Neurophysiology

The nervous system is regarded as the master controlling and communicating system of the body.
Its main functions include:
    - Sensory input: Receiving information from sensory organs.
    - Integration: Processing and interpreting sensory input to make decisions.
    - Motor output: Reacting to integrated stimuli by activating effector organs (muscles or glands).

The nervous system is divided into two main parts:
    - Central Nervous System (CNS):
        - Acts as the integration and command center, including the brain and spinal cord.
    - Peripheral Nervous System (PNS):
        - Composed of all neural tissue outside the CNS.

Sensory (afferent) division:
    - Transmits sensory information to the CNS.
        - Somatic afferent fibers: Carry signals from skin, skeletal muscles, and joints.
        - Visceral afferent fibers: Transmit signals from visceral organs.
Motor (efferent) division:
    - Carries signals from the CNS to effectors (muscles and glands).
        - Somatic nervous system: Controls voluntary movements by innervating skeletal muscles.
        - Autonomic nervous system (ANS): Regulates involuntary functions.

Neurons:
    - Structural units composed of a cell body, axon, and dendrites.
        - Long-lived, amitotic, and have a high metabolic rate.
        - Plasma membrane functions in electrical signaling and cell-to-cell signaling during development.
Neuroglia (glial cells):
- Supportive cells in the nervous system with various functions.

Astrocytes:
    - Most abundant, versatile, and highly branched glial cells.
    - Functions:
        - Support and brace neurons.
        - Guide migration of young neurons.
        - Control the chemical environment and form the blood-brain barrier.
Microglia:
- Phagocytic cells that protect against pathogens.
Ependymal cells:
- Line central cavities of brain and spinal cord; secrete cerebrospinal fluid (CSF).
Oligodendrocytes:
- Form myelin sheaths around CNS axons.

Schwann cells (neurolemmocytes):
- Form myelin sheaths around peripheral axons.
Satellite cells:
- Support neurons in the PNS.

Structure of Neurons:
    - Composed of dendrites, cell body (soma), axon, and axonal terminals.
    - Dendrites are short, tapering processes that receive signals.
    - Axons are slender processes that generate and transmit action potentials and secrete neurotransmitters.
Neuron Classification:
    - Structural classification:
        - Multipolar: Most common type, many processes.
        - Bipolar: Two primary processes, often sensory neurons.
        - Unipolar: Single short process that splits into two branches, found in PNS ganglia.
    - Functional classification:
        - Sensory (afferent): Transmit impulses toward the CNS.
        - Motor (efferent): Carry impulses away from the CNS.
        - Interneurons (association neurons): Relay signals within the CNS.

Neurons are highly irritable; action potentials are electrical impulses carried along the axon.
They maintain the same strength regardless of the stimulus.
An action potential reflects the flow of ions rather than electrons, with potentials created due to the differences in ion concentrations inside and outside the cell membrane.

Types of plasma membrane ion channels:
    - Passive (leakage) channels: Allow ions to flow based on concentration gradients.
    - Chemically gated channels: Open in response to specific chemicals (neurotransmitters).
    - Voltage-gated channels: Open in response to a change in membrane potential.
    - Mechanically gated channels: Open in response to physical deformation of the receptor.

The resting membrane potential in a neuron is approximately -70 mV.
It results from differences in ion concentration and membrane permeability, particularly to $ext{K}^+$ ions.
The sodium-potassium pump maintains ionic concentration by moving $3$ $ext{Na}^+$ out and $2$ $ext{K}^+$ into the cell, contributing to the membrane potential.

Changes to the membrane potential lead to three events:
    - Depolarization: Inside of the cell becomes more positive.
    - Repolarization: Returns the membrane potential to its resting state.
    - Hyperpolarization: Membrane potential becomes more negative than resting.

Graded potentials are short-lived, localized changes in membrane potential.
Their magnitude varies directly with the strength of the stimulus, and they decrease in intensity with distance.
Sufficiently strong graded potentials can initiate action potentials.

APs are brief reversals of membrane potential with an amplitude of approximately 100 mV, differ from graded potentials by being an all-or-nothing event.
Specific phases include:
    - Resting state: Na+ and K+ channels are closed.
    - Depolarization phase: Na+ channels open, leading to rapid inflow of Na+.
    - Repolarization phase: Na+ channels close, K+ channels open, restoring negativity.
    - Hyperpolarization: K+ continues to leave the cell, causing increased negativity.
Movement across a myelinated axon occurs at nodes of Ranvier, increasing conduction speed (saltatory conduction).

Absolute refractory period: No new action potential can be generated, ensuring separate impulses.
Relative refractory period: Following the absolute period, a stronger-than-normal stimulus can initiate an action potential.

Nerve fibers classified based on diameter, degree of myelination, and conduction velocity:
    - Group A fibers: Large diameter, myelinated, fast conduction (e.g., skin, skeletal muscle).
    - Group B fibers: Intermediate diameter and myelinated.
    - Group C fibers: Small diameter, unmyelinated, slow conduction (e.g., found in the CNS).

Synapses mediate information transfer between neurons or from neurons to effector cells (muscles or glands).
Types of Synapses:
    - Electrical synapses: Less common, allow direct current flow.
    - Chemical synapses: Most prevalent, utilize neurotransmitters for signaling.
        - Composed of the axonal terminal of the presynaptic neuron and receptor regions on the postsynaptic neuron.
    - Transmission across the synaptic cleft involves neurotransmitter release and binding.
    - Termination of neurotransmitter effects through degradation, reuptake, or diffusion.

Two primary types:
- Excitatory Postsynaptic Potentials (EPSPs): Lead to a greater likelihood of action potentials; utilize chemically gated channels.
- Inhibitory Postsynaptic Potentials (IPSPs): Make the interior of neurons more negative, decreasing the likelihood of action potential generation.
Summation of EPSPs and IPSPs can influence neuron firing.

Over 50 different neurotransmitters have been identified, classified into various categories including:
    - Acetylcholine (ACh): First identified neurotransmitter, involved in muscle activation; degraded by acetylcholinesterase (AChE).
    - Biogenic amines: Includes dopamine, norepinephrine, and serotonin.
    - Amino acids: Includes GABA, glycine, and glutamate.
    - Peptides: Include substance P (pain mediator) and endorphins (natural painkillers).
Neurotransmitter effects can be either excitatory or inhibitory based on the type of receptor present on the postsynaptic neuron.