Neuronal Structure and Electrical Properties: Study Notes for Anatomy & Physiology

Basic Properties of Neurons

Overview of Neurons

Neurons are specialized, excitable cells that form the fundamental units of the nervous system. They are responsible for receiving, processing, and transmitting information through electrical and chemical signals.

• Excitable cells: Neurons can generate and propagate action potentials (rapid changes in membrane potential).

• Synaptic transmission: Neurons communicate with each other via synapses, using neurotransmitters to relay signals.

• Main structural parts:

  • Dendrites: Receive input from other neurons.

  • Soma: The cell body, containing the nucleus and organelles.

  • Axon: Conducts output signals to synaptic terminals.

Neuron Morphologies

Types and Structures of Neurons

Neurons exhibit diverse morphologies, which influence their function and connectivity.

• Unipolar/bipolar neurons: Simple structure, fewer inputs; often found in sensory pathways.

• Multipolar/pyramidal neurons: Highly branched dendritic trees, capable of integrating thousands of synaptic inputs; common in the cerebral cortex.

• Dendritic spines: Small protrusions on dendrites that serve as major sites of synaptic contact and plasticity.

Glial Cells (10:1 ratio vs neurons)

Types and Functions of Glial Cells

Glial cells outnumber neurons and provide essential support for neuronal function.

• Oligodendrocytes/Schwann cells: Form myelin sheaths around axons, increasing the speed of electrical conduction.

• Astrocytes: Regulate the chemical environment, provide structural support, and maintain the blood-brain barrier.

• Microglia: Act as immune defense cells, removing debris and responding to injury.

Electrical Properties of Neurons

Key Electrical Parameters

Neuronal signaling relies on the movement of ions across the cell membrane, governed by several electrical properties:

• Voltage (Vm): The difference in electric potential across the membrane, measured in millivolts (mV).

• Current (Im): The flow of ions (e.g., Na+, K+, Ca2+, Cl-) across the membrane. Units: pA/nA.

• Resistance (Rm): Opposition to current flow. Reciprocal is conductance (G), measured in nS (nanosiemens).

• Ohm's Law:

$V = I \times R$ $V = \text{Voltage},\ I = \text{Current},\ R = \text{Resistance}$

• Capacitance (Cm): The ability to store charge across the membrane. Units: pF (picofarads). Determined by membrane surface area and thickness. Myelin decreases capacitance, resulting in faster conduction.

• Membrane time constant (τ): $\tau = R_m \times C_m$

  • Determines how quickly the membrane potential (Vm) can change in response to current.

  • Small τ = fast response; large τ = slow, integrates inputs.

Resting Membrane Potential (RMP)

Establishment and Calculation of RMP

The resting membrane potential is the baseline electrical charge difference across the neuronal membrane, typically around -60 mV.

• Equilibrium potential (Eion): The membrane voltage at which the electrical and concentration (chemical) forces for a specific ion are balanced.

• Nernst equation: Used to calculate the equilibrium potential for a given ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{out}}{[\text{ion}]_{in}} \right)$ Where: $R$ = gas constant, $T$ = temperature (Kelvin), $z$ = valence of the ion, $F$ = Faraday's constant, $[\text{ion}]_{out}$ = extracellular concentration, $[\text{ion}]_{in}$ = intracellular concentration.

• For K+: Intracellular concentration is high, extracellular is low; $E_K \approx -80$ mV.

Driving Force & Multiple Ions

Membrane Potential and Ion Permeability

In real neurons, the resting membrane potential is determined by the relative permeabilities of several ions, primarily Na+, K+, and Cl-.

• RMP is a weighted average of the equilibrium potentials of permeant ions.

• Goldman-Hodgkin-Katz (GHK) equation:

$V_m = \frac{g_{Na}E_{Na} + g_{K}E_{K} + g_{Cl}E_{Cl}}{g_{Na} + g_{K} + g_{Cl}}$ Where $g_{ion}$ is the conductance for each ion.

• At RMP (~ -60 mV):

  • Na+: Inward driving force (wants to enter the cell).

  • K+: Outward driving force (wants to leave the cell).

  • Cl-: Near equilibrium.

• Total current balances out (net Im = 0).

Key Takeaways

Summary of Nervous System Structure and Function

• CNS: Composed of the brain and spinal cord; protected by bone, meninges, cerebrospinal fluid (CSF), and the blood-brain barrier (BBB).

• Spinal cord processes reflexes; the brain consists of the cerebrum, cerebellum, and brainstem.

• Cortex: Divided into sensory (posterior), motor (anterior), and association (integration) areas.

• Neurons are excitable and communicate via synapses; glial cells provide support.

• Vm, Im, Rm, Cm define neuronal electrical behavior.

• RMP is established by the K+ gradient and is tuned by Na+ and Cl- conductance, maintained by the Na+/K+ pump.

• Maintaining RMP consumes >50% of the brain's metabolic energy.

• Without active ion pumping, RMP is lost and neurons cannot signal, leading to functional failure.