10.4-Muscle - Neuromuscular junction

Question 1

What are the excitable membranes

Answer 1

– Are found in skeletal muscle fibers and neurons
– Depolarization and repolarization events produce
action potentials (electrical impulses)

Question 2

What is the neuromuscular junction

Answer 2

Synapse between a neuron and a skeletal muscle fiber

Question 3

Which neurotransmitter is released a the axon terminal of the motor neuron

Answer 3

The neurotransmitter is acetylcholine (ACh)

Question 4

What is the function of ACh

Answer 4

ACh binds to and opens a chemically gated Na+
channel on the muscle fiber
▪ Na+ enters cell and depolarizes motor end plate
▪ An action potential is generated

Question 5

Explain the Excitation-Contaction

Answer 5

1. Neural Control
   A skeletal muscle fiber contracts when stimulated by a motor neuron
   at a neuromuscular junction. The stimulus arrives in the form of an
   action potential at the axon terminal.
2. Excitation
   The action potential causes the release of ACh into the synaptic cleft,
   which leads to excitation—the production of an action potential in the
   sarcolemma.
3. Release of Calcium Ions
   This action potential travels along the sarcolemma and down T
   tubules to the triads. This triggers the release of calcium ions+2Ca
   from the terminal cisternae of the sarcoplasmic reticulum
4. Contraction Cycle Begins
   The contraction cycle begins when the calcium ions+2 Ca bind to troponin, resulting in the exposure of the active sites on the thin
   filaments. This allows cross-bridge formation and will continue as
   long as AT P is available.
5. Sarcomere Shortening
   As the thick and thin filaments interact, the sarcomeres shorten,
   pulling the ends of the muscle fiber closer together.
6. Generation of Muscle Tension
   During the contraction, the entire skeletal muscle shortens and
   produces a pull, or tension, on the tendons at either end.

Question 6

What happens when the contraction cycle starts

Answer 6

1) The Contraction Cycle Begins. The contraction cycle involves a series of interrelated
steps. It begins with the arrival of calcium ions+2 Ca within the zone of overlap in a
sarcomere.
(2) Active-Site Exposure. Calcium ions bind to troponin, weakening the bond between
actin and the troponin–tropomyosin complex. The troponin molecule then changes position,
rolling the tropomyosin molecule away from the active sites on actin and allowing
interaction with the energized myosin heads
3) Cross-Bridge Formation. Once the active sites are exposed, the energized myosin
heads bind to them, forming cross-bridges.
4) Myosin Head Pivoting. After cross-bridge formation, the energy that was stored in the
resting state is released as the myosin head pivots toward the M line. This action is called
the power stroke; when it occurs, the bound ADP and phosphate group are released.
5) Cross-Bridge Detachment. When another A TP binds to the myosin head, the link
between the myosin head and the active site on the actin molecule is broken. The active
site is now exposed and able to form another cross-bridge.
6) Myosin Reactivation. Myosin reactivation occurs when the free myosin head splits ATP
into ADP and P. The energy released is used to recock the myosin head.

Question 7

The duration of contraction depends on

Answer 7

– Duration of neural stimulus
– Presence of free calcium ions in cytosol
– Availability of ATP

Question 8

What is rigor mortis

Answer 8

– Fixed muscular contraction after death
– Results when
▪ ATP runs out and ion pumps cease to function
▪ Calcium ions build up in cytosol