Neural cell Fact File
Structure and Function of a Neuron
Key Components:
• Dendrites: Branch-like extensions that receive electrical signals from other
neurons. They increase the surface area for communication.
• Cell Body (Soma): Contains the nucleus and organelles. Processes incoming
signals and initiates the neuron's response.
• Axon: A long, singular projection that carries the electrical impulse away from
the cell body toward the axon terminals.
• Myelin Sheath: Fatty insulation around the axon, produced by glial cells
(oligodendrocytes in CNS, Schwann cells in PNS). Increases speed of impulse
conduction.
• Nodes of Ranvier: Gaps in the myelin where action potentials are regenerated,
allowing saltatory conduction.
• Axon Terminals: Small branches at the end of the axon that release
neurotransmitters into the synaptic cleft.
• Synapse: The junction between neurons where neurotransmitter-based
communication occurs.
Types of Neurons
1. Sensory Neurons (Afferent)
• Carry signals from sensory receptors to CNS.
• Long dendrites, short axons.
• Example: Touch receptors in the skin sending signals to the spinal cord.
2. Motor Neurons (Efferent)
• Carry instructions from CNS to effectors (muscles/glands).
• Long axons, short dendrites.
• Example: Stimulating muscle contraction in the leg.
3. Interneurons (Relay Neurons)
• Connect neurons within CNS.
• Integrate, process, and relay information.
• Example: Spinal cord interneurons mediating reflex arcs.
Glial Cells and Their Functions
1. Astrocytes
• Star-shaped, maintain blood-brain barrier.
• Regulate nutrient and ion flow to neurons.
• Recycle neurotransmitters.
2. Oligodendrocytes
• Found in the CNS.
• Form myelin sheath for multiple axons simultaneously.
3. Schwann Cells
• Found in the PNS.
• Myelinate one axon segment per cell.
4. Microglia
• Act as immune cells in the CNS.
• Phagocytose pathogens and debris.
• Involved in neuroinflammation.
5. Ependymal Cells
• Line brain ventricles and spinal cord.
• Produce and circulate cerebrospinal fluid (CSF).
The Action Potential
Resting Potential:
• Neuron at rest: ~ -70mV.
• Maintained by Na⁺/K⁺ pump and leaky K⁺ channels.
Depolarization:
• Stimulus opens voltage-gated Na⁺ channels.
• Na⁺ rushes in, membrane potential becomes positive.
Repolarization:
• Na⁺ channels close, K⁺ channels open.
• K⁺ exits, restoring negative potential.
Hyperpolarization & Refractory Period:
• Membrane becomes briefly more negative than resting.
• Prevents immediate firing again.
All-or-None Principle:
• A threshold (~ -55mV) must be reached to fire.
• Stronger stimuli = higher frequency, not stronger signals.
Synaptic Transmission and Neurotransmitters
1. Electrical Signal to Chemical Signal:
• Action potential reaches axon terminal.
• Triggers Ca²⁺ influx via voltage-gated channels.
• Vesicles release neurotransmitters into synaptic cleft.
2. Neurotransmitter Action:
• Bind to receptors on postsynaptic membrane.
o Excitatory (e.g., glutamate): Depolarizes next neuron.
o Inhibitory (e.g., GABA): Hyperpolarizes, reducing likelihood of firing.
3. Clearance:
• Enzymatic breakdown (e.g., acetylcholinesterase).
• Reuptake into presynaptic neuron.
• Diffusion away from cleft.
Major Neurotransmitters and Their Roles
Neurotransmitter Type Function Example System
Main excitatory transmitter in
Glutamate Excitatory Learning & memory
CNS
GABA Inhibitory Reduces neuronal excitability Anxiety regulation
Parkinson’s,
Dopamine Modulatory Reward, motivation, movement
addiction
Serotonin Modulatory Mood, sleep, appetite Depression, SSRIs
Acetylcholine Excitatory Muscle contraction, learning Alzheimer’s disease
