Week 2:Neurophysiology

Introduction to Neurophysiology

• Neurophysiology studies the nervous system's workings, including thoughts, actions, and sensations.

• The nervous system consists of billions of neurons communicating via electrical signals and chemical messages, influencing reflexes and learning processes.

• Main focus: fundamental principles governing nervous system functions.

Definition of Neurophysiology

• Neurophysiology involves studying electrical changes across neuron plasma membranes and accompanying physiological processes.

• Ion channels facilitate the movement of ions, creating a resting membrane potential of -70 mV, indicating negative charges inside the cell compared to the outside.

Types of Ion Channels in Neurons

• Various ion channels exist, including:

  • Leak Channels: Randomly open and close, found in most neuron parts.

  • Ligand-gated Channels: Respond to chemical stimuli; found in sensory and motor neurons.

  • Mechanically-gated Channels: Respond to physical stimuli (touch, pressure).

  • Voltage-gated Channels: Respond to membrane potential changes.

Resting Membrane Potential Mechanisms

• At resting potential, the outside of the membrane is positive while the inside is negative due to ion distribution.

• Mechanisms affecting resting potential:

  • Selective Permeability: Membrane allows specific ion movements, with K+ having higher permeability than Na+.

  • Unmovable Anions: Negative ions can't leave the cell, maintaining negativity inside.

  • Na+/K+ Pumps: Maintain potential by actively transporting Na+ out and K+ in, requiring ATP.

Action Potential

• Action potentials (AP) are bursts of electrical activity enabling neuron communication.

• Phases of an AP:

  1. Depolarizing Phase: Inside becomes less negative, reaching zero and then positive due to Na+ influx.

  2. Repolarizing Phase: Back to resting potential (-70 mV) due to K+ outflow.

  3. After-hyperpolarizing Phase: Temporary overshoot beyond resting potential.

Action Potential vs. Graded Potential

• Graded Potentials: Small deviations, short-distance communication, resulting from ion channel activity.

• Action Potentials: Rapid, all-or-nothing response, traveling longer distances, maintain strength over distance.

Refractory Periods

• After an AP, a neuron faces difficulty reaching a new AP due to refractory periods:

  • Absolute Refractory Period: No stimulus can generate a new AP.

  • Relative Refractory Period: A stronger stimulus can generate an AP.

Continuous vs. Saltatory Conduction

• Continuous Conduction: Occurs in unmyelinated fibers, slower, involves step-by-step depolarization/repolarization.

• Saltatory Conduction: Occurs in myelinated fibers, faster, AP jumps between nodes of Ranvier.

• Factors Affecting Speed: Myelination, axon diameter, and temperature also influence conduction speed.

Synapse Structure and Function

• A synapse is where neurons communicate:

  • Presynaptic Neuron: Releases neurotransmitters.

  • Postsynaptic Neuron: Responds to neurotransmitters.

• Types of Synapses:

  1. Electrical Synapses: Direct ionic communication, faster, lack integration capabilities.

  2. Chemical Synapses: Use neurotransmitters for signaling, involved in learning and memory.

Mechanisms of Synapse Transmission

• Transmission involves neurotransmitter release upon arrival of an AP at the synaptic knob.

• Steps:

  1. AP opens voltage-gated Ca2+ channels, causing Ca2+ influx.

  2. Ca2+ triggers neurotransmitter release via exocytosis.

  3. Neurotransmitters bind to postsynaptic receptors, leading to ion channel changes and potential conversion (EPSP or IPSP).

Types of Neurotransmitters

1. Acetylcholine: Excitatory/inhibitory effects.

2. Biogenic Amines: Derived from amino acids (e.g., serotonin, dopamine).

3. Amino Acids: Glutamate (excitatory) and GABA (inhibitory).

4. Small Molecules: Nitric oxide, purines.

5. Neuropeptides: Chains of amino acids affecting neurotransmission.

Neuromodulators

• Chemical signals modifying nerve impulse transmission; longer-lasting effects than neurotransmitters.

• Examples: nitric oxide, enkephalins, endorphins, and hormones.

Fate of Neurotransmitters

• Methods for neurotransmitter removal from synaptic cleft:

  1. Diffusion: Moving away to dissipate effects.

  2. Enzymatic Degradation: Breakdown by enzymes (e.g., acetylcholinesterase).

  3. Reuptake: Reabsorption by presynaptic neuron or glial cells.

Summation of Postsynaptic Potentials

• Summation: Process where excitatory and inhibitory potentials combine.

  • Spatial Summation: Input from different sites at once.

  • Temporal Summation: Rapid successive inputs from one neuron.

Neural Circuits

• Composed of neural pools performing specific functions, controlled by neural circuits types:

  1. Divergent Circuits: Single neuron broadcasts to many.

  2. Converging Circuits: Multiple pathways funnel into one.

  3. Reverberating Circuits: Circular feedback loops between neurons, maintaining prolonged activation.

  4. Parallel After-discharge Circuits: Single input leads to multiple pathways before reconverging.

Summary Points

• Muscle contraction regulation applies to various synapse types. Acetylcholine operates uniquely at muscle junctions.

• Neural pool integration ensures a coordinated response.

• Conduit forces of calcium ions highlight neurotransmitter release efficiency.