Membrane Potential

Graded Potentials

Graded potentials are temporary changes in the membrane potential of a cell, typically occurring in response to a stimulus. These changes are crucial for initiating action potentials and for cellular communication in the nervous system.

• Local Change: The membrane potential is most altered at the site of stimulation, with the effect decreasing as distance from the stimulus increases.

• Passive Spread: The effect spreads passively due to local currents within the cell membrane.

• Types of Change: Graded potentials may involve depolarization (making the inside of the cell less negative) or hyperpolarization (making it more negative).

• Stimulus Strength: Stronger stimuli produce greater changes in membrane potential and affect a larger area of the membrane.

Depolarization, Repolarization, and Hyperpolarization

These terms describe the phases of membrane potential changes in response to stimuli:

• Depolarization: A chemical stimulus causes the membrane potential to become less negative (e.g., from -70 mV toward -60 mV).

• Repolarization: Removal of the stimulus allows the membrane potential to return to its resting value.

• Hyperpolarization: Some stimuli cause the membrane potential to become more negative than the resting potential (e.g., toward -80 mV).

Functions of Graded Potentials

• Graded potentials often trigger specific cell functions, such as exocytosis of glandular secretions.

• At the neuromuscular junction, acetylcholine (ACh) causes a graded potential at the motor end plate, leading to muscle contraction.

Action Potentials

Overview and Properties

Action potentials are rapid, propagated changes in membrane potential that travel along excitable membranes, such as neurons and muscle cells. They are essential for nerve impulse transmission.

• Initiation: Begin at the initial segment of the axon.

• Propagation: Affect the entire excitable membrane and do not diminish as they move away from the source.

• Threshold: Stimulated by a graded potential that depolarizes the axolemma to a threshold value (typically -60 to -55 mV).

Generation of Action Potentials

1. Depolarization to Threshold: A graded depolarization large enough to open voltage-gated sodium channels.

2. Activation of Voltage-Gated Na+ Channels: Sodium ions rush into the cytosol, causing rapid depolarization and making the inner membrane surface positive.

3. Inactivation of Na+ Channels and Activation of K+ Channels: At +30 mV, sodium channels close and potassium channels open, allowing K+ to exit the cell and initiating repolarization.

4. Return to Resting Membrane Potential: Potassium channels close as the membrane returns to its resting potential, sometimes resulting in brief hyperpolarization.

All-or-None Principle

• Any stimulus that brings the membrane to threshold will trigger an action potential.

• All action potentials are identical in magnitude and duration, regardless of stimulus strength.

Refractory Periods

• Absolute Refractory Period: Voltage-gated Na+ channels are open or inactivated; the membrane cannot respond to further stimulation.

• Relative Refractory Period: Begins when Na+ channels regain resting condition; only a strong stimulus can initiate another action potential.

Ion Exchange and Restoration

• Sodium-Potassium Exchange Pump: Restores ion concentrations after an action potential. For each ATP used, two K+ ions are imported and three Na+ ions are exported.

Propagation of Action Potentials

Action potentials move along the axon by two main mechanisms:

• Continuous Propagation: Occurs in unmyelinated axons; the action potential affects one segment at a time and travels slowly (about 1 m/sec).

• Saltatory Propagation: Occurs in myelinated axons; the action potential jumps from node to node, traveling faster and using less energy.

Axon Types and Conduction Speed

Additional info: Critical information (e.g., signals for survival or injury prevention) is transmitted via fast Type A fibers.

Synaptic Transmission

Structure and Types of Synapses

A synapse is a specialized site where a neuron communicates with another cell. The presynaptic neuron sends the message, and the postsynaptic neuron receives it.

• Electrical Synapses: Direct physical contact via gap junctions; ions pass directly between cells, allowing rapid transmission.

• Chemical Synapses: Most common; use neurotransmitters to transmit signals across a synaptic cleft.

Types of Chemical Synapses

• Neuromuscular Junction: Between neuron and skeletal muscle.

• Neuroglandular Junction: Between neuron and gland cell.

Neurotransmitters

• Chemical messengers stored in synaptic vesicles in the axon terminal.

• Released into the synaptic cleft and bind to receptors on the postsynaptic membrane.

• Broken down by enzymes and reabsorbed for reuse.

Cholinergic Synapses

• Release acetylcholine (ACh) at neuromuscular junctions, many CNS synapses, and all neuron-to-neuron synapses in the PNS.

• ACh is removed from the synaptic cleft by acetylcholinesterase (AChE), which breaks it into acetate and choline.

Synaptic Delay and Fatigue

• Synaptic Delay: A brief delay (0.2–0.5 ms) occurs due to calcium influx and neurotransmitter release.

• Synaptic Fatigue: Occurs when neurotransmitter recycling cannot keep up with demand, weakening the synaptic response.

Neurotransmitters and Neuromodulators

Classes and Effects

• Excitatory Neurotransmitters: Cause depolarization and promote action potentials.

• Inhibitory Neurotransmitters: Cause hyperpolarization and suppress action potentials.

Major Classes

• Biogenic Amines: Norepinephrine (NE) (excitatory), Dopamine (excitatory or inhibitory), Serotonin (affects attention/emotion), GABA (inhibitory).

• Amino Acids

• Neuropeptides: Small peptide chains, including opioids (enkephalins, endorphins, dynorphins).

• Dissolved Gases: Nitric oxide (NO), carbon monoxide (CO).

Mechanisms of Action

• Direct Effect: Opening or closing chemically gated ion channels (e.g., ACh, glutamate).

• Indirect Effect via G Proteins: Neurotransmitter activates a G protein, which then activates an enzyme (e.g., adenylate cyclase), producing a second messenger such as cyclic AMP ().

• Indirect Effect via Intracellular Enzymes: Lipid-soluble gases diffuse through membranes and activate enzymes inside the cell.

Information Processing in Nervous Tissue

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Graded depolarization of the postsynaptic membrane.

• Inhibitory Postsynaptic Potential (IPSP): Graded hyperpolarization of the postsynaptic membrane.

Summation

• Temporal Summation: Rapid, repeated stimuli at a single synapse.

• Spatial Summation: Simultaneous stimuli at multiple synapses.

Facilitation and Inhibition

• Accumulation of EPSPs can facilitate a neuron, making it easier to reach threshold.

• Neuromodulators and hormones can alter membrane sensitivity, shifting the balance between EPSPs and IPSPs.

Presynaptic Modulation

• Presynaptic Inhibition: Decreases neurotransmitter release from the presynaptic membrane.

• Presynaptic Facilitation: Increases neurotransmitter release from the presynaptic membrane.

Frequency Coding

• Information can be encoded by the frequency of action potentials received.

• Maximum rate is reached when the relative refractory period is eliminated.

Summary of Information Processing

• Neurotransmitters may have excitatory or inhibitory effects.

• Neuromodulators can alter the rate of neurotransmitter release or postsynaptic response.

• Neurons may be facilitated or inhibited by chemicals other than neurotransmitters or neuromodulators.

• Response of postsynaptic neuron can be altered by facilitation, inhibition, or modification of neurotransmitter release.