Source: Psychology 1100 Key Concepts and Terms
Tags: neurons, action potential, all or nothing principle, myelin sheath, synapses, neurotransmitters, forebrain, midbrain, hindbrain, CNS, PNS, nervous system divisions, biological psychology, Ohio State PSY 1100
Difficulty: Intermediate Prerequisites: Comfortable with the research methods material. Some basic biology (cell structure, ions) is helpful but not essential.
This unit moves from how we study the mind to the physical machinery that produces it. The nervous system is the biological infrastructure behind every psychological process in the course, from perception to memory to emotion. Understanding how a single neuron fires, how neurons communicate, and how the brain is organised gives you the vocabulary for nearly every topic that follows. Exam questions here tend to be detail-heavy: know the sequence of the action potential and the parts of each brain division.
The nervous system divides into the central (brain and spinal cord) and peripheral (everything else) systems. Within the brain, three major divisions (forebrain, midbrain, hindbrain) handle different functions. At the cellular level, neurons communicate via electrical impulses (action potentials) and chemical signals (neurotransmitters) across tiny gaps called synapses.
Divisions of the nervous system
Central nervous system (CNS): the brain and spinal cord. Peripheral nervous system (PNS): somatic (voluntary movement) and autonomic (involuntary functions), which further divides into sympathetic (fight or flight) and parasympathetic (rest and digest). Think of the CNS as headquarters and the PNS as the field network connecting it to the rest of the body.
Action potential process
The electrical impulse by which neurons transmit signals. The sequence:
A stimulus reaches a resting neuron, which sits at a negative resting potential.
Sodium (Na+) channels open, allowing Na+ ions to rush into the cell (depolarisation).
If the voltage reaches threshold, the action potential fires.
At peak voltage, sodium channels close and potassium (K+) channels open; K+ flows out.
The neuron becomes hyperpolarised (briefly more negative than resting).
During the refractory period, ion pumps restore resting conditions: Na+ back outside, K+ back inside.
In simple terms, the neuron charges up, fires, overshoots, then resets.
All or nothing principle
Once the electrical impulse reaches threshold, it fires at full strength and travels the entire length of the axon without losing intensity. Think of it like a light switch: it is either on or off. There is no halfway firing.
Myelin sheath
A layer of fatty cells that encases and insulates most axons, speeding up the transmission of nerve impulses. In simple terms, myelin is insulation around a wire. Gaps in the myelin (nodes of Ranvier) let the signal jump along faster.
Synapses
The tiny gaps between neurons, also called synaptic gaps or synaptic clefts. Think of it as the space where one neuron's message is handed off to the next.
Neurotransmitters
Chemical messengers released from a neuron's terminal buttons that cross the synaptic cleft to transmit a signal to another cell. In simple terms, neurotransmitters are the chemical "words" neurons use to talk to each other across the synapse.
Forebrain parts
The brain's largest division and most forward section. Key structures: thalamus, hypothalamus, hippocampus, amygdala. Think of the forebrain as the executive suite: it handles higher-order processing, emotion, memory, and relaying sensory information.
Midbrain parts
Located between the forebrain and hindbrain. Key structures: substantia nigra, reticular formation. In simple terms, the midbrain helps with movement coordination and arousal/alertness.
Hindbrain parts
The rear, lower portion of the brain. Key structures: medulla, cerebellum, pons. Think of the hindbrain as life support and coordination: the medulla controls breathing and heart rate, the cerebellum handles balance and motor coordination, and the pons helps regulate sleep.
The CNS (brain + spinal cord) is the command centre.
The PNS connects the CNS to limbs and organs.
Somatic division: voluntary muscle movements.
Autonomic division: involuntary processes (heart rate, digestion).
Sympathetic branch: activates the body for emergencies.
Parasympathetic branch: calms the body after the emergency passes.
Neurons are the basic signalling units of the nervous system.
At rest, the inside of the neuron is negatively charged relative to the outside (resting potential, roughly -70 mV).
When stimulated, sodium channels open, Na+ floods in, and the cell depolarises.
If depolarisation reaches threshold, the action potential fires (all or nothing).
Potassium channels then open, K+ leaves, and the cell briefly hyperpolarises before returning to rest.
The refractory period prevents the neuron from firing again immediately, which ensures signals travel in one direction.
The action potential travels down the axon to the terminal buttons.
Neurotransmitters are released into the synaptic cleft.
They bind to receptors on the receiving (postsynaptic) neuron, either exciting or inhibiting it.
Excess neurotransmitter is cleared by reuptake or enzymatic breakdown.
Myelinated axons transmit signals much faster than unmyelinated ones.
Diseases that damage myelin (e.g., multiple sclerosis) impair signal transmission, producing motor and sensory problems.
Hindbrain: survival basics. Medulla (breathing, heart rate), cerebellum (coordination, balance), pons (sleep, bridging signals).
Midbrain: movement and alertness. Substantia nigra (dopamine production, movement, linked to Parkinson's disease), reticular formation (arousal, attention filtering).
Forebrain: higher functions. Thalamus (sensory relay station for everything except smell), hypothalamus (hunger, thirst, body temperature, hormones), hippocampus (memory formation), amygdala (emotion, especially fear).
Understanding the action potential is the basis of how anaesthetics work: local anaesthetics block sodium channels so neurons cannot fire, which is why you feel no pain during dental procedures. Damage to myelin sheaths in multiple sclerosis shows what happens when signal transmission breaks down. Knowledge of brain divisions matters in clinical settings; damage to the hippocampus, for instance, impairs the ability to form new memories.
Students often think a stronger stimulus produces a stronger action potential. It does not, because of the all or nothing principle. A stronger stimulus increases the rate of firing, not the intensity of each individual impulse.
The synapse is a gap, not a connection. Neurons do not physically touch; they communicate chemically across the cleft.
The thalamus relays almost all sensory information, but smell is the exception. Olfactory signals go directly to the cortex, bypassing the thalamus.
"Sympathetic" sounds soothing, but the sympathetic nervous system is the one that activates you for emergencies. The parasympathetic system is the calming one.
⚠️ Be prepared to describe the action potential process step by step, in order.
⚠️ Know the all or nothing principle and be able to explain what it means for signal strength vs. firing rate.
⚠️ Expect a question asking you to match brain structures to their divisions (forebrain, midbrain, hindbrain).
⚠️ Distinguish the sympathetic and parasympathetic branches of the autonomic nervous system.
⚠️ Know the role of neurotransmitters and the basic sequence of synaptic transmission.
True or False: A stronger stimulus produces a larger action potential.
Fill in the blank: The fatty layer that insulates axons and speeds up signal transmission is called the __________.
True or False: The hippocampus is part of the hindbrain.
Fill in the blank: Chemical messengers that cross the synaptic cleft are called __________.
True or False: The parasympathetic nervous system activates the body for fight or flight.
(Answers: 1. False; 2. myelin sheath; 3. False, it is part of the forebrain; 4. neurotransmitters; 5. False, that is the sympathetic system.)
Q: Describe the sequence of events during an action potential, from resting state to return to rest.
A: At rest, the neuron is negatively charged. A stimulus opens sodium channels, Na+ rushes in (depolarisation). If threshold is reached, the action potential fires. At the peak, sodium channels close and potassium channels open, K+ flows out (repolarisation), the cell overshoots to hyperpolarisation, and ion pumps restore the resting potential during the refractory period.
Q: What does the all or nothing principle mean for neural communication?
A: A neuron either fires at full strength or does not fire at all. Stimulus intensity is coded by the rate of firing (how many action potentials per second), not by the size of each action potential.
Q: Name the three divisions of the brain and give one structure from each.
A: Forebrain (e.g., thalamus), midbrain (e.g., substantia nigra), hindbrain (e.g., cerebellum).
Q: What is the role of the myelin sheath?
A: It insulates the axon and speeds up neural transmission. The signal "jumps" between gaps in the myelin (nodes of Ranvier), which is faster than continuous conduction along an unmyelinated axon.
Q: How do the sympathetic and parasympathetic nervous systems differ?
A: The sympathetic system activates the body for emergencies (increased heart rate, dilated pupils). The parasympathetic system returns the body to a calm, resting state (slowed heart rate, resumed digestion).
This material connects forward to sensation and perception: sensory systems rely on neurons, action potentials, and neurotransmitters to convert stimuli into experiences. It also connects to the consciousness and stress units, because the HPA axis and sleep stages depend on the brain structures introduced here (hypothalamus, reticular formation).
action potential, depolarisation, repolarisation, hyperpolarisation, resting potential, threshold, sodium channels, potassium channels, refractory period, all or nothing principle, myelin sheath, nodes of Ranvier, synapse, synaptic cleft, neurotransmitters, terminal buttons, CNS, PNS, somatic, autonomic, sympathetic, parasympathetic, forebrain, thalamus, hypothalamus, hippocampus, amygdala, midbrain, substantia nigra, reticular formation, hindbrain, medulla, cerebellum, pons, PSY 1100, Ohio State psychology midterm