Review Sheet 13: Neuron Anatomy and Physiology
Neurons, the fundamental units of the nervous system, are specialized cells responsible for transmitting information through electrical and chemical signals. Understanding their anatomy and physiology is critical to grasping how the brain processes sensory input, controls movements, and enables cognition. This review sheet breaks down the structure of neurons, their functional mechanisms, and the processes that enable communication within the nervous system.
Anatomy of a Neuron
Neurons are polarized cells with distinct regions that support their role in signal transmission.
- Cell Body (Soma): The central part of the neuron containing the nucleus and organelles necessary for survival and protein synthesis.
- Dendrites: Branch-like structures extending from the cell body that receive signals from other neurons. These inputs are integrated in the soma.
- Axon: A long, cable-like projection that transmits electrical impulses (action potentials) away from the cell body toward target cells.
- Myelin Sheath: A fatty insulating layer produced by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS) that wraps around the axon, speeding up signal conduction.
- Axon Terminals (Synaptic Knobs): Specialized endings of the axon that release neurotransmitters into synapses to communicate with other neurons or effector cells.
Key Variations:
- Unipolar Neurons: Found in sensory pathways; a single process extends from the cell body, dividing into a central axon and peripheral dendrite.
- Bipolar Neurons: Have two processes (one axon, one dendrite); rare and primarily in sensory systems (e.g., retina).
- Multipolar Neurons: Most common type, with multiple dendrites and one axon; dominate the brain and spinal cord.
Physiology of Neurons: How Signals Travel
Neurons communicate via electrical and chemical processes.
1. Resting Membrane Potential
At rest, neurons maintain a negative charge inside (-70 mV) due to:
- Ion Gradient: Higher potassium (K⁺) inside and sodium (Na⁺) outside the cell.
- Selectively Permeable Membrane: Leak channels allow K⁺ to diffuse out, while Na⁺ diffuses in slowly.
- Sodium-Potassium Pump: Actively transports 3 Na⁺ out and 2 K⁺ in, maintaining the gradient.
2. Action Potential
An action potential is a rapid, all-or-none electrical impulse.
Steps:
- Depolarization: A stimulus opens voltage-gated Na⁺ channels, allowing Na⁺ influx. The membrane potential reaches +30 mV.
- Repolarization: Na⁺ channels close, and K⁺ channels open, allowing K⁺ efflux. The membrane potential returns toward -70 mV.
- Hyperpolarization: Excess K⁺ efflux briefly makes the membrane more negative than resting potential.
- Refractory Period: A brief recovery phase where the neuron cannot fire another action potential immediately.
Key Players:
- Voltage-Gated Ion Channels: Open/close in response to membrane potential changes.
- Sodium-Potassium Pump: Restores ion balance after firing.
