Neurons Knowledge-Based Study

Study notes

1. Overview of Neurons

   Neurons are highly specialized cells that serve as the fundamental structural and functional units of the nervous system. Their primary function is to receive, integrate, and transmit information throughout the body via electrical impulses and chemical signals.

   Key Structural Components

   • Cell Body (Soma): Contains the nucleus and essential cellular organelles such as mitochondria, ribosomes, and the Golgi apparatus. It carries out general metabolic functions and synthesizes neurotransmitters.
   • Dendrites: Highly branched, tapering extensions projecting from the soma. Dendrites act as the receptive field, receiving incoming signals from sensory receptors or neighboring neurons and conducting them toward the cell body.
   • Axon: A single, elongated fiber arising from a specialized region of the soma called the axon hillock. The axon conducts nerve impulses away from the cell body toward target cells.
   • Myelin Sheath: An insulating lipid-rich layer that encloses many axons in both the peripheral and central nervous systems (formed by Schwann cells and oligodendrocytes, respectively). It dramatically accelerates the speed of impulse conduction.
   • Nodes of Ranvier: Regularly spaced, unmyelinated gaps along the axon where ion exchange occurs, enabling rapid electrical propagation.
   • Axon Terminals: Specialized, branched endings of the axon that contain synaptic vesicles filled with neurotransmitters, forming junctions with other cells.

   Classification of Neurons

   Neurons can be classified according to their structural characteristics or their physiological roles within neural pathways.

   Functional Categories

   • Sensory (Afferent) Neurons: Carry impulses from sensory receptors (such as light, touch, or temperature receptors) toward the central nervous system (CNS).
   • Motor (Efferent) Neurons: Transmit impulses away from the CNS to effector organs, such as muscles and endocrine or exocrine glands, triggering a physiological response.
   • Interneurons (Association Neurons): Located entirely within the central nervous system, interneurons connect sensory and motor pathways and process, integrate, and interpret incoming information.

   Direction of Electrical Flow

   Within an individual neuron, the standard pathway for electrical transmission is strictly unidirectional: incoming signals are received at the dendrites, integrated at the cell body (soma) and axon hillock, propagated along the axon, and discharged at the axon terminals.

   Resting Membrane Potential

   When a neuron is not transmitting a signal, its plasma membrane maintains a voltage difference known as the resting membrane potential (typically around -70 mV). This electrical polarization is primarily established and maintained by:

   • Active transport via the sodium-potassium pump ($Na^+/K^+$ ATPase), which continuously pumps three $Na^+$ ions out of the cell for every two $K^+$ ions pumped in.
   • Differential membrane permeability, as resting potassium leak channels allow $K^+$ to diffuse out of the cell more freely than $Na^+$ can enter.
   • The presence of trapped, negatively charged intracellular proteins and phosphate ions.

   Action Potential and Propagation

   When a stimulus depolarizes the axon hillock past a critical level known as the threshold potential (typically around -55 mV), voltage-gated ion channels open in sequence:

   1. Depolarization: Voltage-gated $Na^+$ channels open rapidly, allowing a massive influx of $Na^+$ ions, causing the interior of the cell to become positively charged (reaching approximately +30 mV).
   2. Repolarization: Voltage-gated $Na^+$ channels inactivate, and slower voltage-gated $K^+$ channels open, allowing $K^+$ to rush out of the cell and restoring negative membrane polarity.
   3. Hyperpolarization: Extended $K^+$ outflow briefly makes the membrane more negative than the resting level before resting potential is restored.

   Impulses follow the all-or-none principle: a stimulus either triggers a complete action potential if it reaches threshold, or none at all. In myelinated neurons, action potentials jump from one node of Ranvier to the next, a process called saltatory conduction, which is far faster and more energy-efficient than continuous conduction along unmyelinated fibers.

   The Synapse

   A synapse is a specialized junction through which a neuron communicates with an adjacent target cell, such as another neuron, muscle fiber, or gland cell. Most physiological synapses in the human nervous system are chemical synapses.

   Steps in Chemical Transmission

   1. Arrival of Action Potential: An electrical impulse reaches the presynaptic axon terminal, causing local depolarization of the terminal membrane.
   2. Calcium Influx: Depolarization opens voltage-gated $Ca^{2+}$ channels, driving an influx of calcium ions into the presynaptic terminal.
   3. Exocytosis of Neurotransmitters: Elevated intracellular calcium triggers synaptic vesicles to fuse with the presynaptic membrane, releasing chemical messengers called neurotransmitters into the microscopic gap known as the synaptic cleft.
   4. Receptor Binding: Neurotransmitters diffuse across the synaptic cleft and bind specifically to receptor proteins located on the postsynaptic membrane.
   5. Postsynaptic Response: Receptor binding opens ion channels, producing either an excitatory postsynaptic potential (depolarization) or an inhibitory postsynaptic potential (hyperpolarization).
   6. Termination of Signal: The neurotransmitter is rapidly cleared from the cleft via enzymatic degradation, active reuptake by presynaptic transport proteins, or diffusion away from the synapse.

   Because neurotransmitter vesicles are confined to the presynaptic terminal and functional receptors are situated predominantly on the postsynaptic membrane, chemical transmission across a synapse is strictly unidirectional.