Nerve and Muscle Physiology

• Physiology examines the function of living organisms, from molecular events in cells to whole-body integrated functions.
• Nerve physiology studies neurons and the nervous system, particularly electrical and chemical signal transmission.
• Muscle physiology studies muscle function, including contraction mechanisms, energy use, and different muscle type

Nerve Physiology

• Neurons serve as the nervous system's fundamental units, responsible for transmitting information.
• Neurons use electrical and chemical signals to communicate.
• Electrical signals travel within a neuron, whereas chemical signals transmit information across neurons.

Resting Membrane Potential

• The resting membrane potential represents the electrical potential difference across a neuron's plasma membrane when not actively signaling.
• Neurons typically have a resting membrane potential of -70 mV, indicating a negative charge inside relative to the outside.
• The resting membrane potential arises mainly from uneven ion distribution (Na+, K+, Cl-) and selective membrane permeability.
• Na+/K+ pumps maintain concentration gradients by actively transporting 3 Na+ ions out and 2 K+ ions into the cell.
• Ion channels facilitate passive ion diffusion across the membrane along electrochemical gradients.
• K+ leak channels remain open at rest, allowing K+ outflow and contributing to the negative resting membrane potential.

Action Potential

• An action potential is a rapid, temporary shift in a neuron's membrane potential, enabling long-distance communication.
• The action potential is initiated when the membrane potential reaches a threshold, usually around -55 mV.
• Depolarization occurs when Na+ channels open in response to a stimulus, allowing Na+ influx and causing a more positive membrane potential.
• Repolarization involves Na+ channel inactivation and K+ channel opening, allowing K+ efflux and restoring the negative membrane potential.
• Hyperpolarization briefly makes the membrane potential more negative than usual because of continued K+ outflow.
• Action potentials propagate down the axon without weakening because of the regenerative nature of voltage-gated ion channels.

Synaptic Transmission

• Synaptic transmission involves neuron communication with another cell (neuron, muscle, or gland) across a synapse.
• Action potential arrival at the axon terminal triggers voltage-gated Ca2+ channel opening.
• Ca2+ influx causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane.
• Neurotransmitters are released into the synaptic cleft and bind to postsynaptic membrane receptors.
• Neurotransmitter binding to receptors causes excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs).
• EPSPs depolarize the postsynaptic membrane, increasing action potential likelihood.
• IPSPs hyperpolarize the postsynaptic membrane, decreasing action potential likelihood.
• Neurotransmitters are removed from the synaptic cleft through degradation, reuptake, or diffusion to stop the signal.

Muscle Physiology

• Muscle tissue supports movement, posture maintenance, and heat generation.
• Skeletal, smooth, and cardiac tissues constitute the three primary muscle types.

Skeletal Muscle

• Skeletal muscle connects to bones and facilitates voluntary movements.
• Long, cylindrical, multinucleated muscle fibers (cells) compose skeletal muscle.
• Muscle fibers contain myofibrils comprised of repeating sarcomere units.
• Sarcomeres, the basic contractile units, consist of actin (thin) and myosin (thick) filaments.
• The sliding filament theory explains muscle contraction by myosin filaments sliding past actin filaments, shortening the sarcomere and creating force.
• Action potentials in motor neurons stimulate acetylcholine (ACh) release at the neuromuscular junction.
• ACh binds to muscle fiber membrane receptors, causing depolarization and initiating a muscle fiber action potential.
• The action potential spreads along the sarcolemma and T-tubules, prompting Ca2+ release from the sarcoplasmic reticulum.
• Ca2+ binds to troponin, removing tropomyosin from actin-binding sites and allowing myosin to bind to actin and begin cross-bridge cycling.
• ATP hydrolysis powers myosin to pull actin filaments, shortening the sarcomere.

Smooth Muscle

• Smooth muscle resides in internal organ walls (e.g., blood vessels, digestive tract) and controls involuntary movements.
• Spindle-shaped smooth muscle cells contain a single nucleus.
• Smooth muscle lacks the striated appearance of skeletal muscle due to non-sarcomeric actin and myosin filament arrangement.
• Contraction starts from various stimuli, encompassing; hormones, neurotransmitters, and local factors.
• Ca2+ influx activates myosin light chain kinase (MLCK).
• MLCK phosphorylates myosin, enabling actin binding and cross-bridge cycling initiation.

Cardiac Muscle

• Cardiac muscle, found exclusively in the heart, pumps blood.
• Branched cardiac muscle cells contain a single nucleus.
• Cardiac muscle's contraction is involuntary, but striated like skeletal muscle.
• Intercalated discs with gap junctions connect cardiac muscle cells, enabling rapid electrical signal spread.
• Cardiac muscle cell action potentials feature a prolonged plateau phase (Ca2+ influx), contributing to long contraction duration.
• The plateau phase prevents tetanus.
• Specialized sinoatrial (SA) node pacemaker cells control rhythmic heart contractions by creating spontaneous action potentials.

Study of nerve and muscle physiology, covering neuron function, signal transmission, and resting membrane potential. It also explores muscle contraction mechanisms and energy usage. Understand electrical and chemical signals in neurons.