Makindo Medical Notes

🧠 Anatomy of Neurons

Neurons are the fundamental units of the nervous system, responsible for transmitting electrical ⚡ and chemical 🧪 signals throughout the body. They form complex networks that allow sensation, movement, memory, and cognition. Each neuron has three main parts: the cell body, dendrites, and axon.

🔎 Parts of a Neuron

• Cell Body (Soma):
  • Contains the nucleus 🧬 and organelles (mitochondria, ribosomes, ER) for metabolism and protein synthesis.
  • Controls gene expression, essential for maintenance and repair.
  • Clinical Pearl: Loss of cell body integrity = neuronal death (seen in neurodegenerative diseases like Alzheimer’s).
• Dendrites:
  • Tree-like branches 🌳 that receive input from other neurons.
  • Conduct graded potentials toward the soma.
  • High dendritic branching = greater synaptic input (e.g., Purkinje cells in the cerebellum).
• Axon:
  • Long projection transmitting action potentials away from soma ➝ target cells.
  • Length varies (few mm ➝ over 1 m in motor neurons).
  • Ends in axon terminals where neurotransmitters are released.
  • Clinical Pearl: Axonal injury ➝ Wallerian degeneration distally.

🖼️ Images

⚙️ Specialized Structures of Neurons

• Myelin Sheath:
  • Fatty insulation produced by Schwann cells (PNS) and oligodendrocytes (CNS).
  • Speeds conduction via saltatory conduction at the Nodes of Ranvier.
  • Clinical Pearl: Demyelination (e.g., Multiple Sclerosis, Guillain-Barré) ➝ slowed or blocked conduction.
• Nodes of Ranvier:
  • Gaps between myelin segments where ion channels cluster.
  • Action potentials "jump" node-to-node ➝ rapid conduction.
• Axon Terminals:
  • Contain vesicles filled with neurotransmitters.
  • Release chemicals into the synaptic cleft ➝ signal to next neuron, muscle, or gland.
  • Key site for drug action (e.g., SSRIs act at serotonin synapses).

🔀 Types of Neurons

Neurons are classified by function: sensory, motor, or interneurons.

• 👁️ Sensory (Afferent) Neurons:
  • Carry info from receptors (skin, eyes, ears) ➝ CNS.
  • Convert stimuli ➝ electrical signals.
  • Example: retinal photoreceptors for vision.
• 💪 Motor (Efferent) Neurons:
  • Transmit CNS signals ➝ muscles & glands.
  • Control voluntary (skeletal) and involuntary (smooth, cardiac) activity.
  • Clinical Pearl: Motor neuron death = ALS (amyotrophic lateral sclerosis).
• 🧩 Interneurons:
  • Connect sensory and motor neurons within CNS.
  • Responsible for reflexes, decision-making, and higher cognition.
  • Make up ~99% of neurons in CNS.

⚡ Functions of Neurons

• Signal Transmission: Carry electrical impulses and chemical messages ➝ movement, sensation, thought.
• Synaptic Transmission: Release neurotransmitters at synapses ➝ stimulate next neuron/target.
• Integration: Summation of excitatory & inhibitory inputs at axon hillock determines action potential firing.
• Plasticity: Adaptation & rewiring in response to experience or injury ➝ basis of learning & memory.

🧪 Neurotransmitters

Neurotransmitters are the “chemical language” of the brain.

• Excitatory: 🔥 Glutamate, Acetylcholine ➝ depolarize target cells.
• Inhibitory: ❄️ GABA, Glycine ➝ hyperpolarize target cells.
• Modulatory: 🎭 Dopamine, Serotonin, Norepinephrine ➝ alter tone, mood, attention.
• Clinical Pearl: Dopamine loss in substantia nigra ➝ Parkinson’s disease.

📉 Nerve Impulse (Action Potential)

• Resting Potential: –70 mV due to Na⁺/K⁺ gradients.
• Depolarization: Na⁺ influx ➝ inside becomes positive.
• Repolarization: K⁺ efflux ➝ restores negativity.
• Refractory Period: Prevents back-propagation, ensures one-way conduction.
• Clinical Pearl: Local anaesthetics (e.g., lidocaine) block Na⁺ channels ➝ prevent action potentials.