Download Action Potential and Neuromuscular Junction

Contraction of skeletal muscles
• Muscles are stimulated by nerves
The place 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 or 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 sequence
1. 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.
Myogram of Kinds of Contraction
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