Ch. 10: The Muscular System

What are somatic motor neurons?

They are nerve cells that activate skeletal muscle fibers, and their cell bodies reside in the brain or spinal cord.

What is the purpose of the Na⁺/K⁺ pump?

It pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions in, establishing and maintaining a concentration gradient.

What is the resting membrane potential?

It is the difference in voltage across the cell membrane, with the inside of the cell being more negative than the outside, typically around -70 mV.

What causes a graded potential?

It occurs when a ligand binds to a receptor and opens ligand-gated channels, allowing Na⁺ to flow into the cell.

What is a ligand-gated channel?

A type of ion channel that opens in response to the binding of a chemical signal (ligand), such as a neurotransmitter, to a receptor on the channel.

How does a ligand-gated channel open?

When a specific ligand binds to the receptor site on the channel, causing a conformational change that allows ions to flow through the channel.

How does a ligand-gated channel close?

When the ligand detaches from the receptor, causing the channel to return to its original shape and stop allowing ions to flow.

Where is a ligand-gated channel located?

In the cell membrane, particularly at synapses, where they mediate neurotransmission.

What is a voltage-gated channel?

A type of ion channel that opens in response to a change in voltage, allowing ions to pass through. It is essential for the propagation of action potentials.

How does a voltage-gated channel open?

When the membrane potential reaches a certain threshold, causing a conformational change in the channel that allows ions to flow through.

How does a voltage-gated channel close?

When the membrane potential returns to its resting state, causing the channel to revert to its closed configuration and stop ion flow.

Where is a voltage-gated channel located?

Along the axon of neurons and muscle fiber sarcolemma, which act much faster than ligand-gated channels.

Define threshold.

The minimum amount of depolarization (e.g., movement toward a more positive charge) required to initiate an action potential.

What is the threshold for an action potential?

The threshold is around -65 mV, and if it’s reached, an action potential will be triggered.

What happens during depolarization?

Voltage-gated Na⁺ channels open, Na⁺ rushes into the cell, and the membrane potential increases to +30 mV.

What happens during repolarization?

Voltage-gated Na⁺ channels close, K⁺ channels open, and K⁺ rushes out of the cell, bringing the membrane potential back to negative values.

What is the refractory period?

A short period after an action potential when the cell cannot fire another signal until the Na⁺/K⁺ pump restores concentration gradients.

How does an action potential propagate?

The action potential moves along the axon or sarcolemma in one direction, triggering further depolarization along the membrane.

What happens when action potentials are repeated?

Repeated action potentials can increase the strength or duration of the muscle’s response, such as in muscle contraction.

What is the role of the neuromuscular junction?

It is the site where the motor neuron’s action potential is transferred to the muscle fiber’s action potential.

What is acetylcholine (ACh)?

It is the excitatory neurotransmitter used at the NMJ to stimulate muscle fibers.

What happens when an action potential reaches the presynaptic terminal?

Voltage-gated Ca²⁺ channels open, causing Ca²⁺ to enter, which triggers the release of ACh from synaptic vesicles into the synaptic cleft.

What is the motor end plate?

It is the postsynaptic membrane of the muscle fiber, which contains ACh receptors that bind ACh to initiate muscle contraction.

What happens after ACh binds to its receptor?

Ligand-gated Na⁺ channels open, Na⁺ flows into the muscle fiber, creating a local depolarization that can trigger an action potential.

How is ACh removed from the synaptic cleft?

ACh is broken down by acetylcholinesterase (AChE) into acetic acid and choline, preventing continued stimulation.

What is myasthenia gravis?

A disease characterized by a shortage of ACh receptors due to an autoimmune disorder, leading to muscle weakness.

What is tetanus toxin?

It targets regulatory neurons and causes a constant release of ACh, leading to spastic paralysis and respiratory failure.

What is botulism toxin?

It blocks the release of ACh, causing flaccid paralysis and potential respiratory failure.

What is excitation-contraction coupling?

It is the process by which an action potential triggers the release of Ca²⁺, which allows actin and myosin to interact and cause muscle contraction.

What happens when the action potential reaches the T-tubules?

It triggers a depolarization that causes voltage-gated Ca²⁺ channels in the sarcoplasmic reticulum to open and release Ca²⁺.

How does Ca²⁺ contribute to muscle contraction?

Ca²⁺ binds to troponin, changing its shape and exposing the myosin-binding site on actin, which allows contraction to begin.

What is cross-bridge formation?

It is when the myosin head binds to actin, linking the actin and myosin filaments.

What is the power stroke?

It is when the myosin head pivots and pulls actin over itself, narrowing the bare zone and contributing to muscle contraction.

What is the recovery stroke?

It is when the myosin head detaches from actin, returns to its high-energy state, and is ready to form another cross-bridge.

How does the sarcomere shorten?

Through repeated cross-bridge cycles, myosin heads walk along actin filaments, shortening the sarcomere and contracting the muscle.

What happens when muscle relaxation occurs?

Ca²⁺ is pumped back into the sarcoplasmic reticulum, and the myosin heads detach from actin, causing the sarcomeres to lengthen.

How does rigor mortis occur, and what role does ATP play in the process?

It begins 3-4 hours after death, peaks at 12 hours, and fades within 48-60 hours. It happens when calcium builds up in muscle cells, causing myosin cross-bridges to form. Since ATP production stops, these cross-bridges can't detach, leading to stiffness. As muscle proteins break down, rigor mortis eventually fades.

What is a motor unit?

A motor neuron and all the muscle fibers it innervates. When the motor neuron fires, all the muscle fibers it controls contract simultaneously.

What is the relationship between motor unit size and control?

Muscles that require fine control, like those in the eyes and fingers, have small motor units, while larger muscles with less precise movements, like the hip and thigh muscles, have larger motor units.

How are muscle contractions graded?

By increasing the firing rate of motor neurons or by recruiting more motor units, thus increasing the force of contraction.

What is muscle tone?

The slight, prolonged contractions of a small number of motor units that keep muscles firm, stable, and ready to respond. It helps maintain posture and stabilize joints.

What are the three main energy needs of a muscle fiber during contraction?

ATP is required for cross-bridge movement and detachment, for operating the calcium pump in the sarcoplasmic reticulum, and for maintaining ion gradients via the Na⁺/K⁺ pump.

How is ATP regenerated in muscle fibers?

Through creatine phosphate (CP), anaerobic glycolysis, and aerobic respiration, with each method providing energy at different rates.

What is the role of creatine phosphate in ATP regeneration?

Creatine phosphate rapidly regenerates ATP through direct phosphorylation of ADP, providing energy for about 15 seconds of intense activity (like a 100-meter dash).

What is the formula for creatine phosphate (CP) regenerating ATP?

CP + ADP → Creatine + ATP

How does anaerobic glycolysis generate ATP?

By breaking down glucose into pyruvate, which is converted into lactic acid. This process provides energy for 1-2 minutes of intense activity.

What is the formula for anaerobic glycolysis?

C₆H₁₂O₆ → pyruvate → 2 lactic acid + 2 ATP (net) + heat

What happens to lactic acid produced in anaerobic glycolysis?

Lactic acid diffuses out of muscle cells and enters the bloodstream, where it can be converted back into glucose by the liver, heart, or kidneys.

What is aerobic respiration, and how does it generate ATP?

Aerobic respiration uses oxygen to break down glucose in mitochondria, producing 36-38 ATP per glucose molecule, making it the most efficient energy source for prolonged exercise.

What is the formula for aerobic respiration?

C₆H₁₂O₆ + O₂ → CO₂ + H₂O + 38 ATP (net) + heat

What is oxygen debt?

The extra oxygen needed after exercise to replenish oxygen reserves, convert lactic acid to glucose, and restore ATP and creatine phosphate levels.

What causes muscle fatigue?

It can result from central nervous system fatigue, ATP depletion, altered calcium release, or synaptic fatigue at the neuromuscular junction.

What is synaptic fatigue?

When acetylcholine (ACh) is depleted at the neuromuscular junction due to rapid and sustained action potentials, neurotransmission becomes less effective, leading to impaired muscle contraction.

What are slow-twitch muscle fibers?

They have a slower response time, better blood supply, more mitochondria, and greater fatigue resistance, making them suited for long-term, sustained activity.

What are fast-twitch muscle fibers?

They have a faster response time, fewer mitochondria, and a less developed blood supply, making them suited for short bursts of vigorous activity, though they fatigue quickly.

How do muscle fiber types vary across the body?

Large postural muscles tend to have more slow-twitch fibers, while muscles in the upper limbs have more fast-twitch fibers. The ratio of fiber types is genetically determined.