Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell when it 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: Occurs at a specific concentration and is not a direct contributor to the resting potential.

Equation:

Where is the membrane potential, is the gas constant, is temperature, is Faraday's constant, and and are the extracellular and intracellular potassium concentrations, respectively.

All-or-None Principle

The all-or-none principle states that once the threshold is reached, an action potential will occur fully; if the threshold is not reached, no action potential occurs. This applies to all excitable membranes.

• Threshold: Minimum stimulus required to trigger an action potential.

• Action Potential: Rapid change in membrane potential that propagates 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 the threshold for an action potential.

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

Hyperpolarization and Action Potentials

Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, often due to prolonged opening of chloride channels. Action potentials are generated when depolarization reaches the threshold.

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Refractory Period

The refractory period is the time after an action potential during which a neuron is unable to fire another action potential. This ensures one-way propagation of the action potential and limits the frequency of firing.

• Absolute Refractory Period: No action potential can be generated.

• Relative Refractory Period: A stronger-than-normal stimulus is required to generate an action potential.

Pacemaker Cells

Pacemaker cells are specialized muscle cells (such as those in the heart) that spontaneously generate action potentials without external stimuli, regulating rhythmic activities.

• Example: Cardiac pacemaker cells control the heartbeat.

Endothelial Cells and Erythrocytes

Endothelial cells line blood vessels and interact with erythrocytes (red blood cells) to maintain blood flow and the blood-brain barrier.

• Function: Regulate exchange of substances between blood and tissues.

Sensory Receptors and Neural Pathways

Purely Sensory Cranial Nerve

The optic nerve is an example of a cranial nerve that is purely sensory, transmitting visual information from the eye to the brain.

• Function: Vision.

Referred Pain and Myocardial Infarction

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

• Mechanism: Sensory nerves from different areas converge in the spinal cord.

Somatosensory Cortex Representation

The somatosensory cortex allocates more space to body regions with higher sensory input, such as the upper lip, fingers, and tongue.

• Example: Homunculus representation in the brain.

Free Nerve Endings: Temperature and Pain

Free nerve endings are responsible for detecting changes in temperature and pain. They are unencapsulated and found throughout the body.

• Function: Sensory detection of harmful stimuli.

Osmoreceptors

Osmoreceptors detect changes in osmotic pressure or solute concentration in body fluids. They help regulate water balance and are classified as interoceptors.

• Example: Regulation of thirst and fluid intake.

Tonic Receptors

Tonic receptors are sensory receptors that adapt slowly to stimuli and continue to respond during prolonged stimulation. They are important for detecting continuous stimuli, such as light or pressure.

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

Summary Table: Types of Sensory Receptors

Additional info: Some context and terminology have been expanded for clarity and completeness, including the addition of equations and a summary table for receptor types.