Muscle Physiology: Motor Units, Synapses, and Contraction Mechanics

What is the difference between a motor neuron and a motor unit?

A motor neuron is a nerve cell that transmits signals from the central nervous system to muscles, while a motor unit consists of a single motor neuron and all the muscle fibers it innervates.

What is the difference between large and small motor units? Where will you find each?

Large motor units consist of many muscle fibers and are found in muscles that require powerful contractions, such as the quadriceps. Small motor units have fewer muscle fibers and are found in muscles requiring fine motor control, such as those in the fingers.

What is the point of connection between a neuron and a muscle fiber?

The point of connection is called the neuromuscular junction.

What are the structures of a synapse?

A synapse consists of the presynaptic terminal, synaptic cleft, and postsynaptic membrane.

How is a nerve impulse transferred across a neuromuscular junction?

A nerve impulse triggers the release of acetylcholine from the presynaptic terminal into the synaptic cleft, which then binds to receptors on the postsynaptic membrane of the muscle fiber, leading to muscle contraction.

What is acetylcholine and acetylcholinesterase?

Acetylcholine is a neurotransmitter that transmits signals across the neuromuscular junction, while acetylcholinesterase is an enzyme that breaks down acetylcholine to terminate the signal.

What is the resting membrane potential of a skeletal muscle cell?

The resting membrane potential of a skeletal muscle cell is typically around -70 to -90 millivolts, indicating a negative charge inside the cell relative to the outside.

What ions are pumped to the interior and exterior of a cell to maintain the resting membrane potential?

Sodium (Na+) ions are pumped out of the cell, while potassium (K+) ions are pumped into the cell to maintain the resting membrane potential.

What is the difference between ion pumps and ion gates?

Ion pumps actively transport ions across the membrane using energy (ATP), while ion gates are channels that allow ions to passively flow through the membrane in response to specific signals.

What is an end-plate potential?

An end-plate potential is a localized depolarization of the muscle fiber membrane at the neuromuscular junction, caused by the binding of acetylcholine to its receptors.

What is the difference between a ligand-regulated ion gate and a voltage-regulated ion gate?

A ligand-regulated ion gate opens in response to the binding of a specific molecule (ligand), while a voltage-regulated ion gate opens in response to changes in membrane potential.

What is the difference in opening speed between sodium gates and potassium gates?

Sodium gates open rapidly in response to depolarization, while potassium gates open more slowly, contributing to the repolarization phase of the action potential.

What is an action potential?

An action potential is a rapid, temporary change in the membrane potential of a cell, leading to the propagation of an electrical signal along the neuron or muscle fiber.

Describe the steps of the excitation phase from the release of ACh from the synaptic knob to the opening of the first voltage-regulated ion channels in the muscle cell.

1. Acetylcholine (ACh) is released from the synaptic knob. 2. ACh binds to receptors on the muscle fiber membrane. 3. This binding causes depolarization, leading to the opening of voltage-regulated sodium channels. 4. Sodium ions rush into the cell, further depolarizing the membrane.

What is the function of the troponin-tropomyosin complex?

The troponin-tropomyosin complex regulates muscle contraction by blocking the binding sites on actin filaments, preventing myosin from attaching until calcium ions bind to troponin, causing a conformational change.

Describe the steps of the excitation-contraction coupling phase from the creation of an action potential wave on the muscle cell to the binding of myosin to actin.

1. An action potential travels along the muscle fiber membrane and down the T-tubules. 2. This triggers the release of calcium ions from the sarcoplasmic reticulum. 3. Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin. 4. Myosin heads attach to the exposed sites on actin, initiating contraction.

Describe the repeating cycle of steps of the contraction phase.

The contraction phase involves the following steps: 1. Myosin heads bind to actin, forming cross-bridges. 2. Myosin heads pivot, pulling actin filaments toward the center of the sarcomere. 3. ATP binds to myosin, causing it to release actin. 4. ATP is hydrolyzed, re-cocking the myosin head for another cycle.

What can stop the cycle of muscle contraction?

The cycle can be stopped by the cessation of acetylcholine release, leading to the closure of ion channels and the reabsorption of calcium ions by the sarcoplasmic reticulum.

Describe the steps of the relaxation phase from the cessation of ACh release to the return of the muscle to its resting length.

1. Acetylcholine release stops, and ACh in the synaptic cleft is broken down by acetylcholinesterase. 2. Calcium ions are pumped back into the sarcoplasmic reticulum. 3. The troponin-tropomyosin complex returns to its resting state, blocking actin binding sites. 4. The muscle fiber relaxes and returns to its resting length.

What are the 4 factors that return the muscle to its resting length?

The four factors are: 1. Elastic recoil of the connective tissue. 2. Gravity. 3. Antagonistic muscle contraction. 4. The passive tension of the muscle fibers.

What is the relationship between how stretched a muscle cell is and how much tension it can generate when stimulated?

The relationship is described by the length-tension relationship; optimal stretching allows for maximal overlap of actin and myosin filaments, leading to greater tension generation. Too much or too little stretch reduces tension.

What is muscle tone and how does it affect this relationship?

Muscle tone is the continuous and passive partial contraction of muscles, which helps maintain posture and readiness for action. It affects the length-tension relationship by keeping muscles slightly stretched, optimizing their ability to generate tension when stimulated.