Chapter 2 Notes: Biology and Psychology
Introduction and real-world relevance
Film reference: In Concussion, Will Smith portrays Bennet Amalu, who performed an autopsy on Mike Webster, a former Pittsburgh Steelers center.
Findings: Webster exhibited bizarre behaviors; tau proteins were found in his brain, which killed cells in areas linked to executive functions (problem-solving, decision making), mood, and emotions.
Broader pattern: The pattern of brain degeneration seen in Webster’s brain has been found in donated brains of many football players, leading to the diagnosis Chronic Traumatic Encephalopathy (CTE).
Clinical effects of CTE: dementia, memory loss, impaired reasoning, mood disturbances, and depression severe enough to contribute to suicide.
Epidemiology in football players: Mayo Clinic study found CTE in 21 of 66 men with exposure to contact sports (e.g., football) and absent in 198 men without such exposure; Mez et al., 2017 reported CTE in 177 of 202 former football players.
Real-world response: The National Football League (NFL) faced fallout, initially resisting, then introducing protective helmet improvements and concussion protocols to rest or remove injured players.
Societal impact: Football fans face heightened awareness of risks; adjustments are being made in youth leagues to reduce dangerous tackling; parents influence sports choices for children.
The nervous system: overview
The nervous system is a complex network that enables reception of information from the world and initiation of actions.
It consists of the brain, the spinal cord, and other nerves that connect to sensory organs, muscles, and glands.
It is primarily made up of neurons, but also contains glial cells.
The nervous system can be divided into two major parts:
Central nervous system (CNS): brain and spinal cord.
Peripheral nervous system (PNS): nerves outside the CNS, linking the CNS to the body’s sense organs, muscles, and glands.
The CNS and PNS cooperate to produce perception, thought, planning, language, and behavior.
Developmental note: The nervous system becomes more complex as myelination increases and neural networks expand.
Neurons and glial cells
Neurons are the functional units of the nervous system; they conduct impulses.
Visual analogy: Neurons resemble trees with trunks (axon) and branches (dendrites).
Neuron count: Humans are born with more than \${100,000,000,000}\$ neurons (primarily in the brain).
Glial cells: support cells that
Remove dead neurons and waste products,
Nourish and insulate neurons,
Play a role in neural transmission (Moderoș and Pourreia, 2019).
Neurons vary by function and location; their length ranges from fractions of an inch (in the brain) to several feet (in the legs).
The anatomy of a neuron
Key components:
Cell body (soma) contains the nucleus, which uses oxygen and nutrients to generate energy.
Dendrites: branching inputs that receive messages from other neurons.
Axon: a long, slender fiber that conducts electrical impulses away from the cell body.
Axon terminals (terminal buttons): swellings at the end of the axon that release neurotransmitters.
Direction of signaling: Messages travel from dendrites/cell body through the axon to the axon terminals, then across the synapse to other neurons, muscles, or glands.
Myelin: many axons are wrapped in a myelin sheath (white, fatty insulation) that speeds conduction and reduces current leakage. Myelin is essential for rapid signaling and motor development (e.g., crawling and walking).
Myelin and disease: In multiple sclerosis, myelin is replaced by hard tissue, disrupting timing of impulses and motor control.
Types of neurons by direction:
Afferent (sensory) neurons: carry information from the periphery to the CNS (e.g., from skin receptors).
Efferent (motor) neurons: carry commands from the CNS to muscles and glands.
The terms are related as sensory (afferent) vs. motor (efferent).
Example: Afferent signals from toe sensation travel to the spinal cord and brain; efferent signals then activate muscles to respond (e.g., moving the foot). In addition, efferent pathways can stimulate glands (e.g., increasing heart rate).
The neural impulse: electrochemical signaling
The nervous impulse is electrochemical: chemical changes within neurons generate electrical signals that travel along the axon.
Resting potential: neurons maintain a resting negative charge inside relative to outside, primarily due to chloride ions inside. Resting potential ≈ \${-70}\$ mV.
Threshold and depolarization:
When a region of the membrane becomes permeable to sodium ions (\${Na^+}\$), it depolarizes and the area becomes positively charged.
Peak depolarization reaches about \${+30 to +40}\$ mV.
After depolarization, other ions are pumped out to restore resting conditions.
Action potential: the electrical impulse that travels along the axon; it is the same strength at every point along the axon (all-or-none principle).
Conduction speed:
Myelinated axons conduct faster than nonmyelinated ones; speeds range roughly from about \${0.89}\$ to \${100.6}\$ m/s (corresponding to 2 mph to 225 mph).
The impulse is renewed at each point along the axon, preserving strength.
Threshold and firing:
A neuron fires only if the cumulative input reaches a certain threshold.
If subthreshold, the signal dissipates and no action potential occurs.
Refractory period:
After firing, a brief recovery period (a few milliseconds) where the neuron is less sensitive to new inputs; sodium channels reset.
Conducting the impulse: firing is the transmission of an impulse from one neuron to another via chemical signaling at the synapse (not a direct electrical jump across the synaptic cleft).
The synapse and neurotransmitters
Structure of the synapse:
Presynaptic neuron ending (axon terminal) with synaptic vesicles containing neurotransmitters.
Synaptic cleft: the fluid-filled gap between the transmitting and receiving neurons.
Postsynaptic neuron with receptor sites on its dendrites.
Neurotransmitter release and action:
When an impulse reaches the axon terminal, neurotransmitters are released into the synaptic cleft.
Neurotransmitters bind to receptor sites on the receiving neuron (lock-and-key analogy).
Some neurotransmitters are broken down or reabsorbed by the presynaptic neuron (reuptake).
Excitatory vs inhibitory:
Neurotransmitters can excite (promote firing) or inhibit (reduce firing) the postsynaptic neuron.
The net effect (sum of excitatory and inhibitory signals) determines whether the postsynaptic neuron fires.
Key neurotransmitters of interest to psychologists:
Acetylcholine (ACh)
Dopamine
Norepinephrine (aka noradrenaline)
Serotonin
Gamma-aminobutyric acid (GABA)
Endorphins
Clinical and behavioral relevance:
Imbalances in neurotransmitters are linked to disorders such as depression, schizophrenia, and anxiety.
Neurotransmitters: overview and examples
Acetylcholine (ACh)
Role: involved in muscle contraction; excitatory at neuromuscular junctions, inhibitory in the heart.
Historical pharmacology: curare blocks ACh receptors, causing paralysis by preventing muscle contraction; botulism inhibits ACh release, producing similar effects.
Memory: ACh is prevalent in the hippocampus and is important for memory formation; depletion impairs learning and maze navigation in rats.
Dopamine
Roles: reward/pleasure, voluntary movement, learning and memory.
Substances affecting dopamine: nicotine, alcohol, and many drugs elevate dopamine signaling.
Disorders: Parkinson's disease (reduced dopamine; tremors, motor impairment); schizophrenia (excessive dopamine activity in certain brain areas linked to hallucinations and delusions).
Pharmacology: phenothiazines (antipsychotics) block dopamine receptors, potentially causing Parkinson-like side effects which may be mitigated by dose adjustments or alternate medications.
Norepinephrine (noradrenaline)
Roles: both a neurotransmitter and a hormone; involved in arousal, learning, memory, and mood regulation; speeds up heart rate.
Disorders: imbalances linked to mood disorders; stimulants like cocaine and amphetamine increase norepinephrine release and block reuptake.
Serotonin
Roles: emotional regulation, arousal, sleep.
Deficiencies linked to eating disorders, alcoholism, depression, aggression, and insomnia.
Drugs: LSD reduces serotonin action and often increases dopamine activity, contributing to hallucinations.
GABA (gamma-aminobutyric acid)
Role: primary inhibitory neurotransmitter; dampens neural activity to reduce anxiety.
Clinically relevant: tranquilizers and alcohol enhance GABA effects; some anti-anxiety medications increase receptor sensitivity to GABA.
Deficiency linked to depressive states.
Endorphins
Role: endogenous opioids; inhibitory neurotransmitters that dampen pain signals.
Effects: can produce analgesia, improve mood, and contribute to runner’s high; may influence immune function.
The divisions of the nervous system
The nervous system can be divided into two major parts:
Central nervous system (CNS): brain and spinal cord.
Peripheral nervous system (PNS): nerves connecting the CNS to the rest of the body.
The CNS as the "central processing unit" of the nervous system.
The PNS as the body’s input/output network:
It transmits information to and from the CNS via nerves.
Without the PNS, the brain would be unable to receive input or control the body.
The PNS has two main subdivisions:
Somatic nervous system: carries sensory information to the CNS (afferent) and sends voluntary motor commands to muscles (efferent).
Autonomic nervous system (ANS): regulates involuntary functions (glands and internal organs).
The autonomic nervous system (ANS) comprises two branches with largely opposing effects:
Sympathetic division: mobilizes body’s energy during stressful situations (fight or flight; increases heart rate, prepares muscles; energy expenditure).
Parasympathetic division: conserves energy and replenishes reserves (rest and digest; promotes digestion, slows heart rate).
Many organs receive input from both divisions; their effects can average out.
Specific contrasts:
Sympathetic activation tends to increase heart rate and reduce digestion; parasympathetic activation slows heart rate and increases digestion.
Fear or acute stress typically activates the sympathetic system; relaxation and recovery activate the parasympathetic system.
The central nervous system (CNS) details
Central structure: CNS consists of the brain and spinal cord.
The spinal cord as a true information superhighway:
A column of nerves about the thickness of a thumb.
It transmits messages from sensory receptors to the brain and from the brain to muscles and glands throughout the body.
It also handles some reflexive actions via spinal reflexes (local processing) without brain involvement.
Spinal reflexes and neurons involved:
A spinal reflex is an inborn, stereotyped response to a stimulus that can involve as few as two neurons (a sensory neuron and a motor neuron).
Some reflexes involve a third neuron, an interneuron, that links sensory and motor neurons within the spinal cord.
Cross-sectional anatomy:
Gray matter: contains nonmyelinated neurons; involved in reflexes and local processing; shaped like a butterfly in the spinal cord.
White matter: bundles of myelinated axons that carry messages to and from the brain.
Examples of reflexes:
Knee-jerk reflex elicited by a doctor tapping below the knee.
Other reflexes include blinking in response to a puff of air, swallowing when food fills the mouth, and sexual reflexes (urination and defecation).
The CNS enables uniquely human capabilities:
The CNS supports symbolic thought, language, and complex planning that enable humans to adapt, create new environments, and assign names to things.
Connections and implications
Foundational principles:
Neurons communicate via electrochemical signals; signaling depends on membrane potentials, ion flows, and neurotransmitter dynamics.
The all-or-none principle governs neuronal firing: once threshold is reached, the firing magnitude is fixed.
Synaptic transmission combines excitatory and inhibitory inputs to regulate neural activity.
The CNS and PNS together mediate perception, action, and higher-order cognition.
Real-world relevance:
Understanding neural signaling underpins interpretations of learning, memory, and behavior, as well as responses to drugs and brain injuries.
Knowledge of the autonomic nervous system helps explain symptoms in stress and relaxation, digestion, and arousal.
Ethical/philosophical implications:
Advances in neuroscience raise questions about cognitive enhancement, the limits of free will, and responsibility in behavior modification.
Key numerical references and formulas
Resting membrane potential:
Action potential peak:
Activation range of myelinated axons: speeds approx (corresponding to roughly 2 mph to 225 mph)
Potential difference during resting and depolarization:
Resting:
Depolarized:
Refractory period: on the order of a few milliseconds (brief recovery allowing sodium channels to reset)
Summary of major takeaways
The nervous system integrates sensory input, information processing, and motor output through a complex network of neurons and glial cells.
Neurons transmit signals via electrical impulses (action potentials) and chemical signals (neurotransmitters) at synapses; the balance of excitatory and inhibitory signals determines neuronal firing.
The CNS (brain and spinal cord) processes information and coordinates responses, while the PNS (somatic and autonomic) connects the CNS to the body and regulates involuntary functions through the autonomic branches ( sympathetic and parasympathetic )
Reflexes illustrate how the nervous system can respond rapidly and automatically to stimuli, highlighting the spinal cord’s role in basic motor patterns.
Real-world implications of brain injury and neurochemistry underscore the critical link between biology and psychology in understanding behavior, health, and disease.