Biological Psychology: Neurons, Neurotransmitters, and the Endocrine System

Biological Foundations of Psychology

• Principle of Biological Psychology:

  • Everything psychological is biological.

  • Moods, ideas, impulses, sensory experiences, and memories are spurred by underlying biological conditions, biological structures, and chemical processes.

• Physiological and Chemical Reaction to Danger and Fear:

  • Scenario: Being home alone late at night, drifting off to sleep while dreaming about Fritos, followed by a sudden loud banging at the door.

  • Immediate Biological Response: The brain releases an icy typhoon of chemicals, causing an immediate state of hyper-arousal where the heart feels like it is going to explode.

  • Behavioral Impulses: Triggers an immediate urge to flee out the back door or grab a Phillips head screwdriver to stab into the darkness until it sticks into something.

  • Cognitive Processing: Prompts rapid internal evaluation of potential threats—whether it is a Weeping Angel or a neighbor looking to borrow a can of beans—as well as erratic thoughts such as "Woah! Where's the cat?".

  • Chemical Basis: All resulting urges to defend, flee, or evaluate reality stem directly from chemical secretions bathing the brain and nervous system.

Neurons: Structure and Function

• Definition and Role of Neurons:

  • Neurons (nerve cells) are the fundamental building blocks that comprise the nervous system.

  • Possess specialized electrochemical capabilities that allow them to transmit messages across the body.

  • The human brain alone contains billions of individual neurons.

• Size Variations:

  • Range from micro-structures less than $1\,\text{mm}$ in length within the brain to long cells running the entire length of a human leg.

  • Comparison: Prehistoric dinosaurs possessed nerve cells extending up to $150\,\text{feet}$ long.

• Anatomy of a Neuron:

  • Soma (Cell Body):

  • Functions as the neuron's life support system.

  • Houses essential cellular structures, including the nucleus, DNA, mitochondria, and ribosomes.

  • If the soma dies, the entire neuron perishes.

  • Dendrites:

  • Short, bushy, tree-like extensions that receive chemical messages and gossip from neighboring cells.

  • Act as the "listeners" of the neuron, whispering received signals back to the soma.

  • Axon:

  • Long, cable-like extension that acts as the "talker" of the neuron.

  • Transmits electrical impulses away from the soma toward other neurons, muscles, or glands.

  • Encased in many neurons by the myelin sheath, a protective layer of fatty tissue that insulates the axon and accelerates signal transmission speed.

  • Myelin Sheath Degradation: Degeneration of the myelin sheath occurs in conditions such as multiple sclerosis, causing degraded neural signals and subsequent loss of muscle control.

Neural Impulse Transmission and Synaptic Communication

• Action Potential Generation:

  • Neurons transmit signals when stimulated by sensory input or triggered by neighboring neurons.

  • Dendrites pick up external signals and trigger the neuron's action potential (firing impulse).

  • The action potential shoots an electrical charge down the length of the axon to its terminal endings toward neighboring neurons.

• Synapses and the Synaptic Gap:

  • Synapses: The specialized contact points between neighboring neurons.

  • Dendrites are decorated with synapses that closely approach but do not physically touch adjacent axon terminals.

  • Synaptic Gap: The microscopic cleft separating an axon terminal from a receiving dendrite, measuring less than $\frac{1}{1000000}\,\text{inch}$ (less than a millionth of an inch).

• Neurotransmitter Release and Reuptake:

  • Arrival of an action potential at an axon terminal activates chemical messengers called neurotransmitters.

  • Neurotransmitters jump across the microscopic synaptic gap and land on specific receptor sites on the receiving neuron, fitting precisely like a key into a lock.

  • Binding excites or inhibits the receiving neuron's firing trigger.

  • Reuptake: Unbound or excess neurotransmitters detach from receptors and are immediately reabsorbed by the sending neuron that originally released them.

Classification and Function of Neurotransmitters

• General Functions:

  • Neurotransmitters drive physical movement (such as making jazz hands), emotional states, learning, feeling, memory, alertness, and sleep.

  • Endorphins: Pleasure-inducing neurotransmitters flooded in the body after physical exertion (such as running $10\,\text{miles}$), falling in love, or eating a piece of pie.

  • The human body utilizes over $100$ distinct types of neurotransmitters.

• Excitatory Neurotransmitters:

  • Rev up neural activity, increasing the probability that a receiving neuron will fire an action potential.

  • Norepinephrine: Controls alertness and arousal.

  • Glutamate: Primary neurotransmitter involved in memory processes. An over-supply can overstimulate the brain, leading to seizures or migraines (explaining sensitivity to monosodium glutamate / MSG in foods like Ramen).

• Inhibitory Neurotransmitters:

  • Calm neural activity, decreasing the probability of an action potential firing.

  • GABA (gamma-aminobutyric acid): Major inhibitory neurotransmitter in the central nervous system.

  • Serotonin: Regulates mood, hunger, and sleep. Deficits in serotonin are linked to clinical depression; a specific class of antidepressant medications functions by raising brain serotonin levels.

• Dual-Action Neurotransmitters:

  • Act as either excitatory or inhibitory depending on the specific receptor types encountered.

  • Acetylcholine: Enables muscle action and influences learning and memory. Deterioration of acetylcholine-producing neurons occurs in Alzheimer's disease patients.

  • Dopamine: Associated with learning, movement, and pleasurable emotions. Excessive dopamine activity is linked to schizophrenia, as well as addictive and impulsive behavior.

The Endocrine System and Hormonal Regulation

• Hormones vs. Neurotransmitters:

  • Hormones are chemical messengers produced by the endocrine system that act on the brain and peripheral tissues.

  • Some hormones are chemically identical to specific neurotransmitters.

  • Regulate key biological functions: mood, arousal, circadian rhythms, metabolism, immune function monitoring, growth signaling, and sexual reproduction.

  • Primary functional drives center on attraction, appetite, and aggression.

• Speed of Transmission:

  • Nervous System: High-speed communication using rapid electrical impulses and synaptic chemical transfers (analogous to sending a text message).

  • Endocrine System: Slow chemical communication where glands secrete hormones directly into the bloodstream to be ferried to tissues and the brain (analogous to writing a letter, licking a stamp, addressing it, and sending it through the Post Office).

  • Duration: Hormonal signals linger in the body much longer than neural signals, explaining why it takes extended time to calm down following intense fright or anger.

• Major Endocrine Glands:

  • Adrenal Glands:

  • Located adjacent to the kidneys.

  • Secrete adrenaline (epinephrine) during fight-or-flight emergency responses.

  • Adrenaline increases heart rate, blood pressure, and blood sugar levels, mobilizing energy to flee or engage threats (such as punching a charging baboon in the throat).

  • Pancreas:

  • Situated next to the adrenal glands.

  • Secretes insulin and glucagon to monitor sugar absorption, maintaining the body's primary fuel source.

  • Thyroid and Parathyroid Glands:

  • Located at the base of the throat.

  • Thyroid hormones regulate metabolism; parathyroid hormones monitor body calcium levels.

  • Gonads:

  • Testicles secrete male sex hormones, including testosterone and estrogen.

  • Ovaries secrete female sex hormones, including estrogen and progesterone.

  • Pituitary Gland:

  • Pea-sized structure located deep within the bunker of the brain.

  • Functions as the master gland of the endocrine system, releasing secretions that direct all other endocrine glands.

  • Secretes growth hormone (drives physical development) and oxytocin (promotes warm feelings of trust and social bonding).

  • Governed directly by the hypothalamus region of the brain.

Interconnected Feedback Loops and Emergency Responses

• Integrated Cascade During Fright:

  1. Sensory input (visual and auditory) is gathered by the eyes and ears and routed directly to the brain's hypothalamus without immediate conscious cognitive analysis.

  2. The hypothalamus sends signals down the chain of command to the pituitary gland.

  3. The pituitary gland signals the adrenal glands to secrete adrenaline into the bloodstream.

  4. Adrenaline circulates throughout the body, raising heart rate, blood pressure, and blood sugar levels.

  5. Sensory and systemic feedback returns to the brain, allowing cognitive re-evaluation (recognizing a false alarm) and prompting neural instructions to restore calm.

• System Loop Architecture:

  • The interaction between the nervous and endocrine systems operates as a continuous feedback loop.

  • Directional flow: Brain $\rightarrow$ Gland $\rightarrow$ Hormone $\rightarrow$ Brain.