Contraction of skeletal muscles

• Muscles are stimulated by nerves

The place where where the nerve meets the muscle is called the Neuromuscular junction

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!

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

The Junction

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

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.

Sliding

Sliding

• Myosin– 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

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.

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.

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.

Contraction of the muscle

• Steps of Sliding filament sequence1. Myosin head becomes activated as it splits ATP into

ADP + phosphate using ATPase.

2. Ca2+ enters the sarcoplasm from the SpR, binding to troponin moving the tropomyosin.

3. The activated myosin can attach to exposed actin filament

4. 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.

Contraction of the muscle

• Steps of sliding filament sequence continued….

5. When the power stroke is complete, ATP binds to the myosin head and it detaches from actin.

6. ATP splits and returns myosin head to original position.

7. Myosin head is ready to combine further down on the filament.

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

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.

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