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Structure and Functions of Neurons | BA Psychology Notes

Study material and explanations for Unit 2

Structure and Functions of Neurons

The human brain is the most complex biological computer in the known universe. Containing approximately 86 to 100 billion specialized cells, the fundamental structural and functional building block of this communication network is the Neuron (nerve cell).

Unlike other bodily cells (like skin or liver cells), neurons are specialized to generate, transmit, and process rapid electrochemical signals across microscopic and macroscopic distances.

1. Anatomy of a Typical Neuron

A standard multipolar neuron consists of four major structural components:

Anatomy and Signal Flow in a Typical Neuron
01
Dendrites
Branching tree-like fibers that receive incoming electrochemical chemical signals from adjacent neurons.
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02
Soma (Cell Body)
Metabolic hub containing nucleus, mitochondria, and cytoplasm. Integrates incoming electrical inputs.
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03
Axon & Myelin
Long conducting cable carrying action potentials away from soma. Insulated by fatty myelin sheath.
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04
Terminal Buttons
Knob-like endings containing synaptic vesicles that release neurotransmitters into the synapse.

2. In-Depth Component Analysis

A. The Dendrites (Receiving Antennae)

  • Tree-like branching projections covered in tiny bumps called Dendritic Spines.
  • Function: Provide a massive surface area to receive chemical neurotransmitter messages from thousands of neighboring presynaptic neurons.

B. The Soma / Perikaryon (Metabolic Command Center)

  • Contains the Cell Nucleus (housing genetic DNA) and organelles (Mitochondria for ATP energy, Ribosomes for synthesizing protein and neurotransmitters).
  • Contains the Axon Hillock: a conical neck junction where electrical charges accumulate and the decision to fire an action potential is calculated.

C. The Axon (Transmission Cable)

  • A single, slender cylindrical fiber extending from the soma to target cells (can range from a fraction of a millimeter in the brain to over 1 meter long, such as the sciatic nerve extending down to your toes).
  • Function: Conducts the electrical nerve impulse (Action Potential) away from the soma.

D. The Myelin Sheath and Nodes of Ranvier

  • Myelin Sheath: A white, fatty lipid insulation wrapped around the axon in segments. Produced by Schwann cells in the Peripheral Nervous System and Oligodendrocytes in the Central Nervous System.
  • Function: Speeds up neural transmission up to 100 times via Saltatory Conduction (the nerve impulse leaps from one bare gap to the next).
  • Nodes of Ranvier: Uninsulated microscopic gaps between myelin segments where sodium and potassium ion channels are densely concentrated.
  • Clinical Note: In Multiple Sclerosis (MS), the body’s immune system destroys the myelin sheath, causing sluggish neural transmission, muscle weakness, and loss of coordination.

E. Terminal Buttons / Synaptic Knobs

  • Swollen endings at the tips of telodendria (axon branches).
  • House microscopic membrane sacs called Synaptic Vesicles, which store neurotransmitter molecules awaiting release.

3. Classification of Neurons

Functional and Structural Classifications of Neurons
Functional Classification (By Signal Direction)
  • 1. Sensory (Afferent) Neurons: Carry incoming sensory impulses from physical receptors (eyes, ears, skin) inward to the Central Nervous System.
  • 2. Motor (Efferent) Neurons: Carry outgoing instructions from the CNS outward to muscles and glands to execute physical actions.
  • 3. Interneurons (Association Neurons): Found exclusively in the brain and spinal cord. Connect sensory and motor neurons, analyze data, and perform complex cognitive thinking (make up >99% of all neurons).
Structural Classification (By Number of Extensions)
  • 1. Multipolar Neurons: One axon and numerous dendrites. Most common type in brain and spinal motor pathways.
  • 2. Bipolar Neurons: One axon and one dendritic branch exiting opposite ends of soma. Rare, specialized sensory cells (retina of eye, olfactory epithelium).
  • 3. Unipolar / Pseudounipolar: A single process exiting soma that splits into two branches. Common in primary sensory ganglia.

4. Neuroglia (Glial Cells): The Unsung Heroes of the Nervous System

Neurons make up only about half the volume of the brain. The other half consists of Glial Cells (from the Greek word for “glue”), which provide structural, nutritional, and protective support:

Major Types of Glial Cells and Their Essential Duties
1. Astrocytes (Star-Shaped) Anchor neurons to nutrient capillaries, regulate extracellular chemical balance, and form the protective Blood-Brain Barrier (BBB) keeping toxins out of the brain.
2. Oligodendrocytes (Myelin Makers (CNS)) Extend arms to wrap multiple insulating myelin sheaths around central nervous system axons in the brain and spinal cord.
3. Microglia (Immune Defenders) The brain's resident immune scavengers. Phagocytize cellular debris, dead neurons, and invading bacterial pathogens.
4. Schwann Cells (Myelin Makers (PNS)) Form the protective myelin sheath around individual peripheral nerve fibers outside the brain and spine, aiding nerve regeneration.