Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the cell membrane when the cell is not actively sending a signal. It is primarily determined by the distribution of ions, such as sodium (Na+), potassium (K+), and chloride (Cl-), and the selective permeability of the cell membrane.

• Key Contributors: Sodium-potassium pump, ion channels, and membrane permeability.

• Synaptic Transmission: 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:

• Example: In neurons, the resting membrane potential is typically around -70 mV.

All-or-None Principle

The all-or-none principle states that action potentials are only generated if the membrane potential reaches a certain threshold. Once this threshold is reached, the action potential will occur fully; if not, it will not occur at all.

• Application: Applies to all excitable membranes, such as neurons and muscle cells.

• Example: A neuron will not fire an action potential if the stimulus is too weak to reach threshold.

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 and can summate to trigger an action potential.

• Characteristics: Can be depolarizing or hyperpolarizing, and their amplitude depends on the strength of the stimulus.

• Location: Typically found in dendrites and cell bodies.

Hyperpolarization and Action Potentials

Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, often due to the prolonged opening of chloride channels. Action potentials are rapid, all-or-none electrical signals generated by the movement of ions across the membrane.

• Hyperpolarization: Makes it more difficult for the cell to reach threshold and generate an action potential.

• Action Potential: Generated when depolarization reaches threshold, typically involving sodium influx and potassium efflux.

Relative Refractory Period

The relative refractory period is the time after an action potential during which a stronger-than-normal stimulus is required to initiate another action potential.

• Cause: Due to continued efflux of potassium ions (K+).

• Example: Neurons are less excitable during this period.

Pacemaker Cells

Pacemaker cells are specialized muscle cells that spontaneously generate action potentials without external stimuli, regulating rhythmic activities such as the heartbeat.

• Location: Found in the sinoatrial (SA) node of the heart.

• Function: Initiate and regulate the 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.

• Example: Endothelial dysfunction can lead to cardiovascular diseases.

Purkinje Fibers

Purkinje fibers are specialized cardiac muscle fibers that conduct electrical impulses rapidly, ensuring coordinated contraction of the heart.

• Location: Found in the ventricles of the heart.

• Function: Facilitate rapid transmission of action potentials.

Sensory Receptors and Sensory Processing

Purely Sensory Cranial Nerve

Some cranial nerves are classified as purely sensory, meaning they only transmit sensory information.

• Example: The optic nerve (cranial nerve II) is purely sensory, transmitting visual information.

Referred Pain and Myocardial Infarction

Referred pain occurs when pain from one area of the body is perceived in another area. In myocardial infarction (heart attack), pain can be felt in the jaw, neck, or arm.

• Mechanism: Due to convergence of nerve fibers in the spinal cord.

• Example: Heart attack pain may be felt in the left arm.

Somatosensory Cortex Representation

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

• Homunculus: A visual representation of the sensory cortex, showing disproportionate representation.

Sensory Receptors for Temperature and Pain

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

• Location: Found throughout the skin and mucous membranes.

• Function: Transmit signals related to noxious stimuli.

Osmoreceptors

Osmoreceptors detect changes in osmotic pressure or solute concentration in body fluids.

• Location: Primarily in the hypothalamus.

• Function: Regulate thirst and water balance.

Tonic Receptors

Tonic receptors respond to prolonged or continuous stimuli, such as light or pressure, and adapt slowly.

• Example: Photoreceptors in the eye adapt to changes in light intensity.

Table: Types of Sensory Receptors