Neural Structure, Action Potentials, and Neurotransmitters

Neural Structure, Action Potentials, and Neurotransmitters

Biological psychology, or biopsychology, explores the complex link between biological processes and behavior. Every thought, emotion, sensation, and action is rooted in electrical signals and chemical reactions within the nervous system. Understanding the biological foundations of behavior requires examining the microscopic building blocks of the brain: neurons.

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Anatomy of the Neuron

The neuron is the specialized cell responsible for receiving, processing, and transmitting information throughout the body. The human brain contains approximately 86 billion neurons, supported by glial cells (gliocytes), which provide structural support, insulation, nutrition, and waste removal.

  [Dendrites] --> [Soma / Cell Body] --> [Axon (Myelin Sheath & Nodes)] --> [Terminal Buttons] --> Synapse

A typical neuron consists of six primary structural components:

1. Dendrites: Branch-like extensions radiating from the cell body that receive chemical signals (neurotransmitters) from neighboring neurons and conduct electrical impulses toward the soma.
2. Soma (Cell Body): The central processing core of the neuron containing the cell nucleus, cytoplasm, and organelles. It integrates incoming signals from dendrites and maintains cellular metabolic health.
3. Axon: A long, single tube-like fiber extending from the soma that conducts electrical impulses away from the cell body toward terminal buttons. Axons range from less than a millimeter to over three feet in length.
4. Myelin Sheath: A fatty layer of glial tissue (produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system) that wraps around the axon. Myelin acts as an electrical insulator, dramatically increasing the speed of signal transmission.
5. Nodes of Ranvier: Microscopic gaps in the myelin sheath along the axon. Electrical impulses jump from node to node in a process called saltatory conduction, speeding up neural transmission.
6. Terminal Buttons (Axon Terminals): Small knob-like structures at the end of axon branches. They store synaptic vesicles filled with chemical messengers (neurotransmitters) ready for release into the synaptic gap.

Clinical Relevance — Multiple Sclerosis (MS): MS is an autoimmune disorder in which the body's immune system attacks and destroys the myelin sheath surrounding central nervous system axons. Demyelination slows or blocks neural transmission, leading to sensory loss, muscle weakness, coordination failure, and cognitive impairments.

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The Neural Impulse: Electrical Communication Within Neurons

Communication within a single neuron is electrical and depends on the movement of electrically charged ions (sodium $\text{Na}^+$, potassium $\text{K}^+$, and chloride $\text{Cl}^-$) across the semipermeable neuronal membrane.

1. Resting Potential (-70 mV)

When a neuron is inactive or at rest, the interior of the axon carries a net negative electrical charge relative to the extracellular fluid outside. This stable negative charge is known as the resting potential, typically -70 millivolts (-70 mV). The membrane is polarized.

• High concentration of sodium ions ($\text{Na}^+$) outside the neuron.
• High concentration of potassium ions ($\text{K}^+$) and negatively charged proteins inside the neuron.
• Maintained by active sodium-potassium pumps, which continuously pump $3\text{ Na}^+$ ions out for every $2\text{ K}^+$ ions pumped in.

2. Threshold and Depolarization

When dendrites receive chemical signals from adjacent neurons, ion channels open, allowing positive sodium ions ($\text{Na}^+$) to enter the cell. If incoming excitatory signals push the membrane potential past a critical threshold of excitation—typically -55 mV—voltage-gated sodium channels open rapidly.

• Depolarization: The influx of $\text{Na}^+$ ions causes the interior charge to rapidly change from negative to positive, peaking at approximately +30 mV.

3. Action Potential and the All-or-None Law

The rapid electrical reversal (+30 mV) that travels down the length of the axon is called the action potential.

• All-or-None Law: A neuron either fires completely or does not fire at all. Increasing the strength of a stimulus beyond the threshold does not produce a larger or faster action potential. Instead, a stronger stimulus increases the frequency or rate of neuronal firing across a population of neurons.

4. Repolarization and Refractory Periods

Immediately after reaching peak positive voltage (+30 mV), sodium channels close and voltage-gated potassium channels open, allowing $\text{K}^+$ ions to flow out of the cell, restoring the internal negative charge (repolarization).

• Absolute Refractory Period: Brief period immediately following an action potential during which sodium channels are inactivated. The neuron cannot fire another action potential regardless of stimulus strength.
• Relative Refractory Period: Following the absolute refractory period, hyperpolarization occurs (charge drops temporarily below -70 mV). The neuron can fire again, but only if stimulated by an extraordinarily strong signal.

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Synaptic Transmission: Chemical Communication Between Neurons

Communication between neurons occurs across microscopic gaps called synapses (synaptic clefts). When the action potential reaches the terminal buttons, it triggers the release of neurotransmitters.

1. Vesicle Fusion and Exocytosis: The electrical action potential causes synaptic vesicles in the terminal button to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.
2. Receptor Binding: Neurotransmitters diffuse across the cleft and bind to specific receptor sites on the postsynaptic neuron's dendrites in a lock-and-key mechanism.
3. Postsynaptic Potentials: Binding generates either an Excitatory Postsynaptic Potential (EPSP) (depolarizing, making firing more likely) or an Inhibitory Postsynaptic Potential (IPSP) (hyperpolarizing, making firing less likely).
4. Termination of Signal: Neurotransmitters cannot remain bound to receptors indefinitely. Signal termination occurs through:
   • Reuptake: The presynaptic terminal button reabsorbs excess neurotransmitters back into the cell for repackaging or destruction.
   • Enzymatic Degradation: Specialized enzymes break down neurotransmitter molecules in the synaptic cleft (e.g., Acetylcholinesterase breaks down Acetylcholine).

Neuropharmacology: Agonists vs. Antagonists

Drugs and foreign substances affect behavior by modifying synaptic transmission:

• Agonists: Substances that enhance or mimic the action of a neurotransmitter. They can bind directly to receptors to activate them, block reuptake (e.g., SSRIs), or increase neurotransmitter synthesis.
• Antagonists: Substances that inhibit or block the normal action of a neurotransmitter. They bind to receptors without activating them, physically preventing the endogenous neurotransmitter from binding (e.g., Curare or Haloperidol).

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Major Neurotransmitters and Associated Disorders