Receptors

Receptors detect stimulus and convert it into a form that neurons can understand and transmit—either as a graded potential or by modulating synaptic transmission.

 

1. What receptors do (core functions)

1. Detect a specific stimulus

Receptors are specialized proteins (or receptor cells) that respond selectively to:

  • Chemical signals (neurotransmitters, hormones)

  • Mechanical forces (stretch, pressure)

  • Thermal changes (heat, cold)

  • Light (photons)

  • Painful stimuli (nociception)

This specificity is what allows precise signaling.

2. Transduce the stimulus into an electrical signal

This is called sensory transduction.

  • The receptor converts the stimulus into a change in membrane potential

  • This produces a receptor potential (graded, not all-or-none)

  • If large enough, it triggers an action potential in the afferent neuron

📌 Example:
Stretching a muscle spindle opens mechanosensitive ion channels → Na⁺ influx → depolarization → action potential.

2. Receptors in synaptic (neuronal) signaling

At synapses, receptors determine how neurons talk to each other.

A. Ionotropic receptors (fast)

  • Ligand-gated ion channels

  • Neurotransmitter binding directly opens a channel

  • Produces EPSPs or IPSPs

  • Millisecond response

📌 Examples:

  • AMPA (glutamate → Na⁺ influx → excitation)

  • GABA_A (Cl⁻ influx → inhibition)

B. Metabotropic receptors (slow, modulatory)

  • G-protein–coupled receptors

  • Activate second messenger cascades

  • Modulate ion channels, gene expression, synaptic strength

  • Slower but longer-lasting effects

📌 Examples:

  • Muscarinic ACh receptors

  • Dopamine receptors

  • β-adrenergic receptors

3. Receptors determine signal outcome

The same neurotransmitter can have different effects depending on the receptor:

Neurotransmitter Receptor Effect
Acetylcholine Nicotinic Excitatory
Acetylcholine Muscarinic (M2) Inhibitory
Glutamate AMPA Fast excitation
Glutamate NMDA Plasticity, Ca²⁺ entry
GABA GABA_A Inhibitory

➡️ Receptors, not neurotransmitters alone, determine function.

4. Receptors in sensory systems

Different receptor types encode different modalities:

Modality Receptor type
Touch/pressure Mechanoreceptors
Pain Nociceptors
Temperature Thermoreceptors
Vision Photoreceptors
Taste Chemoreceptors
Hearing Hair cell mechanoreceptors

They also encode:

  • Intensity (firing frequency)

  • Duration (adaptation)

  • Location (receptive fields)

5. Clinical relevance (why this matters)

  • Local anesthetics block Na⁺ channels → action potentials fail

  • Antidepressants alter receptor signaling and sensitivity

  • Myasthenia gravis targets nicotinic ACh receptors

  • Parkinson’s disease involves dopamine receptor signaling

  • Benzodiazepines enhance GABA_A receptor function

One-line summary

Receptors detect stimuli or neurotransmitters and convert them into electrical or biochemical signals that determine whether, when, and how neurons communicate.

If you want, I can:

  • Contrast receptor potentials vs action potentials

  • Link this to synaptic integration

  • Apply this to USMLE-style questions or clinical cases

+++++++++++++++++++++++++++++++

 

  • Adrenergic: α1A, α1b, α1c, α1d, α2a, α2b, α2c, α2d, β1, β2, β3
  • Dopaminergic: D1, D2, D3, D4, D5
  • GABAergic: GABAA, GABAB1a, GABAB1δ, GABAB2, GABAC
  • Glutaminergic: NMDA, AMPA, kainate, mGluR1, mGluR2, mGluR3, mGluR4, mGluR5, mGluR6, mGluR7
  • Histaminergic: H1, H2, H3
  • Cholinergic: Muscarinic: M1, M2, M3, M4, M5; Nicotinic: muscle, neuronal (α-bungarotoxin-insensitive), neuronal (α-bungarotoxin-sensitive)
  • Opioid: μ, δ1, δ2, κ
  • Serotonergic: 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F, 5-HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT4, 5-HT5, 5-HT6, 5-HT7
  • Glycinergic: Glycine

++++++++++++

Here is a clean, high-yield contrast between receptor potentials and action potentials, the way it is expected in physiology exams and clinical reasoning.

1. Definitions

Receptor potential

  • A graded change in membrane potential produced when a receptor detects a stimulus

  • Occurs in sensory receptor endings or specialized receptor cells

  • Reflects stimulus strength

Action potential

  • A regenerative, all-or-none electrical signal used for long-distance transmission

  • Occurs in axons

  • Encodes information by frequency, not size

2. Side-by-side comparison

Feature Receptor Potential Action Potential
Nature Graded All-or-none
Amplitude Variable Fixed
Threshold No Yes
Summation Yes (temporal & spatial) No
Propagation Decremental (local) Non-decremental
Direction Local One direction
Refractory period None Absolute & relative
Ion channels Stimulus-gated (mechanical, chemical, thermal) Voltage-gated Na⁺ & K⁺
Location Sensory receptors / dendrites Axon
Encodes Stimulus intensity Information via firing rate

3. Functional relationship (how one leads to the other)

  1. Stimulus activates receptor

  2. Receptor potential forms

  3. If depolarization reaches threshold at the trigger zone:

    • → Action potentials fire

  4. Stronger stimulus → larger receptor potential → higher AP frequency

📌 Key point:

Stimulus intensity is coded by the size of the receptor potential but by the frequency of action potentials.

4. Examples

A. Touch receptor (mechanoreceptor)

  • Light touch → small receptor potential → few APs

  • Firm pressure → large receptor potential → many APs

B. Photoreceptors (exception worth knowing)

  • Light causes hyperpolarization, not depolarization

  • Still graded → modulates neurotransmitter release

C. Pain receptors

  • Strong noxious stimulus → large receptor potential

  • Leads to high-frequency firing → perceived as severe pain

5. Clinical correlations

Condition Effect
Demyelination (MS) AP propagation fails
Local anesthetics Block AP generation
Neuropathy Reduced receptor potentials
Hyperkalemia Alters AP threshold
Na⁺ channel mutations AP abnormalities (epilepsy, pain syndromes)

6. High-yield exam pearls

  • Receptor potentials can summate; action potentials cannot

  • Action potentials do not decrease in size with distance

  • Frequency ≠ amplitude for APs

  • Blocking Na⁺ channels abolishes APs but not receptor potentials

One-sentence takeaway

Receptor potentials are graded local signals that detect stimulus strength, while action potentials are all-or-none signals that transmit information over distance by changing firing frequency.

If you want, I can:

  • Walk through a USMLE-style MCQ

  • Explain adaptation (phasic vs tonic receptors)

  • Link this to EEG vs nerve conduction studies

 

Signaling in the Nervous System

Digital World Medical School
© 2025