Difficulty: Introductory to Intermediate | Prerequisites: Research Methods notes helpful but not required
Tags: neuroscience, neurons, action potential, all-or-nothing principle, myelin sheath, synapse, neurotransmitters, dopamine, serotonin, acetylcholine, forebrain, midbrain, hindbrain, thalamus, hypothalamus, hippocampus, amygdala, cerebellum, medulla, pons, CNS, PNS, sympathetic, parasympathetic, somatic nervous system, autonomic nervous system
This unit explains the biological machinery behind every psychological process you will study in this course. Neurons fire electrical signals, pass chemical messages across synapses, and collectively form a nervous system divided into central and peripheral branches. The brain itself is organised into forebrain, midbrain, and hindbrain structures, each responsible for different functions. Understanding this material gives you the vocabulary to make sense of topics from sensation to stress to mental health.
Neurons communicate via electrical impulses (action potentials) and chemical signals (neurotransmitters crossing synapses). The nervous system splits into the CNS (brain and spinal cord) and PNS (everything else). The brain is divided into forebrain (complex thought, emotion, memory), midbrain (movement, arousal), and hindbrain (vital functions like breathing and balance).
Action potential
An electrical impulse that travels along a neuron's axon. Triggered when a stimulus causes sodium ions to rush in (depolarisation), reaching a threshold. Sodium channels then open fully, the signal propagates, potassium channels open to repolarise the neuron, and a refractory period follows before the next signal.
Think of it as: a chain of dominoes falling down the axon, then resetting.
All-or-nothing principle
A neuron either fires at full strength once the threshold is reached, or it does not fire at all. The strength of the stimulus determines whether the neuron fires, not how strongly it fires.
In simple terms: there is no "half fire." It is on or off.
Myelin sheath
A fatty insulating layer surrounding many axons that speeds up the transmission of nerve impulses. Damage to the myelin sheath (as in multiple sclerosis) impairs neural communication.
Think of it as: the rubber coating on an electrical wire that keeps the signal moving fast.
Synapse
The tiny gap between two neurons where chemical communication occurs. Neurotransmitters are released from the presynaptic neuron, cross the synaptic cleft, and bind to receptors on the postsynaptic neuron.
Neurotransmitters
Chemical messengers that cross synapses to transmit signals between neurons. Key examples include dopamine (movement, reward, pleasure), serotonin (mood, sleep, appetite), and acetylcholine (muscle movement, memory, attention).
Think of it as: the chemical "mail" that carries messages from one neuron to the next.
Thalamus
A forebrain structure that acts as the brain's sensory relay station, routing incoming sensory information to the appropriate cortical area. Nearly all senses pass through the thalamus (smell is the notable exception).
Hypothalamus
A small forebrain structure that regulates hunger, thirst, body temperature, and the endocrine system. Despite its size, it plays a central role in maintaining homeostasis.
Hippocampus
A forebrain structure essential for the formation of new long-term memories. Damage here impairs the ability to create new memories while often leaving older memories intact.
Amygdala
A forebrain structure involved in processing emotions, particularly fear and aggression. It plays a key role in emotional learning and the fight-or-flight response.
Substantia nigra
A midbrain structure involved in movement and reward. Degeneration of dopamine-producing neurons here is the hallmark of Parkinson's disease.
Reticular formation
A midbrain network that regulates arousal, alertness, and consciousness. It acts as a filter for incoming sensory information.
Medulla (medulla oblongata)
A hindbrain structure that controls vital, automatic functions: heartbeat, breathing, and blood pressure.
Think of it as: the brain's life-support control room.
Cerebellum
A hindbrain structure that coordinates voluntary movement, balance, and motor learning. Damage here produces clumsy, uncoordinated movement.
Pons
A hindbrain structure involved in sleep, respiration, and relaying signals between the cerebellum and the rest of the brain.
Central nervous system (CNS)
The brain and spinal cord. Responsible for processing and integrating information.
Peripheral nervous system (PNS)
All nerves outside the brain and spinal cord, divided into the somatic nervous system (voluntary movement) and the autonomic nervous system (involuntary functions).
Sympathetic nervous system
The branch of the autonomic nervous system that prepares the body for action: "fight or flight." Increases heart rate, dilates pupils, diverts blood to muscles.
Parasympathetic nervous system
The branch of the autonomic nervous system that calms the body after stress: "rest and digest." Slows heart rate, promotes digestion, conserves energy.
A neuron at rest holds a negative charge inside its membrane (resting potential, roughly -70 mV).
When stimulated past the threshold, sodium channels open and sodium ions rush in, creating an action potential that travels down the axon.
At the peak, sodium channels close and potassium channels open, repolarising the neuron.
The neuron then enters a refractory period (brief reset before it can fire again).
The all-or-nothing principle means the neuron fires at full strength or not at all. Stimulus intensity is coded by the rate of firing, not the strength of individual impulses.
The myelin sheath insulates the axon and allows the signal to jump between gaps (nodes of Ranvier), dramatically increasing speed.
When the action potential reaches the axon terminal, it triggers the release of neurotransmitters into the synaptic cleft.
These chemical messengers bind to receptors on the postsynaptic neuron.
Effects can be excitatory (increasing the chance of firing) or inhibitory (decreasing it).
Key neurotransmitters to know:
Dopamine: movement, reward, motivation. Linked to Parkinson's (too little) and schizophrenia (too much activity in certain pathways).
Serotonin: mood regulation, sleep, appetite. Low levels linked to depression.
Acetylcholine: muscle contraction, attention, memory. Depleted in Alzheimer's disease.
Forebrain (largest region, higher-order functions):
Thalamus: sensory relay (except smell)
Hypothalamus: homeostasis, hunger, thirst, temperature, endocrine control
Hippocampus: forming new memories
Amygdala: emotion processing, especially fear
Midbrain (relay and motor functions):
Substantia nigra: movement, reward (dopamine production)
Reticular formation: arousal, consciousness, filtering sensory input
Hindbrain (basic survival functions):
Medulla: heartbeat, breathing, blood pressure
Cerebellum: coordination, balance, motor learning
Pons: sleep, respiration, relay between cerebellum and cortex
Central Nervous System (CNS): brain + spinal cord. The command centre.
Peripheral Nervous System (PNS): everything outside the CNS.
Somatic Nervous System: controls voluntary skeletal muscles.
Autonomic Nervous System: controls involuntary functions (organs, glands).
Sympathetic: activates the body (fight or flight). Increases heart rate, redirects blood to muscles, dilates airways.
Parasympathetic: calms the body (rest and digest). Slows heart rate, promotes digestion, conserves energy.
The two branches work in opposition to maintain balance.
Understanding neurotransmitters is the basis of psychopharmacology: SSRIs treat depression by increasing serotonin availability at the synapse. Knowledge of myelin sheath damage explains the progressive symptoms of multiple sclerosis. The sympathetic/parasympathetic distinction is used constantly in stress management and clinical anxiety treatment.
Students often think a stronger stimulus produces a stronger action potential. It does not. The all-or-nothing principle means the signal is always the same strength; what changes is the firing rate (how many times per second the neuron fires).
Students sometimes think the synapse is a physical connection between two neurons. It is a gap. Communication across it is chemical, not electrical.
Students often confuse the sympathetic and parasympathetic systems. A useful mnemonic: sympathetic = "stress" (both start with S); parasympathetic = "peace" (both start with P).
Students frequently mix up the hippocampus and hypothalamus. The hippocampus is for memory; the hypothalamus is for homeostasis (hunger, thirst, temperature).
⚠️ Be able to walk through the steps of an action potential in order: resting state, depolarisation, threshold, propagation, repolarisation, refractory period.
⚠️ Know the key neurotransmitters and what happens when they are too high or too low (dopamine, serotonin, acetylcholine).
⚠️ Be able to match each brain structure to its function (thalamus = sensory relay, hippocampus = memory, amygdala = emotion, etc.).
⚠️ Understand the nervous system hierarchy: CNS vs PNS, somatic vs autonomic, sympathetic vs parasympathetic.
True or false: The action potential varies in strength depending on the stimulus. (False. All-or-nothing principle.)
Fill in the blank: The ______ is the brain's sensory relay station for all senses except smell. (Thalamus)
True or false: The parasympathetic nervous system activates the fight-or-flight response. (False. That is the sympathetic system.)
Fill in the blank: The fatty insulating layer around axons that speeds up signal transmission is the ______. (Myelin sheath)
True or false: Neurotransmitters are electrical signals. (False. They are chemical messengers.)
Q: Describe the sequence of events in an action potential.
A: At rest, the neuron has a negative internal charge. A stimulus depolarises the membrane. If the threshold is reached, sodium channels open and sodium rushes in, propagating the impulse along the axon. At peak voltage, sodium channels close and potassium channels open, repolarising the neuron. A refractory period follows before the neuron can fire again.
Q: A patient has damage to the hippocampus. What symptoms would you predict?
A: Difficulty forming new long-term memories (anterograde amnesia). Older memories may remain intact, since they are stored elsewhere in the cortex.
Q: Explain the difference between the sympathetic and parasympathetic nervous systems, with an example.
A: The sympathetic system prepares the body for action (e.g. increasing heart rate when you see a threat). The parasympathetic system returns the body to a calm state afterward (e.g. slowing heart rate once the threat passes). They work in opposition to maintain balance.
Q: Why does damage to the myelin sheath impair neural function?
A: The myelin sheath insulates the axon and allows the action potential to travel quickly by jumping between nodes of Ranvier. Without it, signals slow down or fail to propagate properly, as seen in multiple sclerosis.
Neural communication underpins everything in the sensation and perception unit (sensory receptors convert stimuli into action potentials). The sympathetic nervous system and HPA axis are central to the stress and health psychology unit. Neurotransmitter imbalances are a recurring theme in abnormal psychology and psychopharmacology.
action potential steps, all-or-nothing principle, resting potential, depolarisation, repolarisation, refractory period, threshold, myelin sheath, nodes of Ranvier, multiple sclerosis, synapse, synaptic cleft, neurotransmitters, dopamine, serotonin, acetylcholine, excitatory, inhibitory, forebrain structures, thalamus, hypothalamus, hippocampus, amygdala, midbrain, substantia nigra, reticular formation, hindbrain, medulla oblongata, cerebellum, pons, central nervous system, peripheral nervous system, somatic nervous system, autonomic nervous system, sympathetic nervous system fight or flight, parasympathetic nervous system rest and digest, General Psychology OSU