Neuroscience Chapter: Membrane Potential

Membrane Structure and Function

• Membranes contain leakage channels that are always open, allowing ions to flow in and out.
• Sodium ions (Na+) have both a chemical and electrical gradient driving them into the cell.
• Na+/K+ pumps actively transport Na+ out of cells against its gradient, maintaining resting membrane potential.
• The Na+/K+ pump moves 3 Na+ out for every 2 potassium ions (K+) brought in.

Graded Potentials

• Resting membrane potential primarily depends on leakage channels.
• Gated ion channels open/close in response to stimuli (e.g., light, sound, chemicals).
• Graded potentials result from deviations in resting membrane potential and can either depolarize or hyperpolarize the neuron.
• Amplitudes of graded potentials vary, hence the term "graded".
• Graded potentials arise on dendrites and cell bodies, unlike action potentials which occur along axons.
• These potentials do not require a threshold potential, can fade over distance, and can sum algebraically.

Action Potentials

• Action potentials are triggered when graded potentials reach the threshold potential of approximately -55mV.
• Voltage-gated Na+ and K+ channels are essential for the generation of action potentials.
• Once threshold is achieved, Na+ channels open, causing rapid depolarization as Na+ flows into the cell.
• Following depolarization, K+ channels open leading to repolarization as K+ exits the cell.
• Action potentials are self-propagating, creating a chain reaction along the axon.

Refractory Periods

• An absolute refractory period ensures action potentials only travel in one direction, preventing backflow.
• Frequencies of action potentials can increase to about 1000 per second but not their size (amplitude).

Saltatory Conduction

• Occurs in myelinated axons where action potentials jump between nodes of Ranvier, increasing transmission speed.

Muscle Fiber Structure

• Muscle fibers are multi-nucleated cells formed from fused muscle cells containing myofibrils, which are bundles of myofilaments.
• Special names for muscle cell organelles: sarcolemma (membrane), sarcoplasm (cytoplasm), and sarcoplasmic reticulum (SR).

Myofibrils and Sarcomeres

• Myofibrils consist of actin (thin) and myosin (thick) filaments arranged in a banded pattern.
• Key structures: Z-lines, A-bands (actin and myosin overlap), I-bands (actin only), H-bands (myosin only), M-line (middle), and sarcomere (functional unit).

Myosin and Actin Interaction

• Myosin molecules have globular heads with binding sites for actin and ATP. They bind to Z-lines via titin proteins.
• Actin molecules create a helical structure and possess binding sites blocked by the regulatory proteins troponin and tropomyosin.
• Calcium ions (Ca2+) released from the sarcoplasmic reticulum enable muscle contraction by allowing myosin to bind to actin.

Motor Units and Neuromuscular Junction

• A motor unit consists of one motor neuron and all muscle fibers it innervates, functioning collectively.
• The 'all-or-nothing' principle dictates that when a motor neuron fires, all fibers in the unit contract.
• Each muscle fiber connects at a neuromuscular junction, which functions similarly to chemical synapses with acetylcholine as the neurotransmitter.

This quiz covers the structure and function of membranes, focusing on leakage channels, the role of sodium ions, and the functioning of Na+/K+ pumps. Additionally, it explores graded potentials, their characteristics, and how they differ from action potentials in neurons.