Ch. 3: Biological Processes: Communicating Internally: Connecting World and Brain
Major Types of Neurons and Neural Support
Neurons are individual cells in the nervous system that receive, transmit, and integrate information using electrochemical language—a combination of electrical and chemical processes. The nervous system relies on three primary types of neurons to bridge environmental inputs with internal processing and motor outputs:
Sensory Neurons: Make initial contact with the environment and carry incoming messages toward the spinal cord and brain. Environmental stimuli, such as heat, excite receptor regions on sensory neurons to initiate transmission.
Interneurons: Represent the most plentiful cell type in the nervous system. Interneurons make no direct contact with the external environment; instead, they transfer information between internal processing sites, such as conveying signals from sensory neurons to motor neurons or up to the brain.
Motor Neurons: Carry commands away from the central nervous system to muscles and glands that directly execute physical responses.
Automatic responses known as reflexes are governed primarily by spinal cord pathways and require no direct input from the brain. If the spinal cord is severed, communication with the brain is blocked—eliminating conscious pain perception—yet the reflex pathway remains functional, allowing physical responses like finger withdrawal to still occur.
Glial cells provide essential structural and metabolic support by removing waste, filling empty spaces, and facilitating efficient neural communication. While historical estimates suggested a glial-to-neuron ratio of , current evidence indicates the ratio is closer to (von Bartheld et al., 2016). Glial cells produce the myelin sheath, an insulating layer wrapped around axons that protects the cell and accelerates signal transmission. Additionally, glial cells modulate localized neuronal activity, contributing to learning and memory (De Pitta et al., 2016), body weight regulation (García-Cáceres et al., 2019), and pathologies such as brain cancer, Parkinson's disease, multiple sclerosis, schizophrenia, and bipolar disorder.
Structural Components of Neurons
Neurons possess four primary structural components designed for receiving, integrating, and transmitting signals:
Dendrites: Branch-like extensions that act as the primary receivers of incoming information from other neurons. A single neuron can feature thousands of dendritic branches.
Soma: The central cell body that contains genetic material and serves as the metabolic center. Received inputs are integrated and summed within the soma.
Axon: A long, single tail-like extension that transmits electrical signals known as action potentials toward target cells. The axon originates at a specialized trigger region called the axon hillock.
Terminal Buttons: Small swellings located at the branched ends of the axon that release chemical messengers into the extracellular space.
Neurons do not make physical contact with one another. Signals cross the synapse, a microscopic gap separating the terminal buttons of the transmitting neuron from the dendrites or soma of the receiving neuron.
Electrochemical Transmission and Neural Firing
Information flows through a neuron in a fixed, unidirectional sequence:
Communication occurs electrically within an individual neuron and chemically across the synapse between neurons.
Resting Potential: Unstimulated neurons maintain a stable electrical charge differential across their membrane, termed the resting potential. The intracellular fluid exhibits a negative charge between and relative to the extracellular environment. This differential is established by an uneven distribution of positively charged sodium () and potassium () ions, negatively charged chloride () ions, and large intracellular protein molecules. A membrane-bound sodium-potassium pump actively maintains high extracellular sodium/chloride concentrations and high intracellular potassium concentrations.
Action Potential Generation: Stimulation from adjacent neurons alters membrane permeability. Excitatory inputs cause depolarization by opening membrane channels to allow an influx of positive sodium ions, shifting the internal charge toward zero. Inhibitory inputs induce hyperpolarization by promoting potassium efflux or chloride influx, making the internal charge more negative.
Threshold and All-or-None Response: Excitatory and inhibitory potentials sum at the axon hillock. If net depolarization reaches a critical threshold, an action potential is triggered. Action potentials operate on an all-or-none basis; once initiated, the impulse travels completely down the axon at a constant magnitude regardless of stimulus intensity.
Conduction Speed: Axon diameter and myelination determine transmission speed, which ranges from to . Myelinated axons undergo saltatory conduction (derived from Latin saltare, "to jump"), in which the action potential jumps between uninsulated gaps in the myelin sheath called nodes of Ranvier.
Major Neurotransmitters and Their Functions
Arrival of an action potential at the terminal buttons prompts synaptic vesicles to release neurotransmitters into the synapse. These chemical messengers diffuse across the gap and bind to receptor sites on the postsynaptic membrane. The resulting response—excitatory or inhibitory—is dictated by the properties of the receptor molecule rather than the neurotransmitter itself.
Glutamate: The primary excitatory neurotransmitter in the brain, essential for fundamental cognitive functions including learning and memory.
Acetylcholine: The principal neurotransmitter at motor neuron-muscle junctions, triggering muscle contractions required for voluntary movement. Severe degradation of acetylcholine-producing cells is linked to cognitive decline in Alzheimer's disease (Kandimalla & Reddy, 2017).
Dopamine: A mixed-effect neurotransmitter involved in motor control, posture, sleep, and reward systems. Underproduction of dopamine leads to the motor deficits characteristic of Parkinson's disease, which is treated with the precursor L-dopa. Excessive or dysregulated dopamine activity is linked to schizophrenia (Maia & Frank, 2017; Schmack et al., 2018).
Serotonin: Primarily inhibitory, influencing sleep, dreaming, mood, appetite, and general arousal. Serotonin dysregulation is associated with clinical depression, obsessive-compulsive disorder, and autism spectrum disorder (C. L. Muller et al., 2016).
Gamma-Aminobutyric Acid (GABA): The primary inhibitory neurotransmitter responsible for dampening neural excitation and regulating anxiety.
Pharmacological Impacts on Neural Communication
Exogenous substances modify neural communication by functioning as agonists or antagonists:
Agonists: Facilitate or mimic neurotransmitter activity. Nicotine binds to acetylcholine receptors to stimulate bodily functions like heart rate. Cocaine functions as a dopamine agonist by inhibiting reuptake, causing dopamine to persist in the synapse.
Antagonists: Inhibit or block neurotransmitter activity. Pharmacological treatments for schizophrenia act as dopamine antagonists to reduce symptoms (Howes et al., 2015). Curare blocks acetylcholine receptors, resulting in muscle paralysis and asphyxiation. Botox blocks the release of acetylcholine to prevent local muscle contractions.
Endorphins: Endogenous morphinelike neuropeptides that function as internal analgesics. Discovered through brain receptor binding assays (Pert & Snyder, 1973; S. H. Snyder, 2017), endorphins are released during stress and physical exertion, playing a key role in pain mitigation and reinforcement phenomena like the runner's high (Pert, 2002; Szabo et al., 2019).
Neural Networks and Firing Dynamics
Complex behavior and mental processing result from coordinated activation patterns across vast networks of interconnected neurons rather than isolated cellular firing.
Firing Rates and Refractory Periods: Information intensity is encoded by a neuron's firing rate—the number of action potentials generated per unit of time. Following an impulse, a cell enters a brief refractory period during which it cannot generate subsequent action potentials. Many neurons display baseline spontaneous firing rates, allowing information to be encoded through both increases and decreases in firing frequency.
Artificial Neural Networks: Computational models simulating neural network connections demonstrate that distributed processing systems can tolerate localized node damage while maintaining functional output, mirroring the adaptive resilience of biological neural networks.