Source: Nervous System Functions & Structure Guide
Tags: action potential, graded potential, EPSP, IPSP, depolarization, repolarization, hyperpolarization, threshold, resting membrane potential, summation, spatial summation, temporal summation, neurotransmitter, reuptake, degradation, synapse, reflex, reflex arc, monosynaptic, polysynaptic
Difficulty: Intermediate Prerequisites: Nervous system organisation and neuron basics (Part 1 of these notes). Familiarity with ions (Na+, K+), membrane potential, and basic neuron anatomy (axon, dendrite, cell body, synaptic terminal).
Once you know how the nervous system is organised and what neurons look like, the next question is: how do those neurons transmit information? This unit covers the electrical events that allow neurons to communicate, from small local signals (graded potentials) through full-blown action potentials, to the chemical handoff at synapses and the clearance of neurotransmitters afterwards. It finishes with reflexes, which tie the whole sensory-integration-motor loop together in its fastest, most automatic form. This is where the physiology starts, and it is heavily tested.
Neurons communicate using graded potentials (small, local voltage changes) that can sum together to trigger action potentials (all-or-nothing electrical spikes). At the synapse, the electrical signal becomes chemical: neurotransmitters cross the gap and are then cleared by reuptake, degradation, or diffusion. Reflexes are the simplest complete circuits, wired for speed, running from receptor through the spinal cord to effector with minimal processing.
Graded potential
A local change in membrane potential whose size varies with the strength of the stimulus. Graded potentials diminish with distance from their origin.
Think of it as a ripple in a pond: stronger near the source, weaker as it spreads.
Excitatory postsynaptic potential (EPSP)
A graded depolarisation at a postsynaptic membrane that brings the neuron closer to threshold, increasing the likelihood of firing an action potential.
In simple terms, an EPSP is a "go" signal that nudges the neuron toward firing.
Inhibitory postsynaptic potential (IPSP)
A graded hyperpolarisation at a postsynaptic membrane that moves the neuron further from threshold, decreasing the likelihood of an action potential.
In simple terms, an IPSP is a "stop" signal that makes firing less likely.
Spatial summation
The combination of graded potentials arriving from multiple synapses at different locations on the neuron at the same time.
Think of it as several people pushing a car from different angles simultaneously.
Temporal summation
The combination of graded potentials arriving from the same synapse in rapid succession, before the earlier signals have faded.
Think of it as one person pushing the car repeatedly, each shove adding to the last before the car rolls back.
Action potential
A rapid, all-or-nothing reversal of membrane potential that propagates along the axon. Once threshold is reached, the action potential fires at full strength regardless of how strong the stimulus was.
In simple terms, this is the nerve impulse: the signal that travels the length of the neuron.
Threshold
The critical level of depolarisation (typically around –55 mV) at which voltage-gated sodium channels open and an action potential is triggered.
Think of it as the tipping point: below it nothing happens, at or above it the neuron fires.
Depolarisation
The phase of the action potential in which sodium ions (Na+) rush into the cell, driving the membrane potential from negative toward positive.
Repolarisation
The phase in which sodium channels close and potassium ions (K+) exit the cell, returning the membrane potential toward its resting negative state.
Hyperpolarisation
A brief overshoot below resting membrane potential caused by continued potassium efflux before K+ channels close. The membrane temporarily becomes more negative than its resting value.
Resting membrane potential (RMP)
The stable baseline voltage across the neuron's membrane when the cell is not being stimulated, typically around –70 mV. Maintained by the sodium-potassium pump and leak channels.
Reuptake
A neurotransmitter clearance mechanism in which the presynaptic neuron (or neighbouring astrocytes) transports released neurotransmitter molecules back into the cell for reuse.
In simple terms, the sender hoovers the chemical messenger back up to use again later.
Degradation (enzymatic)
A neurotransmitter clearance mechanism in which enzymes in the synaptic cleft break down the neurotransmitter. The classic example is acetylcholinesterase breaking down acetylcholine.
Diffusion
A neurotransmitter clearance mechanism in which neurotransmitter molecules simply drift away from the synaptic cleft, diluting their concentration and ending their effect.
Reflex
A rapid, involuntary, and predictable motor response to a stimulus. Reflexes operate through a reflex arc and do not require conscious thought.
Reflex arc
The neural pathway that mediates a reflex. In its simplest form: receptor → afferent neuron → integration centre (CNS) → efferent neuron → effector.
Monosynaptic reflex
A reflex arc with a single synapse, directly connecting the sensory neuron to the motor neuron in the spinal cord. The knee-jerk (patellar) reflex is the textbook example.
Polysynaptic reflex
A reflex arc involving one or more interneurons between the sensory and motor neurons, allowing more complex processing and responses.
Ipsilateral
Occurring on the same side of the body as the stimulus.
Contralateral
Occurring on the opposite side of the body from the stimulus.
Graded potentials are small, local changes in membrane voltage caused by the opening of ligand-gated or mechanically-gated ion channels.
Their amplitude is proportional to stimulus strength: a stronger stimulus produces a larger graded potential.
They decay with distance (decremental conduction), so on their own they cannot travel far.
Their importance lies in summation: graded potentials from different sources or repeated stimuli can combine at the axon hillock to reach threshold.
Two types of graded potentials at synapses:
EPSPs depolarise the membrane (bring it closer to threshold). Caused by the influx of positive ions, typically Na+.
IPSPs hyperpolarise the membrane (push it further from threshold). Caused by influx of Cl– or efflux of K+.
Two types of summation:
Spatial summation: EPSPs (and IPSPs) from multiple synapses at different locations on the neuron arrive simultaneously and are added together at the axon hillock.
Temporal summation: repeated signals from the same synapse arrive so quickly that the effects stack before the earlier ones decay.
The neuron essentially tallies EPSPs and IPSPs. If the net result at the axon hillock reaches threshold, an action potential fires. If not, the signal dies locally.
The action potential proceeds in a fixed sequence:
Resting state: the neuron sits at resting membrane potential (approximately –70 mV). Voltage-gated Na+ and K+ channels are closed.
Threshold: if graded potentials depolarise the membrane to roughly –55 mV, voltage-gated Na+ channels begin to open.
Depolarisation: Na+ channels open rapidly. Sodium rushes into the cell, driving the membrane potential sharply positive (toward +30 mV). This is the rising phase of the spike.
Repolarisation: Na+ channels inactivate (close). Voltage-gated K+ channels open, and potassium flows out of the cell, returning the membrane toward negative values.
Hyperpolarisation (undershoot): K+ channels are slow to close, so potassium continues to leave briefly, pushing the membrane potential slightly below resting level.
Return to RMP: the sodium-potassium pump and leak channels restore the resting distribution of ions, bringing the membrane back to –70 mV.
The action potential is all-or-nothing: once threshold is reached, it fires at full amplitude. Stimulus strength is coded by the frequency of action potentials, not their size.
Once a neurotransmitter has been released into the synaptic cleft and has bound to its receptors, the signal must be terminated promptly. Three mechanisms handle this:
Reuptake: transport proteins on the presynaptic membrane (or on neighbouring astrocytes) actively pump the neurotransmitter back into the cell. This is the primary clearance route for many neurotransmitters including noradrenaline, serotonin, and dopamine. Many psychiatric medications (SSRIs, for instance) work by blocking reuptake.
Enzymatic degradation: enzymes in the cleft break the neurotransmitter into inactive fragments. The most-tested example is acetylcholinesterase (AChE), which rapidly splits acetylcholine into acetate and choline.
Diffusion: neurotransmitter molecules drift out of the synaptic cleft into the surrounding extracellular fluid, where their concentration becomes too low to have an effect.
All three mechanisms serve the same purpose: ensuring that each signal is brief and discrete, so the synapse is ready to transmit the next one.
Reflexes are automatic, involuntary responses that protect the body or maintain homeostasis. They bypass conscious processing, which is why they are fast.
Types by effector:
Somatic reflexes: involve skeletal muscles. Examples: knee-jerk reflex, withdrawal reflex.
Visceral (autonomic) reflexes: involve smooth muscle, cardiac muscle, or glands. Examples: regulation of blood pressure, pupil dilation.
Reflex arc complexity:
Monosynaptic: one synapse. Sensory neuron synapses directly on motor neuron. Fast and simple. The patellar (knee-jerk) reflex is the classic example.
Polysynaptic: one or more interneurons sit between sensory and motor neurons. Allows more complex, coordinated responses.
Additional reflex characteristics:
Ipsilateral: the response occurs on the same side as the stimulus.
Contralateral: the response occurs on the opposite side.
Innate reflexes: present from birth, hardwired. Example: the withdrawal (flexor) reflex.
Acquired reflexes: learned through experience. Example: the complex motor patterns involved in riding a bicycle.
Two commonly tested reflex examples:
Flexor (withdrawal) reflex: a painful stimulus triggers the withdrawal of a limb. This is polysynaptic and ipsilateral. Importantly, it can be overridden by higher brain centres (you can force yourself to hold a hot mug rather than drop it).
Crossed extensor reflex: works alongside the withdrawal reflex. While the ipsilateral limb flexes and withdraws, the contralateral limb extends to support balance. This is why you do not fall over when you step on something sharp.
The mechanism of neurotransmitter reuptake is the basis for an entire class of medications. Selective serotonin reuptake inhibitors (SSRIs) block the reuptake of serotonin at synapses, increasing its availability in the cleft, and are among the most widely prescribed antidepressants.
Nerve agents and certain pesticides (organophosphates) work by inhibiting acetylcholinesterase, preventing the clearance of acetylcholine at neuromuscular junctions. The result is continuous, uncontrolled muscle contraction.
Clinicians test reflexes (the patellar reflex, for example) as a quick screen of spinal cord and peripheral nerve function. An absent or exaggerated reflex can point to specific levels of spinal cord damage.
Students often think a stronger stimulus produces a larger action potential. It does not. Action potentials are all-or-nothing. A stronger stimulus increases the frequency of firing, not the amplitude.
EPSPs and IPSPs are frequently confused with action potentials. Graded potentials are local, variable in size, and decremental. Action potentials are propagated, all-or-nothing, and non-decremental. They are fundamentally different events.
Students sometimes assume that reflexes cannot be modified or overridden. Somatic reflexes (like the withdrawal reflex) can be consciously suppressed by higher brain centres. Visceral reflexes, however, are largely beyond voluntary control.
"Monosynaptic" does not mean "one neuron." It means one synapse, which involves at least two neurons (sensory and motor).
⚠️ Be able to list the phases of the action potential in order: resting state → threshold → depolarisation → repolarisation → hyperpolarisation → return to RMP. This is a staple exam question in almost every format (multiple choice, ordering, short answer).
⚠️ Know the ions involved at each phase. Na+ in during depolarisation, K+ out during repolarisation. Exam questions will ask which channel opens or closes at each stage.
⚠️ Understand the difference between spatial and temporal summation. A common question gives a scenario and asks you to identify which type is occurring.
⚠️ The three neurotransmitter clearance mechanisms (reuptake, degradation, diffusion) are frequently tested, especially with a clinical tie-in asking how a drug works.
⚠️ Reflex arc components (receptor, afferent neuron, integration centre, efferent neuron, effector) are a classic ordering or labelling question. Know the sequence.
⚠️ Be able to distinguish monosynaptic from polysynaptic reflexes and give an example of each.
True or false: A stronger stimulus produces a bigger action potential. False. Action potentials are all-or-nothing. Stimulus strength is coded by firing frequency.
Fill in the blank: The type of summation in which signals from multiple synapses arrive simultaneously is called _______ summation. Spatial.
True or false: Acetylcholinesterase is an example of neurotransmitter reuptake. False. Acetylcholinesterase is an example of enzymatic degradation. Reuptake involves transport proteins pulling the neurotransmitter back into the presynaptic cell.
Fill in the blank: A reflex arc with a single synapse connecting the sensory neuron directly to the motor neuron is called a _______ reflex. Monosynaptic.
True or false: The crossed extensor reflex occurs on the same side as the stimulus. False. The crossed extensor reflex is contralateral, occurring on the opposite side to support balance during withdrawal.
Q: What is the difference between a graded potential and an action potential?
A: A graded potential is a local, variable-amplitude change in membrane potential that decreases with distance. An action potential is an all-or-nothing, self-propagating electrical signal that maintains its full amplitude along the entire length of the axon.
Q: Describe the sequence of ion movements during an action potential.
A: At threshold, voltage-gated Na+ channels open, and sodium rushes into the cell (depolarisation). Na+ channels then inactivate while voltage-gated K+ channels open, allowing potassium to exit the cell (repolarisation). Continued K+ efflux briefly drives the membrane potential below resting level (hyperpolarisation). The sodium-potassium pump then restores resting membrane potential.
Q: Explain how spatial summation differs from temporal summation.
A: Spatial summation occurs when graded potentials from multiple synapses at different locations on the neuron combine simultaneously. Temporal summation occurs when graded potentials from the same synapse arrive in rapid succession, stacking before earlier signals decay.
Q: Name and briefly describe the three mechanisms by which neurotransmitters are cleared from the synaptic cleft.
A: Reuptake (the presynaptic neuron or astrocytes transport the neurotransmitter back into the cell), enzymatic degradation (enzymes in the cleft break the neurotransmitter down, e.g. acetylcholinesterase breaking down acetylcholine), and diffusion (neurotransmitter molecules drift away from the cleft).
Q: Trace the pathway of a simple monosynaptic reflex arc, naming each component.
A: Receptor detects stimulus → afferent (sensory) neuron carries signal to spinal cord → single synapse in the spinal cord (integration centre) → efferent (motor) neuron carries command to effector → effector (muscle) responds.
Q: What is the crossed extensor reflex, and why is it functionally important?
A: The crossed extensor reflex is a contralateral response that accompanies the ipsilateral withdrawal (flexor) reflex. When one limb withdraws from a painful stimulus, the opposite limb extends to bear the body's weight and maintain balance.
Graded potentials and action potentials appear again in the muscular system, where the motor end plate potential at the neuromuscular junction triggers muscle contraction. Neurotransmitter clearance connects to pharmacology topics across the course, particularly where drugs modify synaptic transmission (anaesthetics, antidepressants, muscle relaxants). Reflexes tie directly into spinal cord anatomy and clinical assessment, and the autonomic reflex pathway resurfaces in cardiovascular regulation (baroreceptor reflex) and digestive physiology.
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