Source: General Psychology, The Ohio State University, Chapter 5
Tags: EPSP, IPSP, action potential, neurotransmitters, agonist, antagonist, ACh, dopamine, GABA, autonomic nervous system, sympathetic, parasympathetic, reflex arc, Ian Waterman
Difficulty: Intermediate | Prerequisites: Chapter 1 notes (behaviourism's emphasis on stimulus-response), Chapter 3 notes (biological adaptations)
This chapter moves from the level of genes and behaviour down to the cellular machinery that makes behaviour possible: neurons, neurotransmitters, synapses, and the divisions of the nervous system. It covers how signals travel within and between neurons, how drugs and poisons alter neurotransmitter activity, and how the autonomic nervous system regulates bodily functions. The reflex arc and the case of Ian Waterman illustrate what happens when these systems break down. If the terminology feels dense, start with the key terms and then work through the core content.
Neurons communicate through electrical signals (action potentials) and chemical signals (neurotransmitters crossing the synapse). EPSPs push a neuron toward firing; IPSPs push it away from firing. Drugs and poisons work by mimicking, boosting, or blocking neurotransmitters (agonists increase activity, antagonists decrease it). The autonomic nervous system has two branches: sympathetic (fight or flight) and parasympathetic (rest and digest). Reflexes are automatic responses mediated by spinal arcs, and the case of Ian Waterman shows how critical sensory feedback from the spinal cord is for normal movement.
Excitatory postsynaptic potential (EPSP)
A graded change in a neuron's membrane potential toward depolarisation (more positive), caused by excitatory neurotransmitters. Makes the neuron more likely to fire. In simple terms, an EPSP is a small push toward "go."
Inhibitory postsynaptic potential (IPSP)
A graded change in a neuron's membrane potential toward hyperpolarisation (more negative), caused by inhibitory neurotransmitters. Makes the neuron less likely to fire. In simple terms, an IPSP is a small push toward "stop."
Action potential
An all-or-nothing electrical signal that travels down the axon once the neuron's threshold is reached. Unlike EPSPs and IPSPs, it does not decrease with distance. Takes roughly 2–3 milliseconds to travel. Think of it as the neuron's "fire" signal. Once triggered, it goes the full distance at full strength.
Axon hillock
The region of the neuron where EPSPs and IPSPs are summed. If the net result reaches threshold, an action potential is initiated.
Depolarisation
A shift in a neuron's membrane potential toward a more positive charge. The direction of an EPSP.
Hyperpolarisation
A shift in a neuron's membrane potential toward a more negative charge. The direction of an IPSP.
Absolute refractory period
The brief interval during and immediately after an action potential when it is impossible to generate another action potential, regardless of stimulus strength. Typically lasts about one millisecond. In simple terms, the neuron needs a moment to reset before it can fire again.
Relative refractory period
The interval after the absolute refractory period when a new action potential can be generated, but only if the stimulus is stronger than usual.
Acetylcholine (ACh)
A neurotransmitter involved in both learning/memory and muscle movement. ACh binds to gated ion channels and depolarises the neuron by allowing Na+ to flow in.
GABA (gamma-aminobutyric acid)
The primary inhibitory neurotransmitter. Opens Cl- channels that hyperpolarise the neuron.
Dopamine
A neurotransmitter involved in reward, motivation, and movement. Plays a central role in the brain's reward pathway.
Agonist
A drug or poison that increases the activity of one or more neurotransmitters. Can work directly (binding to the receptor in place of the neurotransmitter) or indirectly (stimulating the release of the neurotransmitter or blocking its breakdown/reuptake).
Antagonist
A drug or poison that decreases the activity of one or more neurotransmitters. Can work directly (binding to the receptor and blocking the neurotransmitter from attaching) or indirectly (inhibiting the release of the neurotransmitter).
Autonomic nervous system (ANS)
The division of the peripheral nervous system that controls involuntary bodily functions: heart rate, digestion, respiratory rate, pupil dilation, sexual arousal, and others.
Sympathetic nervous system
The "fight or flight" branch of the ANS. Activates the body during arousal or stress: dilates pupils, speeds heart rate, speeds breathing, inhibits digestion, produces sweaty palms.
Parasympathetic nervous system
The "rest and digest" branch of the ANS. Returns the body to its resting state: constricts pupils, slows heart rate, slows breathing, stimulates digestion, dry palms.
Reflex arc
The neural pathway that mediates a reflex. When integration occurs in the spinal cord, the reflex is a spinal reflex.
Endocrine system
The glandular system that secretes hormones into the bloodstream. Works with the ANS in response to stress and regulates functions including growth, metabolism, reproduction, and blood sugar.
How postsynaptic potentials work
Both EPSPs and IPSPs are produced by altering the relative concentration of ions between the inside and outside of the cell membrane
Both are graded potentials: their size varies depending on the quantity of neurotransmitter released
Both travel passively and decrease with distance (they fade as they spread from the synapse)
Both take roughly 15–30 milliseconds
EPSP specifics
Excitatory neurotransmitters (e.g., ACh) cause the membrane to depolarise (become more positive)
ACh binds to gated ion channels and allows Na+ to flow into the neuron
IPSP specifics
Inhibitory neurotransmitters (e.g., GABA) cause the membrane to hyperpolarise (become more negative)
GABA opens Cl- channels that make the interior of the neuron more negative
Summation and the action potential
EPSPs and IPSPs are summed at the axon hillock
If the net result reaches the neuron's threshold, an action potential is initiated
The action potential is all-or-nothing: once initiated, it does not decrease in strength as it travels down the axon
It takes approximately 2–3 milliseconds to travel the length of the axon
Refractory periods
Absolute refractory period: during and immediately after an action potential, it is impossible to generate another one. Lasts roughly one millisecond. This sets an upper limit on how fast a neuron can fire.
Relative refractory period: a new action potential can be generated, but requires a stronger-than-usual stimulus
Agonists (increase neurotransmitter activity)
Direct agonists bind to the receptor in place of the neurotransmitter. Indirect agonists stimulate the release of the neurotransmitter, block its breakdown, or block its reuptake.
Examples:
Black widow spider venom: agonist for ACh. Causes continuous release of ACh, flooding the synapse
Sarin (nerve gas): agonist for ACh. Inhibits the enzyme that breaks ACh down in the synapse, causing convulsions and death
L-dopa: increases production of dopamine
Amphetamine: stimulates release of dopamine from axon terminals
Cocaine: blocks reuptake of dopamine (more dopamine stays in the synapse)
Antagonists (decrease neurotransmitter activity)
Direct antagonists bind to receptors and stay there, physically blocking the neurotransmitter from attaching. Indirect antagonists inhibit the release of neurotransmitters.
Examples:
Botulinum toxin (botox): antagonist that blocks the release of ACh at muscle junctions, leading to paralysis and (in large doses) death
Curare: antagonist that occupies ACh receptor sites, preventing ACh from binding. Paralyses the body
Anti-psychotic drugs: block dopamine receptor sites, preventing dopamine from sending its message
Overall function
The ANS is part of the peripheral nervous system
Regulates internal bodily functions that are critical for survival: heart rate, digestion, respiratory rate, pupil dilation, sexual arousal
Works with the endocrine glandular system in response to stress
Sympathetic nervous system (fight or flight)
Activates the body when it is aroused or under threat
Dilates pupils
Speeds heart rate
Speeds breathing
Inhibits digestion
Produces sweaty palms
Parasympathetic nervous system (rest and digest)
Controls the body during its normal resting state
Constricts pupils
Slows heart rate
Slows breathing
Stimulates digestion
Dry palms
The endocrine system
Endocrine glands secrete hormones carried through the bloodstream to target sites
Controlled by the hypothalamus, which directs the pituitary gland
Pituitary gland secretes hormones that direct other endocrine glands
Key glands: thyroid (growth), adrenal (fight or flight hormones), pancreas (digestion and blood sugar)
The medulla oblongata coordinates autonomic functions
What is a reflex?
A reflex is an automatic, sudden, involuntary response to a stimulus. When the integration takes place in the spinal cord, it is a spinal reflex.
Steps of the reflex arc:
Sensory receptor: responds to a stimulus by producing a generator or receptor potential
Sensory neuron: axon conducts impulses from the receptor to the integrating centre
Integrating centre: one or more regions within the CNS that relay impulses from sensory neurons to motor neurons
Motor neuron: axon conducts impulse from the integrating centre to the effector
Effector: the muscle or gland that responds to the motor nerve impulse
The case of Ian Waterman
A viral infection destroyed the sensory nerves connecting Waterman's muscles and joints to his brain
He lost his sense of touch and proprioception (the ability to sense limb position without looking)
He became entirely dependent on vision to know where his limbs were
His case demonstrated that the brain and spinal cord are more functionally separate than commonly assumed: without sensory feedback from the body, even basic movement becomes extraordinarily difficult
Students often confuse EPSPs and IPSPs. Remember: EPSP = Excitatory = depolarisation (more positive, pushes toward firing). IPSP = Inhibitory = hyperpolarisation (more negative, pushes away from firing).
"An action potential varies in strength depending on the stimulus." It does not. Action potentials are all-or-nothing. What varies is the rate of firing, not the strength of each individual action potential.
"Agonists are always drugs and antagonists are always poisons." Both categories include drugs and poisons. The distinction is about whether the substance increases or decreases neurotransmitter activity, not whether it is helpful or harmful.
Students sometimes think the sympathetic and parasympathetic systems are active at different times. In reality, both are always active; the balance between them shifts depending on the situation.
⚠️ Know the differences between EPSP, IPSP, and action potential. Be able to describe how each works, what neurotransmitters are involved, and how they are summed at the axon hillock.
⚠️ Be able to define agonist and antagonist, explain direct vs. indirect mechanisms, and give at least two examples of each.
⚠️ Know the two divisions of the ANS (sympathetic and parasympathetic) and list the bodily responses each controls. The "dilated vs. constricted pupils / sweaty vs. dry palms" contrasts are classic exam material.
⚠️ Know the five steps of the reflex arc in order.
⚠️ Be ready to explain what Ian Waterman's case tells us about the importance of sensory feedback and spinal reflexes for normal movement.
⚠️ Understand absolute vs. relative refractory periods and why the absolute refractory period limits neural firing rate.
True or False: An IPSP makes a neuron more likely to fire.
Fill in the blank: The neurotransmitter GABA opens __________ channels to hyperpolarise the neuron.
True or False: Cocaine is an agonist for dopamine because it blocks dopamine reuptake.
Fill in the blank: The __________ nervous system dilates pupils and speeds heart rate during a threat.
True or False: An action potential decreases in strength as it travels down the axon.
Answers: 1. False (IPSPs inhibit firing). 2. Cl- (chloride). 3. True. 4. Sympathetic. 5. False (action potentials are all-or-nothing and do not decrease with distance).
Q: Describe the differences between IPSPs, EPSPs, and action potentials.
A: EPSPs are graded depolarisations (more positive) caused by excitatory neurotransmitters like ACh. IPSPs are graded hyperpolarisations (more negative) caused by inhibitory neurotransmitters like GABA. Both are graded, travel passively, and fade with distance. They are summed at the axon hillock. If the net result reaches threshold, an all-or-nothing action potential is triggered, which travels the full length of the axon without decreasing (2–3 ms).
Q: What are relative and absolute refractory periods, and why does the absolute refractory period matter?
A: The absolute refractory period occurs during and just after an action potential, when no new action potential can be generated regardless of stimulus strength (~1 ms). The relative refractory period follows, during which a new action potential is possible but requires a stronger stimulus. The absolute refractory period sets the maximum firing rate of the neuron.
Q: Define agonist and antagonist and give two examples of each.
A: Agonists increase neurotransmitter activity. Examples: black widow venom (floods synapse with ACh), cocaine (blocks dopamine reuptake). Antagonists decrease neurotransmitter activity. Examples: botulinum toxin (blocks ACh release at muscle junctions), curare (occupies ACh receptor sites, preventing binding).
Q: What are the divisions of the autonomic nervous system and what do they control?
A: The sympathetic nervous system (fight or flight) activates the body under stress: dilates pupils, speeds heart rate and breathing, inhibits digestion, sweaty palms. The parasympathetic nervous system (rest and digest) returns the body to rest: constricts pupils, slows heart rate and breathing, stimulates digestion, dry palms. Both work with the endocrine system.
Q: What is a reflex arc, and what does Ian Waterman's case teach us?
A: A reflex arc is the neural pathway for an automatic response: sensory receptor, sensory neuron, integrating centre (in the spinal cord for spinal reflexes), motor neuron, effector (muscle/gland). Ian Waterman lost sensory nerve connections to his muscles and joints after a viral infection, eliminating proprioception. His case shows that sensory feedback from the spinal cord is critical for normal movement and that the brain and spinal cord are more functionally independent than commonly assumed.
The neurotransmitter concepts here connect directly to Chapter 6's material on the brain's reward system (dopamine, the VTA, nucleus accumbens) and drug addiction. The autonomic nervous system links to the emotion and motivation material in Chapter 6 (the sympathetic response underpins the fight-or-flight component of emotion). The agonist/antagonist framework is relevant to any pharmacology discussed later in the course.
EPSP excitatory postsynaptic potential, IPSP inhibitory postsynaptic potential, action potential all-or-nothing, axon hillock summation, depolarisation, hyperpolarisation, absolute refractory period, relative refractory period, acetylcholine ACh, GABA inhibitory, dopamine reward, agonist drug, antagonist drug, black widow venom ACh, sarin nerve gas, cocaine dopamine reuptake, botulinum toxin, curare, autonomic nervous system ANS, sympathetic fight or flight, parasympathetic rest and digest, endocrine system hormones, hypothalamus pituitary, reflex arc steps, spinal reflex, Ian Waterman proprioception, sensory feedback, anti-psychotic dopamine blocker, L-dopa, amphetamine