Physiology: Nerves and Neurons

Physiology: Nerves and Neurons

Resting Membrane Potential

The resting membrane potential (RMP) of a typical neuron is approximately –70 mV (inside negative relative to outside). This potential arises because the membrane is selectively permeable to K⁺ at rest (via leak channels), and the Na⁺/K⁺-ATPase pump maintains concentration gradients: Na⁺ is 145 mM extracellular and 10 mM intracellular; K⁺ is 4 mM extracellular and 150 mM intracellular. K⁺ diffuses out down its concentration gradient, leaving behind negative charges, until the electrical gradient (pulling K⁺ back in) equals the chemical gradient — the equilibrium potential for K⁺ (approximately –90 mV, Nernst equation). The RMP is slightly more positive than –90 mV because of small Na⁺ and Cl⁻ permeabilities.

Nervous System Organisation

The nervous system divides into the central (CNS: brain and spinal cord) and peripheral (PNS: all nerves outside the CNS, including cranial nerves, spinal nerves, and the enteric nervous system). The ENS is embedded in the walls of the gastrointestinal tract and is classified as part of the PNS.

Glial cells outnumber neurons. CNS glia: oligodendrocytes (myelinate up to 50 axons each), astrocytes (blood–brain barrier, metabolic support), microglia (immune surveillance), ependymal cells (line CSF-filled ventricles). PNS glia: Schwann cells (myelinate one axon segment each; form myelin or ensheath unmyelinated fibres), satellite cells (surround cell bodies in ganglia).

Ion Channels

Three major functional classes of ion channels underlie neuronal signalling:

1. Voltage-gated channels: open when membrane potential reaches a threshold; voltage-gated Na⁺ channels open rapidly during depolarisation, voltage-gated K⁺ channels open more slowly during repolarisation.
2. Ligand-gated (ionotropic) channels: open when a specific neurotransmitter binds (e.g., nAChR opens when ACh binds, allowing Na⁺ in).
3. Mechanically-gated channels: open in response to membrane deformation (e.g., in hair cells of the cochlea, Meissner's corpuscles).

Conduction Velocity

Action potential conduction speed is determined by two factors: axon diameter (larger diameter → lower internal resistance → faster conduction) and myelination (myelin increases membrane resistance and decreases capacitance, enabling saltatory conduction between nodes of Ranvier). Unmyelinated C fibres (0.2–1.5 µm) conduct at <2 m/s; myelinated Aα fibres (12–20 µm, motor and proprioception) conduct at 70–120 m/s.