Resting Membrane Potential

Definition and Contributors

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell when it is not actively transmitting signals. This potential is crucial for the function of excitable cells such as neurons and muscle cells.

• Key Contributors: The sodium-potassium pump, ion channels, and selective permeability of the cell membrane maintain the resting membrane potential.

• Typical Value: In neurons, the resting membrane potential is usually around -70 mV.

• Synaptic Transmission: This process is not a direct contributor to the resting membrane potential.

Equation

The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

where is the equilibrium potential, is the gas constant, is temperature, is the charge of the ion, is Faraday's constant, and and are the extracellular and intracellular concentrations of the ion.

Action Potentials

All-or-None Principle

The all-or-none principle states that once the threshold is reached, an action potential is generated and propagated along the membrane without decrement. All excitable membranes follow this rule.

• Threshold: The minimum depolarization required to trigger an action potential.

• Action Potential: A rapid change in membrane potential that travels along the cell membrane.

Graded Potentials

Graded potentials are changes in membrane potential that vary in magnitude and do not follow the all-or-none principle. They occur in response to stimuli of varying strength and can summate to reach threshold.

• Location: Typically found in dendrites and cell bodies.

• Function: Important for initiating action potentials.

Hyperpolarization and Action Potentials

Prolonged opening of chloride channels or potassium channels can cause hyperpolarization, making the neuron more difficult to excite and less likely to generate action potentials.

• Hyperpolarization: Membrane potential becomes more negative than the resting potential.

• Effect: Reduces neuronal excitability.

Relative Refractory Period

During the relative refractory period, a stronger than usual stimulus is required to generate an action potential due to the efflux of potassium ions ().

• Absolute Refractory Period: No action potential can be generated regardless of stimulus strength.

• Relative Refractory Period: Action potential can be generated only by a stronger stimulus.

Specialized Cells and Structures

Pacemaker Cells (Cardiac)

Pacemaker cells in cardiac muscle can spontaneously generate action potentials without external input due to specialized ion channel activity.

• Location: Sinoatrial (SA) node of the heart.

• Function: Initiate and regulate heartbeat.

Endothelial Cells and Erythrocytes

Endothelial cells line blood vessels and interact with erythrocytes (red blood cells) to maintain vascular health and blood flow.

• Function: Regulate exchange between blood and tissues.

• Erythrocytes: Transport oxygen and carbon dioxide.

Neural Coordination and Sensory Systems

Motor Coordination

The motor cortex and other brain regions coordinate voluntary movements, especially when tracking moving objects or performing complex tasks.

• Function: Planning, initiating, and controlling movement.

• Example: Eye movement coordination when following a moving object.

Purely Sensory Cranial Nerves

Some cranial nerves are purely sensory, such as the optic nerves, which are responsible for vision.

• Example: Optic nerve (cranial nerve II) transmits visual information from the retina to the brain.

Pain Perception and Myocardial Infarction

Pain from a heart attack (myocardial infarction) can be felt in the chest and may also radiate to other areas such as the arm or jaw due to referred pain.

• Referred Pain: Pain perceived at a location other than the site of the painful stimulus.

• Example: Arm pain during a heart attack.

Temperature and Pain Receptors

Free nerve endings in the skin are responsible for detecting changes in temperature and pain.

• Thermoreceptors: Detect changes in temperature.

• Nociceptors: Detect pain stimuli.

Somatosensory Cortex Representation

The somatosensory cortex allocates more space to body regions with higher sensory input, such as the lips and hands.

• Function: Processes sensory information from the body.

• Example: Greater representation for fingertips than for the back.

Tonic Receptors

Tonic receptors are activated during prolonged or continuous stimuli, such as adaptation to darkness after entering a dark room.

• Function: Maintain response to sustained stimulus.

• Example: Photoreceptors in the eye adjusting to low light conditions.

Summary Table: Key Neural Concepts