Nervous Tissue - Membrane Potentials and Action Potentials

Membrane Potential

• Electrical Signals from Ion Movements

  • Plasma membranes generate electrical signals through ion movements.
  • Membrane potential is crucial for neuron function.

• Resting Membrane Potential

  • The electric potential difference across a resting cell's membrane.

• Graded Potential

  • A temporary, localized change in resting potential caused by stimuli.

• Action Potential

  • An electrical impulse produced by graded potential.
  • Propagates along the axon surface to the synapse.

Resting Membrane Potential Factors

• Selectively Permeable Membranes

  • Different ionic compositions in extracellular fluid (ECF) and intracellular fluid (cytosol).
  • ECF: High concentrations of $Na^+$ and $Cl^-$.
  • Cytosol: High concentrations of $K^+$ and negatively charged proteins.

• Passive and Active Ion Movement

  • Chemical Gradients
  • $K^+$ moves out of the cell; $Na^+$ moves in.
  • Sodium-potassium (Na+/K+) pumps maintain ion concentrations across the membrane: transfers 3 $Na^+$ out and 2 $K^+$ into the cell, using ATP.

Types of Ion Channels

• Active Ion Channels
  1. Chemically gated ion channels (ligand-gated)
  • Open in response to specific chemicals (e.g., acetylcholine, ACh)
  • Found on neuron cell body and dendrites.
  1. Mechanically gated ion channels
  • Respond to membrane distortion. Found in sensory receptors for touch, pressure, or vibration.
  1. Voltage-gated ion channels
  • Respond to changes in membrane potential, found in axons and muscle cells.

Graded Potentials

• Characteristics

  • Changes in membrane potential that do not propagate far from the stimulation site.
  • Result from any stimulus that opens gated channels:
  • Example: chemical stimulus opens chemically gated sodium channels, causing depolarization (more positive inside).

• Mechanism of Action

  • Adjacent areas of the membrane can become depolarized due to local current created by sodium ion influx.
  • The magnitude depends on the strength of the stimulus.

Generation of Action Potentials

• Action Potential Initiation
  1. Depolarization to Threshold
  • Graded depolarization must reach -60 to -55 mV (threshold) to trigger an action potential.
  1. Rapid Depolarization
  • Opening of voltage-gated sodium channels leads to a rush of $Na^+$ into the cell, changing the membrane potential from -60 mV to +30 mV.
  1. Repolarization
  • At +30 mV, sodium channels inactivate, and voltage-gated potassium channels open, causing $K^+$ to exit, returning the potential toward resting levels.
  1. Return to Resting Potential
  • Voltage-gated K+ channels eventually close, resulting in brief hyperpolarization to -90 mV before stabilizing at -70 mV.

Refractory Periods

• Refractory Period: Time during which the membrane will not respond normally to additional stimuli.
  • Absolute Refractory Period: All voltage-gated sodium channels are inactivated; no new action potential can occur.
  • Relative Refractory Period: Begins when sodium channels regain resting condition; a strong stimulus can initiate a new action potential.

Propagation of Action Potentials

• Mechanisms
  1. Continuous Propagation (in unmyelinated axons)
  • Action potentials affect one segment at a time, propagating slowly (1 m/sec).
  1. Saltatory Propagation (in myelinated axons)
  • Action potentials 'jump' from node to node, which speeds up signal transmission (up to 120 m/sec).

Synapses

• Synapse: Site of communication between neurons; involves:

  • Presynaptic Neuron: Sends messages.
  • Postsynaptic Neuron: Receives messages.

• Types of Synapses

  1. Electrical Synapses: Fast, direct physical contact between cells via gap junctions.
  2. Chemical Synapses: Signal transmission occurs across synaptic cleft using neurotransmitters (most common synapse type).

• Neuromuscular Junction: Specific type of chemical synapse between a neuron and skeletal muscle cell.

Neurotransmitters**

• Chemical Messengers: Released into the synaptic cleft and bind to receptors on the postsynaptic membrane, causing changes in permeability and graded potentials.

• Cholinergic Synapses: Events

  1. Action potential depolarizes the axon terminal.
  2. Calcium ions enter and trigger ACh release.
  3. ACh binds to receptors on post-synaptic cell, leading to depolarization or potential action generation.
  4. ACh is broken down by acetylcholinesterase (AChE) to prevent continuous stimulation.

• Synaptic Delay: Occurs between action potential arrival and effect on postsynaptic membrane (0.2-0.5 msec). Fewer synapses reduce response time.