Physiology · MU Sofia

5. Transmembrane potential and resting membrane potential

Covers the transmembrane potential and the resting membrane potential of neurons, around -70 mV, and the ion gradients and permeability that set it. It also explains the equilibrium potential and the Nernst equation, the Goldman equation with its variables, and the Na+/K+ ATPase pump. The note then gives an overview of voltage-gated ion channels, their structure, gating and inactivation, with a figure of an open channel and a voltage recording.

Inside: 2 sections · 6 labelled pictures · 16 flashcards · 9 min

Topic 5 of 104 in the MU Sofia syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 2 sections, about 1,264 words (9 min of reading), with 16 flashcards.

In this note

  1. Introduction
  2. Nerve Cells (Neurons)

Introduction

  • Muscle and Nerve tissues are called excitable tissues. This is because they respond to external stimuli.

Nerve Cells (Neurons)

  • Neurons are specialized cells designed to transmit information throughout the body using electrical impulses known as action potentials. Here’s how they operate:
  • 1. Resting Membrane Potential: Neurons maintain a resting membrane potential around -70 mV due to the differential distribution of ions across their membrane, primarily driven by ion pumps and channels.
  • 2. Action Potential Initiation: When a neuron receives a stimulus strong enough to depolarize the cell membrane to a threshold level (typically around -55 mV), voltage-gated sodium (Na+) channels open. This results in a rapid influx of Na+ ions, causing further depolarization.
  • 3. Propagation of Action Potential: As the membrane depolarizes, adjacent segments of the axon's membrane reach the threshold and open their Na+ channels, propagating the action potential along the axon toward the nerve terminal.
  • 4. Repolarization: After the peak of the action potential, voltage-gated potassium (K+) channels open, allowing K+ to flow out of the neuron. This outflow of K+restores the negative membrane potential (repolarization). Sodium channels also start to inactivate, ceasing the influx of Na+.
  • 5. Hyperpolarization and Refractory Periods: The K+ channels close slowly, often leading to a temporary hyperpolarization (more negative than the resting potential). During the refractory period, the neuron cannot fire another action potential, ensuring that action potentials only travel in one direction and maintaining signal fidelity.
  • 6. Signal Transmission: At the axon terminal, the action potential causes the release of neurotransmitters into the synaptic cleft, which then bind to receptors on the next cell (which could be another neuron, a muscle cell, or a gland cell), propagating the signal.

Muscle Cells

  • Muscle cells (myocytes) are the effectors in the body, responding to neural input by contracting, which facilitates movement and various physiological functions.
  • 1. Excitation-Contraction Coupling: In muscle cells, the process begins when an action potential in a motor neuron triggers the release of neurotransmitters (e.g., acetylcholine for skeletal muscles) at the neuromuscular junction. The neurotransmitters bind to receptors on the muscle cell membrane, triggering an action potential in the muscle cell.
  • 2. Action Potential in Muscle Cell: The action potential travels along the muscle cell membrane and down into the cell through structures called T-tubules. This depolarization leads to the opening of voltage-gated calcium channels in the sarcoplasmic reticulum, a specialized organelle that stores calcium.

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