Biological Psychology: Nerve Cells and Nerve Impulses

Introduction to Biological Psychology

Your mental experiences, thoughts, and behaviors are fundamentally dependent on the intricate activity of a vast number of separate, yet interconnected, cells within your nervous system.
Biological psychology seeks to understand this complex interplay by examining the cellular components and their electrochemical processes.

The human nervous system is primarily composed of two fundamental types of cells:
- Neurons: The primary functional units responsible for transmitting information.
- Glia (Neuroglia): Support cells that assist neurons in various ways.
The human brain alone contains an astonishing approximately 86 ext{ billion} individual neurons.
Distribution of Neurons:
- Cerebral cortex and associated areas: 12 ext{ to } 15 ext{ billion} neurons.
- Spinal cord: 1 ext{ billion} neurons.
- Cerebellum: 70 ext{ billion} neurons.

In the late 1800s, the Spanish neuroscientist Santiago Ramón y Cajal (1852-1934) made a groundbreaking discovery that revolutionized the understanding of the nervous system.
He was the first to conclusively demonstrate that individual cells comprising the nervous system remain distinct and separate entities.
This contradicted the prevailing belief at the time, which suggested that nerve cells merged directly into one another, forming a continuous network. Cajal's work established the fundamental principle of the neuron doctrine.

While neurons share many common structures with other animal cells, they possess a highly distinctive shape that is specialized for information processing and transmission.

Motor Neuron:
- Has its cell body (soma) located in the spinal cord.
- Receives excitatory signals from other neurons.
- Conducts nerve impulses along its axon to stimulate a muscle or a gland, initiating movement or secretion.
Sensory Neuron:
- Highly specialized at one end to detect particular types of stimulation.
- These specialized endings are sensitive to specific sensory modalities such as touch, light, sound, taste, or pain, converting them into electrical signals.

All neurons, regardless of their specific type, generally consist of four main components:
1. Dendrites
2. Soma/Cell Body
3. Axon
4. Presynaptic Terminals

Dendrites are branching fibers that extend from the cell body.
Their surface is lined with synaptic receptors, which are specialized proteins designed to receive chemical signals (neurotransmitters) from other neurons.
Dendrites are primarily responsible for bringing information into the neuron.
Some dendrites also feature small, mushroom-shaped outgrowths called dendritic spines.
- These spines further branch out, significantly increasing the surface area of the dendrite.
- A greater dendritic surface area allows the neuron to form more synaptic connections and, consequently, receive more information from other neurons.

The soma, or cell body, is the central part of the neuron.
It contains the nucleus, mitochondria, and ribosomes, as described earlier.
The soma is responsible for the metabolic work of the neuron, carrying out essential life-sustaining functions.
In many neurons, the surface of the soma is also covered with synapses, allowing it to receive direct input from other neurons.

An axon is a long, thin fiber that extends from the cell body.
Its primary function is to transmit nerve impulses away from the cell body toward other neurons, organs, or muscles.
Many vertebrate axons are covered by a myelin sheath, an insulating material.
- This sheath is composed of fats and proteins and significantly speeds up impulse conduction.
- The myelin sheath is not continuous but has interruptions known as nodes of Ranvier.
At the end points of an axon are presynaptic terminals (also called terminal boutons or axon terminals).
- These specialized structures are responsible for releasing chemicals (neurotransmitters) across the synaptic cleft to communicate with other neurons or effector cells.

To clarify the direction of information flow relative to a given structure, specific terms are used:
- Afferent axon: An axon that brings information into a structure (e.g., a sensory neuron carrying information from the periphery to the central nervous system).
- Efferent axon: An axon that carries information away from a structure (e.g., a motor neuron carrying commands from the central nervous system to a muscle).
- Interneurons (or Intrinsic Neurons): Neurons whose dendrites and axons are completely contained within a single local structure (e.g., within a particular brain region or the spinal cordsegment), facilitating local processing.

Neurons exhibit significant variations in their size, shape, and function.
The unique shape of a neuron is crucial, as it determines its connectivity with other neurons and its specific contribution to the overall functioning of the nervous system.
The function of a neuron is closely related to its shape.
- Example: Purkinje cells, found in the cerebellum, possess an extraordinarily wide and flat dendritic tree that branches extensively within a single plane, allowing them to integrate a vast array of inputs.