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.
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.
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.
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 potentials (AP) are bursts of electrical activity enabling neuron communication.
Phases of an AP:
Depolarizing Phase: Inside becomes less negative, reaching zero and then positive due to Na+ influx.
Repolarizing Phase: Back to resting potential (-70 mV) due to K+ outflow.
After-hyperpolarizing Phase: Temporary overshoot beyond resting 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.
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 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.
A synapse is where neurons communicate:
Presynaptic Neuron: Releases neurotransmitters.
Postsynaptic Neuron: Responds to neurotransmitters.
Types of Synapses:
Electrical Synapses: Direct ionic communication, faster, lack integration capabilities.
Chemical Synapses: Use neurotransmitters for signaling, involved in learning and memory.
Transmission involves neurotransmitter release upon arrival of an AP at the synaptic knob.
Steps:
AP opens voltage-gated Ca2+ channels, causing Ca2+ influx.
Ca2+ triggers neurotransmitter release via exocytosis.
Neurotransmitters bind to postsynaptic receptors, leading to ion channel changes and potential conversion (EPSP or IPSP).
Acetylcholine: Excitatory/inhibitory effects.
Biogenic Amines: Derived from amino acids (e.g., serotonin, dopamine).
Amino Acids: Glutamate (excitatory) and GABA (inhibitory).
Small Molecules: Nitric oxide, purines.
Neuropeptides: Chains of amino acids affecting neurotransmission.
Chemical signals modifying nerve impulse transmission; longer-lasting effects than neurotransmitters.
Examples: nitric oxide, enkephalins, endorphins, and hormones.
Methods for neurotransmitter removal from synaptic cleft:
Diffusion: Moving away to dissipate effects.
Enzymatic Degradation: Breakdown by enzymes (e.g., acetylcholinesterase).
Reuptake: Reabsorption by presynaptic neuron or glial cells.
Summation: Process where excitatory and inhibitory potentials combine.
Spatial Summation: Input from different sites at once.
Temporal Summation: Rapid successive inputs from one neuron.
Composed of neural pools performing specific functions, controlled by neural circuits types:
Divergent Circuits: Single neuron broadcasts to many.
Converging Circuits: Multiple pathways funnel into one.
Reverberating Circuits: Circular feedback loops between neurons, maintaining prolonged activation.
Parallel After-discharge Circuits: Single input leads to multiple pathways before reconverging.
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.