3.2 Cells of the Nervous System

Structure and Function of Neurons

Neurons are the specialized cells that make up your nervous system. They transmit electrical and chemical signals that underlie everything from reflexes to complex thought. Understanding how neurons are built and how they communicate is foundational to the rest of biopsychology.

Components of Neurons

Each neuron has a specific structure designed to receive, process, and send signals.

• Cell body (soma) — Contains the nucleus and organelles that keep the neuron alive and functioning. Think of it as the neuron's control center.
• Dendrites — Branch-like extensions that receive incoming signals from other neurons. More branching means the neuron can receive more input.
• Axon — A long, thin fiber that extends from the cell body and carries electrical impulses away from it, toward other neurons or target cells.
• Axon terminals — Small branches at the end of the axon. These contain synaptic vesicles, tiny sacs filled with chemical messengers called neurotransmitters (like dopamine or serotonin). The axon terminals are where the signal gets passed to the next cell.
• Myelin sheath — A fatty insulating layer that wraps around the axon and speeds up signal transmission. In the peripheral nervous system (PNS), myelin is produced by Schwann cells. In the central nervous system (CNS), it's produced by oligodendrocytes. Gaps in the myelin called nodes of Ranvier allow the signal to "jump" between them, which is why myelinated neurons conduct impulses much faster.

Process of Neural Communication

Neural communication involves both electrical signaling (within a neuron) and chemical signaling (between neurons).

Resting potential is the electrical charge difference across a neuron's membrane when it's not firing. This sits at about $-70 \text{ mV}$, meaning the inside of the neuron is slightly negative compared to the outside. The neuron is "at rest" but ready to fire.

Action potential is what happens when the neuron does fire. If incoming signals push the neuron past its threshold, a brief reversal of charge occurs: the inside of the membrane rapidly becomes positive, then returns to negative. This electrical impulse travels down the axon toward the axon terminals. Action potentials follow an all-or-none principle: the neuron either fires completely or doesn't fire at all. There's no "half" action potential.

Synaptic transmission is how the signal crosses the gap (called the synapse) between one neuron and the next. Here's how it works:

1. The action potential arrives at the axon terminal.
2. Voltage-gated calcium channels open, and calcium ions flow into the terminal.
3. The calcium influx causes synaptic vesicles to release neurotransmitters into the synaptic cleft (the tiny gap between neurons).
4. Neurotransmitters cross the cleft and bind to receptors on the postsynaptic neuron (the receiving neuron).
5. Depending on the type of neurotransmitter and receptor, the postsynaptic neuron either becomes more likely to fire (excitatory postsynaptic potential, or EPSP) or less likely to fire (inhibitory postsynaptic potential, or IPSP).

Neurotransmitter removal — After neurotransmitters have done their job, they need to be cleared from the synaptic cleft. Otherwise, the postsynaptic neuron would be stimulated nonstop. This happens through reuptake (the presynaptic neuron reabsorbs the neurotransmitter) or enzymatic degradation (enzymes in the cleft break the neurotransmitter down).

Psychoactive Substances and Neurotransmitter Systems

Many drugs and medications work by interfering with normal neurotransmitter activity. There are a few main ways this happens:

• Agonists bind to receptors and activate them, mimicking the effect of the natural neurotransmitter. For example, nicotine acts as an agonist at acetylcholine receptors, and morphine acts as an agonist at opioid receptors.
• Antagonists bind to receptors but don't activate them. They block the natural neurotransmitter from having its effect. Naloxone, for instance, is an opioid antagonist used to reverse overdoses: it blocks opioid receptors so morphine or heroin can't activate them.
• Reuptake inhibitors block the reuptake process, so neurotransmitters stay in the synaptic cleft longer and keep stimulating the postsynaptic neuron. SSRIs (selective serotonin reuptake inhibitors) like fluoxetine (Prozac) work this way to treat depression by increasing serotonin activity.
• Synthesis and degradation modulators change how much neurotransmitter gets produced or broken down. Levodopa is a precursor to dopamine used to treat Parkinson's disease (the brain converts it into dopamine). MAOIs prevent the enzyme monoamine oxidase from breaking down neurotransmitters like serotonin and norepinephrine, increasing their availability.

Organization of the Nervous System

Major Divisions

The nervous system has two main parts:

• Central nervous system (CNS) — The brain and spinal cord. This is where information is processed and integrated. It's the decision-making hub.
• Peripheral nervous system (PNS) — All the nerves outside the brain and spinal cord. The PNS carries signals between the CNS and the rest of your body: your organs, muscles, and glands.

Neuroplasticity

Neuroplasticity is the brain's ability to reorganize itself by forming new neural connections throughout life. This is why you can learn new skills, adapt to new environments, and sometimes recover function after brain injuries. Your brain isn't "fixed" after childhood; it keeps rewiring based on your experiences, though this ability does gradually decline with age.