Module 3 PNS: Receptor Physiology (Afferent Division)

What are afferent neurons also called sensory or receptor neurons? Where are they found? What is their structure?

neurons that carry nerve impulses from peripheral receptors or special sense organs to the central nervous system.

• their cell bodies are found in clusters called ganglia immediately external to the spine.

• their axons extend into the spinal cord’s dorsal horn

Structure: small round cell body with a single long dendrite and short axon

• dendrite extends to periphery and acts as an axon; once stimulus is received it conducts an action potential towards the cell body.

What does the afferent division of the peripheral nervous system deal with?

Sensory information

• pain

• vision

• hearing

• equilibrium

• taste

• smell

Receptors of afferent neurons are located at the peripheral ending of the afferent neuron. What do they do? How are they activated? What happens when they are activated?

respond to stimuli from the external and internal environment.

• to be activated: must have their minimum threshold of stimulus met. If a stimuli’s strength doesn’t meet the threshold potential, then it’ll create subthreshold potentials. if it hits threshold, it’ll make an AP.

When activated: afferent neurons use these APs to inform the central nervous system that the threshold stimulus has been reached and a receptor has been activated.

When an afferent neuron sends a signal to the CNS indicating that a stimulus threshold has been reached and a receptor has been activated, a conversion must occur for the signal to be read by the CNS. What is this conversion, and what is it called?

Transduction

• conversion of the environmental signal to an electrical signal (AP).

Different types of stimuli can initiate transduction. The individual action potential’s themselves are all the same. However, the resulting electrical signals aren’t, as if they were the CNS couldn’t distinguish them. What makes these electrical PNS signals different so that the CNS can differentiate them?

Afferent neurons use 4 properties to help the CNS differentiate PNS signals:

• modality

• duration

• intensity

• location

Describe how afferent neurons use modality to help the CNS discern different electrical signals from the PNS.

each type of receptor is specialized to respond to different stimuli. For example:

• photoreceptors → responsive to visible wavelengths of light

• thermoreceptors → sensitive to heat

• mechanoreceptors → responsive to mechanical energy, vibration and acceleration

• chemoreceptors → sensitive to specific chemicals (think tastebuds).

Describe how afferent neurons use intensity to help the CNS discern different electrical signals from the PNS.

action potentials are an all or nothing phenomenon so afferent neurons can’t encode a stimuli’s strength by increasing or decreasing amplitude.

Instead it can trigger an increased frequency of firing.

• nerve cells code intensity of a stimuli by AP frequency

Describe how afferent neurons use location to help the CNS discern different electrical signals from the PNS.

the brain can identify the site of sensory stimulation using the activated afferent fibre’s locations. Afferent neurons can encode the location of a stimulus in three ways:

• receptive field → each neuron has an environment region that its’ sensitive to, so if a stimulus appears in its’ receptive field, the neuron will fire and send it’s location to the brain.

• multiple sensors → our brain can compare inputs from more than one sensor (ex. 2 eyes)

• gradients → (with smell) we can determine location based on gradients (ex. smell becoming more intense= closer to the source).

Describe how afferent neurons use duration to help the CNS discern different electrical signals from the PNS.

Afferent neurons can encode the duration of a stimulus for the brain to process.

• some cells fire for as long as the stimuli is present there.

• others fire briefly at the onset of stimuli and the offset of stimuli.

Receptors come in two forms. What are these? Does activation differ between them?

Specialized ending of an afferent neuron

• connected to the afferent neuron in image

Separate Receptor Cell (closely associated with the peripheral ending of a neuron).

• not connected to afferent neuron in image

Transduction occurs in these receptors, thus receptor activation occurs very similarly in both receptors.

What happens when receptors (both specialized afferent endings and separate receptor cells) are stimulated?

Stimulation alters their membrane permeability, causing the nonselective cation (+ ions) channels to open. When a cation enters a neuron, it depolarizes the membrane.

THUS, stimulation causes depolarization.

What is the difference between receptor potential and generator potential? What are there similarities?

Receptor Potential → change in potential in specialized receptor cells due to incoming signal

Generator Potential → change in potential in ending of afferent neuron due to incoming signal

Similarities

• both graded potentials meaning the amplitude and duration vary depending on the strength and length of incoming stimuli (EPSPs and IPSPs)

• if graded potentials are of significant magnitude, they’ll initiate an AP in the afferent neuron.

Describe how Specialized Afferent Ending Receptors trigger an action potential compared to how the Separate Receptor Cell does.

Specialized Afferent Ending

• receptor potential itself can cause the afferent fibre to reach threshold and trigger an AP (remember stimulation of the cell causes cation channels to open and depolarization)

• apart of afferent fibre

Separate Receptor Cell

• when receptor potential is strong enough, it releases a chemical messenger that diffuses to the afferent neuron

• this opens the chemical-gated Na+ channels

• If the threshold is then reached by this influx of sodium, the afferent nerve fibre initiates and propagates an AP.

Stimuli of the same intensity don’t always bring about the same magnitude of receptor potential. Why does this occur?

Adaptation → Receptors can regulate their responses; they can adapt to the signal by enhancing or lessening their response to the signal.

There are two types of receptors that vary in their speed of adaptation. What are they? Briefly explain both.

1. Tonic Receptors → slow adapting or don’t adapt at all!

   • important in situations where a near constant signal from stimulus is needed

   • ex. pain receptors (CNS requires knowledge of painful and potentially dangerous stimuli)

2. Phasic Receptors → rapidly adapting

   • upon initiation of stimulus, APs are generated.

   • However, receptor will also stop generating APs rapidly even in the presence of the stimulus.

   • once stimulus is removed, phasic receptors respond with depolarization (fire again) called an “off response”

   • important for monitoring change in stimulus intensity

   • ex. think of wearing a ring or watch, when you first put them on you feel them, but very rapidly their sense of touch is gone until you remove them and the off response is activated (mechanoreceptors)