Skeletal Muscle Physiology Notes

Nervous Tissue (Motor Neurons)

• Microscopic Structure of Neuron- Glial Cells (small black spots)

  • Processes (Axons and Dendrites)

  • Cell Body

  • Dendrites

  • Nucleus

  • Axon Terminals

  • Node of Ranvier

  • Schwann's Cells

  • Axon

  • Myelin Sheath

Neuron Structure

• Dendrites: Receive signals.

• Rough ER (Nissl body): Protein synthesis.

• Polyribosomes & Ribosomes: Involved in translation.

• Golgi apparatus: Processes and packages proteins.

• Nucleus: Contains genetic material.

• Nucleolus: Produces ribosomes.

• Membrane: Encloses the cell.

• Microtubule & Neurofibrils: Provide structural support.

• Mitochondrion: Produces ATP.

• Smooth ER: Lipid synthesis and calcium storage.

• Synapse (Axodendritic, Axoaxonic, Axosomatic): Junctions for signal transmission.

• Neurotransmitter: Chemical messenger.

• Receptor: Binds to neurotransmitters.

• Synaptic vesicles: Store neurotransmitters.

• Synaptic cleft: Gap between neurons.

• Axonal terminal: Releases neurotransmitters.

• Axon hillock: Initiates action potentials.

• Myelin Sheath (Schwann cell): Insulates axon.

• Microfilament: Structural support.

Voltage Clamp

• The voltage clamp maintains the membrane potential at a constant value determined by the experimenter.

• It uses feedback amplifiers to control membrane voltage (V_m).

Action Potential Phases

• Resting Potential: Approximately -70 mV.

• Stimulus: Applied at t = 1 ms, raising membrane potential above -55 mV (threshold potential).

• Depolarization: Membrane potential rapidly rises to a peak of +40 mV at t = 2 ms.

• Repolarization: Potential drops and overshoots to -90 mV at t = 3 ms.

• Restoration: Resting potential of -70 mV is re-established by t = 5 ms.

Action Potential Mechanism

• Cells maintain a resting potential of -70mV.

• Depolarization: Sodium ion channels open, allowing sodium ions to enter the cell.

• Repolarization: Potassium ion channels open, allowing potassium ions to exit the cell.

• The influx of sodium ions increases positive charge inside the cell, causing depolarization.

• Sodium channels close at the action potential peak, while potassium continues to leave.

• The efflux of potassium ions decreases the membrane potential, causing hyperpolarization.

• For small voltage increases, potassium current exceeds sodium current, returning voltage to -70 mV.

• If voltage increases past a critical threshold (typically 15 mV higher than resting), sodium current dominates.

• Positive feedback from sodium current activates more sodium channels, causing the cell to fire, producing an action potential.

• The frequency of action potentials is referred to as the firing rate or neural firing rate.

Neuronal Signal Propagation

• As an action potential (nerve impulse) travels down an axon, there is a change in polarity across the membrane.

• In response to a signal, sodium (Na^+) and potassium (K^+) gated ion channels open and close as the membrane reaches its threshold potential.

• Na^+ channels open at the beginning of the action potential, and Na^+ moves into the axon, causing depolarization.

• Repolarization occurs when K^+ channels open and K^+ moves out of the axon, creating a change in polarity.

• The impulse travels down the axon in one direction to the axon terminal to signal other neurons.

Synapse

• Axonal Terminal

• Dendrites

• Electrical Impulses

• Neurotransmitter Molecules

• Receptor

Skeletal Muscle Structure

• Skeletal muscle cells are long, multi-nucleated cylinders separated by connective tissue.

• Each independent cell is stimulated by a branch from a motor neuron.

• Connective endomysium separates cells.

• Total muscle: Groups of fascicles.

• Epimysium: Connective tissue around the muscle.

• Fascicle: Group of muscle cells.

• Perimysium: Connective tissue around fascicles.

• Endomysium: Connective tissue around muscle cell (myofiber).

• Sarcolemma: Muscle cell plasma membrane.

• Nucleus: Multiple nuclei per cell at the periphery.

• Sarcoplasm: Muscle cell cytoplasm.

• Myofibril: Contains actin and myosin filaments.

• Sarcomere: Basic contractile unit.

  Key features:

  • Movement of skeleton.

  • Under voluntary control.

  • Rapid and forceful contractions for short durations.