muscle tissues 3-27 friday

Understanding Skeletal Muscle Anatomy

• Focus on myofibril organization.

• Arrangement of thick and thin myofilaments within skeletal muscle fibers.

Quiz Reminder

• Quiz on skeletal muscle is scheduled for today or over the weekend if necessary.

Neuromuscular Junction (NMJ)

• Connection point between a neuron and a skeletal muscle fiber.

• Neuron (yellow) approaches muscle fiber (pink) without actual contact.

• Synaptic Cleft: the gap (gray area) between the neuron and muscle fiber.

• Synaptic Vesicles: contain neurotransmitters, specifically acetylcholine (ACh).

Mechanism of Impulse Transmission

• When a nerve impulse reaches the NMJ, synaptic vesicles release ACh through exocytosis into the synaptic cleft.

• ACh diffuses across to bind receptors on the muscle fiber's sarcolemma at the motor end plate.

• Binding of ACh generates an impulse in the muscle fiber that travels along the sarcolemma.

Ion Distribution and Membrane Potential

• Neurons and muscle fibers are polarized:

  • More negatively charged ions inside (net negative charge)

  • More positively charged ions outside (net positive charge)

• Membrane potential: charge difference across the membrane, measured in millivolts (mV).

  • Resting potential: approximately -70mV

  • Threshold potential: approximately -55mV

Ion Types and Channels

• Cations: positively charged ions vital for impulses.

  • Sodium (Na⁺): abundant outside the cell.

  • Potassium (K⁺): abundant inside the cell.

• Channel Proteins: proteins in the plasma membrane that create gated channels for ions to move in/out.

  • Chemical-gated channels: open in response to a specific chemical (e.g. ACh opens sodium channels).

  • Voltage-gated channels: open in response to specific membrane potential levels.

Action Potential Phases

• Depolarization:

  • ACh binds to chemical-gated sodium channels, causing them to open.

  • Sodium diffuses INTO the cell.

  • Membrane potential rises towards threshold value (-55mV).

• Threshold Reached:

  • Voltage-gated sodium channels open at threshold potential, causing further sodium influx, which pushes the potential above zero (positive).

  • Membrane becomes depolarized (positive inside).

• Repolarization:

  • Voltage-gated potassium channels open at +20mV.

  • Potassium diffuses OUT of the cell, restoring the negative interior.

  • Membrane potential returns towards -70mV.

Summary of Impulse Propagation

• An action potential causes similar depolarization and repolarization in adjacent membrane segments.

• This is a traveling wave of sodium ions diffusing in, then potassium ions diffusing out, propagating down the muscle fiber or neuron as a muscle impulse or nerve impulse.

Ion Return to Original Distribution

• Following action potential, sodium-potassium pumps restore original ionic conditions by pumping Na⁺ out and K⁺ back into the cell.

Conclusion

• Understanding these mechanisms is critical for comprehending muscle contraction processes.

• Encourage review of the concepts and diagrams presented in class to solidify knowledge for upcoming assessments.