Action Potentials and Muscle Contraction

Flashcards covering action potentials, neurotransmission, and muscle contraction mechanics based on the BMS 595 Human Physiology Lab lecture.

What is an action potential?

A rapid, temporary reversal of membrane potential in excitable cells (neurons and muscle).

What is the purpose of action potentials in cells?

To allow long-distance electrical signaling without loss of strength.

In what type of cells are action potentials generated?

Excitable cells, including neurons, skeletal muscle, cardiac muscle, and some smooth muscle.

What is the typical resting membrane potential of a neuron?

About $-70\,mV$.

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

Approximately $-90\,mV$.

Which two components help maintain the resting membrane potential?

The $Na^+/K^+$ ATPase pump and $K^+$ leak channels.

What does it mean for a cell to be 'polarized' at rest?

It means there is a resting potential across the membrane; the cell is at its baseline electrical state.

What is the definition of 'threshold' in terms of an action potential?

A minimum depolarization (approximately $-55\,mV$ in neurons) needed to trigger an action potential.

What is the 'all-or-none' principle?

The principle that once threshold is reached, the action potential size is always the same, regardless of stimulus strength.

What happens during the depolarization phase of an action potential?

Voltage-gated $Na^+$ channels open and $Na^+$ rushes into the cell (influx), making the membrane positive.

What occurs during the repolarization phase of an action potential?

$Na^+$ channels inactivate, voltage-gated $K^+$ channels open, and $K^+$ exits the cell (efflux), making the membrane negative again.

What is the hyperpolarization (after-potential) phase?

A phase where the membrane becomes more negative than the resting potential because $K^+$ channels close slowly.

What is the absolute refractory period?

A period during which no new action potential is possible because $Na^+$ channels are inactivated.

What is the relative refractory period?

A period where a stronger stimulus is needed to trigger an action potential because some $Na^+$ channels have recovered.

What are the physiological meanings of refractory periods?

They prevent backward (one-way) propagation and limit the maximum firing rate of the cell.

What is the typical amplitude change of an action potential from rest?

An amplitude change of about $100\,mV$.

How long does a typical action potential last in a neuron?

Approximately $1-2\,ms$.

How is information about stimulus intensity encoded in neurons?

By changes in the firing frequency of action potentials, not by their amplitude.

Does an action potential decay as it travels?

No, it is self-regenerating along the membrane and does not decay with distance.

In terms of energy use, why is the action potential considered 'passive'?

The action potential itself uses ion gradients; energy (ATP via the $Na^+/K^+$ pump) is used indirectly for recovery to restore gradients.

What factors increase the speed of conduction of an action potential?

Larger axon diameter and myelination of the axon.

How do neurons communicate messages chemically?

Through neurotransmission across a gap called a synapse.

Which part of a neuron is responsible for receiving signals?

The dendrites.

What part of the neuron integrates signals?

The cell body.

What is the function of the axon in a neuron?

To transmit the action potential.

What is the function of the myelin sheath?

To insulate the axon and make the electrical signal travel faster.

What are synaptic terminals?

Structures that transmit signals across the synapse to the postsynaptic target.

Where is Acetylcholine typically found and what is its function?

At the neuron-to-muscle synapse; it activates muscles.

What is the location and function of Dopamine according to the lecture?

Mid-brain; it is involved in the control of movement.

What is the role of Epinephrine in the body?

It is associated with the sympathetic system and the stress response.

What are the functions of Serotonin?

Mood and sleep.

Where are Endorphins located and what do they affect?

Brain and spine; they affect mood and pain reduction.

What is the role of Nitric Oxide in the brain?

Memory storage.

What are the four basic operations of the nervous system regarding stimuli?

Determine type of stimulus, signal intensity, integrate responses, and initiate/direct operations.

What is the Axon Hillock?

The initial segment of the axon where signals are integrated before being sent as an action potential.

What is a synapse?

The region where an axon terminal communicates with its postsynaptic target cell.

What is saltatory conduction?

The process where action potentials jump between the nodes of Ranvier in myelinated axons.

Why does saltatory conduction occur only at the nodes of Ranvier?

Because only the nodes of Ranvier contain voltage-gated $Na^+$ channels.

How does a degenerated myelin sheath affect conduction?

It causes current leak, which slows conduction speed.

What is the difference between electrical and chemical synapses?

Electrical synapses pass signals through gap junctions, while chemical synapses use neurotransmitters.

Where are chemical synapses primarily located in the Peripheral Nervous System (PNS)?

They carry information from nerves to myocytes (muscle cells) and gland cells.

What is a neuromuscular junction?

A chemical synapse between a motor neuron and a muscle fiber that signals the muscle to contract.

Name three areas where Acetylcholine is used as a neurotransmitter besides the neuromuscular junction.

Autonomic ganglia, caudate nucleus, and the limbic system.

What structural feature allows ions to move freely between cells in an electrical synapse?

Gap junctions.

What is the function of electrical synapses in the retina?

They adjust electrical connections between rod photoreceptors and 'AII' amacrine cells based on light levels.

How do electrical synapses function in the heart?

They help the heart beat in a coordinated and synchronized manner.

What is the purpose of electrical synapses in the gut?

They help with peristaltic movement.

What is the purpose of electrical synapses in the hypothalamus?

They ensure cells fire action potentials simultaneously to release hormones into circulation.

What are neurotransmitter molecules packaged into before release?

Membranous vesicles.

What enters the presynaptic terminal to trigger the fusion of vesicles?

$Ca^{2+}$ ions entering through voltage-dependent channel gates.

In what 'amount' is neurotransmitter released into the synaptic cleft?

In quantized amounts.

What happens after a neurotransmitter binds to a receptor on the postsynaptic membrane?

Activated receptors trigger a postsynaptic event, such as opening an ion channel or a G-protein-coupled signal cascade.

What are the three ways neurotransmitters are cleared from the synapse?

Diffusion, enzymatic degradation, or active uptake into cells.

What is the main source of $Ca^{2+}$ for skeletal muscle contraction?

The Sarcoplasmic Reticulum ($SR$).

What is the main source of $Ca^{2+}$ for smooth muscle contraction?

Extracellular $Ca^{2+}$ and the Sarcoplasmic Reticulum ($SR$).

Compare the stimulus for skeletal muscle vs. smooth muscle contraction.

Skeletal is always neural (motor neuron); Smooth can be neural, hormonal, local factors, or stretch.

What structure replaces T-tubules in smooth muscle?

Caveolae.

What is the key coupling mechanism in skeletal muscle?

The voltage sensor (DHPR) directly opens the RyR on the sarcoplasmic reticulum.

What is the regulatory protein in skeletal muscle?

Troponin on actin.

What is the regulatory protein in smooth muscle?

Calmodulin in the cytoplasm.

How does calcium activate contraction in skeletal muscle?

$Ca^{2+}$ binds to troponin, which moves tropomyosin out of the way.

How does calcium activate contraction in smooth muscle?

$Ca^{2+}$ binds to calmodulin, which then activates Myosin Light Chain Kinase ($MLCK$).

What is the myosin activation requirement in smooth muscle?

Myosin must be phosphorylated.

Compare the contraction speed and fatigue resistance of skeletal vs. smooth muscle.

Skeletal is fast with variable fatigue resistance; Smooth is slow/sustained with very high fatigue resistance.

What mnemonic describes skeletal muscle contraction mechanisms?

'$Ca^{2+}$ moves the cover!' (referring to Troponin).

What mnemonic describes smooth muscle contraction mechanisms?

'$Ca^{2+}$ turns on the motor!' (referring to Myosin/$MLCK$).

What are the three stages of a skeletal muscle twitch?

Latent period, contraction phase, and relaxation period.

What is the definition of the 'latent period' in a muscle twitch?

The time between the application of a stimulus and the onset of contraction.

What physiological events occur during the latent period?

The impulse travels along the sarcolemma and down T-tubules to the Sarcoplasmic Reticulum, and calcium is released.

What occurs during the 'contraction phase' of a muscle twitch?

$Ca^{2+}$ binds to troponin, exposing binding sites on actin, leading to peak tension.

What happens during the 'relaxation period' of a muscle twitch?

$Ca^{2+}$ ions are no longer present, and the muscle returns from peak tension to baseline.

What is a motor unit?

A motor neuron and the group of skeletal muscle fibers it innervates.

How many motor neurons innervate a single muscle fiber?

A single muscle fiber is innervated by only one motor neuron.

Can an entire muscle receive input from multiple motor neurons?

Yes, an entire muscle may receive input from hundreds of different motor neurons.

How does the nervous system increase contraction force?

Through the recruitment of additional motor units.

What is asynchronous recruitment?

A process where different motor units take turns maintaining tension to avoid fatigue.

What is the difference between isotonic and isometric contractions?

Isotonic contractions create force and move a load; isometric contractions create force without moving a load.

What is a concentric action in isotonic contraction?

A shortening action.

What is an eccentric action in isotonic contraction?

A lengthening action.

In an isometric contraction, what happens to the sarcomeres and elastic elements?

Sarcomeres shorten while the series elastic elements stretch, resulting in little overall length change.

When does an isotonic contraction occur?

When the force of contraction is greater than the load and tension remains constant.

When does an isometric contraction occur?

When the load is greater than the force of the muscle contraction.

True or False: In isotonic contraction, the muscle shortens and moves the load.

True.

True or False: In isometric contraction, the muscle overall length changes.

False.

How does the axon hillock contribute to signaling?

It acts as the integration center for input signals before they become an output signal.

What textbook is cited as a reference for human physiology in the transcript?

Guyton and Hall Textbook of Medical Physiology by John E. Hall.