Multipolar Neurons

Overview and Importance

Multipolar neurons are a major type of neuron in the central nervous system (CNS), crucial for controlling skeletal muscle coordination and communication within the CNS.

• Cell body / perikaryon / soma: Contains the nucleus and is the metabolic center of the neuron.

• Neurofibrils: Cytoskeleton of intermediate filaments; microtubules are also present, providing structural support and transport.

• Nissl bodies: Rough endoplasmic reticulum (ER); very important for synthesis of neurotransmitters.

• Nucleus: Contains genetic material; active in cell cycle phases such as mitosis.

• Dendrites: Fibers that conduct impulses toward the cell body; typically short and highly branched, increasing surface area for synaptic input.

Axon Structure and Function

• Axon hillock: The region where the axon begins; known as the trigger zone where action potentials are initiated.

• Myelin sheath: Formed by Schwann cells (PNS) or oligodendrocytes (CNS); insulates the axon and increases the speed of impulse propagation.

• Axoplasm: The cytoplasm of the axon, containing mitochondria and other organelles to sustain the sodium-potassium pump and other metabolic activities.

• Axolemma: The plasma membrane of the axon, involved in action potential conduction.

• Terminal branches / telodendria: Highly branched ends of the axon, forming synapses with other cells.

• Axon (presynaptic terminal / synaptic knob): The end of the axon that contacts the target cell.

• Synaptic vesicles: Contain neurotransmitters (NTs) such as acetylcholine (ACh).

• NTs: Released via exocytosis and bind to receptors on the postsynaptic cell, creating new nerve and muscle impulses.

Membrane Potentials and Ion Distribution

Resting Membrane Potential (RMP)

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest, typically around -70 mV in neurons.

• Cations: Na+ and K+ are the principal cations; Na+ is higher in the extracellular fluid (ECF), K+ is higher in the intracellular fluid (ICF).

• Anions: PO43-, SO42-, and proteins are abundant in the ICF.

• Polarized membrane: Results from unequal ion distribution and selective permeability.

Principles of Electricity in Neurons

• Membrane potential: Charge separation creates voltage ().

• Resistance: Non-polar PM acts as an insulator, inhibiting flow.

• Current: Flow of charge (), facilitated by channels and proteins.

• Ohm's Law:

Measuring RMP

• Measured in millivolts (mV) using a voltmeter.

• Normal average RMP: mV (inside the PM).

Ion Channel Types

Types and Functions

• Ion channels: Control movement of excitable cells.

• Ligand / Chemical Gated Ion Channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

• Voltage Gated Ion Channels: Open in response to changes in membrane potential.

• Leakage Ion Channels: Always open, important for maintaining RMP.

• Mechanically Gated Ion Channels: Open and close in response to mechanical stimuli (e.g., pressure).

Factors Maintaining the RMP

Role of Potassium (K+)

• K+ is the principal ICF cation; more K+ leakage channels than Na+ leakage channels.

• K+ moves outside to equilibrium, creating a negative charge inside the cell.

Role of Sodium (Na+)

• Na+ is the principal ECF cation; fewer Na+ leakage channels.

• Na+ voltage-gated channels open briefly during stimulation.

Role of Anions

• Proteins and other anions are abundant in the ICF and contribute to the negative charge inside the cell.

Na+/K+ Pump

• Maintains high K+ in ICF and high Na+ in ECF.

• Pumps 3 Na+ out and 2 K+ in per ATP, against the concentration gradient.

Action Potentials

Initiation and Propagation

• The electrical changes occurring in one area of a PM, exposed to a threshold stimulus, allow propagation of the impulse along a nerve fiber.

• Threshold Potential: The value at which an action potential is produced; average is -55 mV.

Phases of Action Potential

• Depolarization / rising phase: Initiated by a threshold stimulus; voltage-gated Na+ channels open, Na+ flows in, and the membrane potential becomes more positive.

• Repolarization / falling phase: At +30 mV, voltage-gated K+ channels open, K+ flows out, restoring the negative membrane potential.

• After-hyperpolarization / after-potential: Excessive outflow of K+ causes a brief hyperpolarization before the membrane returns to resting levels.

Refractory Period

• Absolute refractory period: When Na+ is moving in, the Na+ channel cannot reset, and the cell cannot respond to another stimulus.

• Relative refractory period: When K+ is high and the cell is reestablishing RMP, Na+ channels can open again only in response to intense stimuli.

Synaptic Transmission at Chemical Synapses

Mechanism and Steps

• Electrical synapses are gap junctions between cells, found in cardiac and some smooth muscle cells.

• Depolarization of the axon terminus triggers opening of voltage-gated Ca2+ channels.

• Ca2+ influx causes fusion of vesicles with the PM via exocytosis.

• Neurotransmitter (e.g., ACh) is released into the synaptic cleft and binds its receptor on the target cell.

• Receptor binding opens ion channels, leading to depolarization and action potential transmission.

• NT is then destroyed by an enzyme (e.g., ACh via acetylcholinesterase) or reabsorbed back into the neuron.

Summary Table: Key Features of Neuronal Physiology