tetanus – two stimuli are applied but the second one is delayed until after the refractory period
Isotonic – when muscle shortens and pull on another structure
Isometric – minimal shortening of muscle tension is increased greatly