Biology (9610) | Receptors

Welcome to the World of Receptors!

Hi there! This chapter is all about how your body detects the complex world around you, from the gentle touch of a feather to the vibrant colours of a sunset. This is the very first step in the entire Control process—getting the information needed to react!

We will be focusing on two fascinating examples: a receptor responsible for detecting pressure (the Pacinian corpuscle) and the receptors that allow you to see (the cells in the human retina). Understanding these principles is key to understanding the nervous system as a whole.

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3.4.2 Receptors: The First Step in Control

What is a Receptor?

In the context of the nervous system, a receptor is a specialised cell or group of cells that detects a specific type of change, known as a stimulus.

Receptors act as transducers. Think of a transducer as a device that converts one form of energy into another. In biology:

• They convert the energy of the stimulus (like pressure, light, heat, or sound)
• Into electrical energy (a nerve impulse).

Key Principle 1: Receptor Specificity

The syllabus states that receptors respond only to specific stimuli.

• Each type of receptor is highly specialised to detect one particular type of energy/stimulus.
• Example: The Pacinian corpuscle is excellent at detecting pressure, but it won't react to light. Similarly, the photoreceptors in your eye (rods and cones) only react to light, not sound.

Analogy: Imagine a receptor is like a specific keyhole. Only the correct key (the specific stimulus, e.g., pressure) can open the lock (activate the receptor).

Key Principle 2: Establishing a Generator Potential

The second crucial concept is that stimulation of a receptor leads to the establishment of a generator potential.

• When a receptor is stimulated, the energy input causes a change in the membrane potential of the sensory neurone ending.
• This initial, local change in potential difference across the membrane is called the generator potential.
• If the generator potential is large enough (i.e., it reaches the threshold potential), it triggers a full-blown nerve impulse (an action potential) in the sensory neurone, which then travels to the central nervous system.

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3.4.2.1 The Pacinian Corpuscle: A Pressure Sensor

The Pacinian corpuscle is an excellent example of a mechanoreceptor—a receptor that responds to mechanical forces, specifically pressure and vibration.

Basic Structure

The Pacinian corpuscle has a distinctive structure, often described as like a tiny onion cut in half:

• It consists of a single sensory neurone ending, found deep within the corpuscle.
• This ending is surrounded by multiple concentric layers (like the rings of an onion) made of connective tissue called lamellae.
• Between the lamellae, there is viscous gel.
• Crucially, the membrane of the sensory neurone ending contains special stretch-mediated sodium ion channels.

Step-by-Step Mechanism: How Pressure Becomes Electricity

Don't worry if the term "stretch-mediated sodium ion channels" sounds intimidating! It just means that when the membrane stretches, these channels physically open up.

Here is the sequence of events when pressure is applied to the skin:

1. A pressure stimulus (mechanical energy) is applied to the skin.
2. The mechanical force deforms (changes the shape of) the Pacinian corpuscle. The lamellae are squashed together.
3. This deformation causes the plasma membrane of the sensory neurone ending to stretch.
4. The stretching physically opens the stretch-mediated sodium ion channels in the membrane.
5. Sodium ions (Na+) rapidly diffuse into the sensory neurone ending down their concentration gradient.
6. The influx of positive Na+ ions causes the membrane potential to become less negative (depolarisation). This change is the generator potential.
7. If enough pressure is applied, the generator potential reaches the threshold potential.
8. Once the threshold is reached, an action potential (nerve impulse) is generated and travels along the sensory neurone to the CNS.

Did you know? Pacinian corpuscles are particularly sensitive to high-frequency vibrations because these cause rapid changes in the pressure that constantly deform the corpuscle.

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3.4.2.2 The Human Retina: Seeing the World

The retina is the light-sensitive layer at the back of the eye. It contains specialised light receptors called photoreceptors. We need to focus on how the differences between the two main types of photoreceptors—rods and cones—explain our vision.

Rods vs. Cones: Structure and Function

Feature	Rods	Cones
Function	Responsible for vision in low light (scotopic vision).	Responsible for vision in bright light (photopic vision) and colour vision.
Location	Concentrated in the peripheral retina.	Concentrated in the fovea (the central spot of the retina).
Optical Pigment	Contain one pigment: Rhodopsin.	Contain three types of pigments (Iodopsin), each sensitive to a different wavelength (Red, Green, Blue).

Memory Trick: Think C for Cones = Colour.

Differences Explained by Pigments and Sensitivity

1. Sensitivity to Light

Rods have high sensitivity:

• Rhodopsin is bleached (broken down by light) even by very low levels of light energy.
• Therefore, rods are crucial for seeing in the dark or dim conditions.
• They provide monochromatic vision (seeing only in shades of grey) because only one type of pigment is used.

Cones have low sensitivity:

• Iodopsin requires a much higher light intensity to be bleached and generate an impulse.
• Therefore, cones are ineffective in dim light but essential for detailed, bright, and colour vision.

2. Visual Acuity (Detail) and Colour

Cones provide high visual acuity and colour:

• Since there are three types of cones (each absorbing a different wavelength), signals from these allow the brain to distinguish colour.
• When you look directly at an object, the light falls on the fovea, which is packed mainly with cones, giving you sharp, detailed vision (high visual acuity).

Rods provide low visual acuity:

• Since they are monochromatic, rods cannot distinguish between colours.
• Rods are less effective at resolving fine detail. Why? This is explained by their connections in the optical nerve (convergence).

Differences Explained by Neural Connections (Convergence)

The way photoreceptors connect to bipolar cells and then to ganglion cells (which form the optical nerve) dictates their overall performance:

Rod Connections: High Sensitivity, Low Acuity

Rods show a high degree of convergence:

• Many rod cells (sometimes hundreds) are connected to a single bipolar cell, which then connects to a single optical nerve fibre (ganglion cell).
• Advantage (High Sensitivity): A very weak light stimulus hitting several rods at once can trigger a cumulative effect (a form of spatial summation) sufficient to depolarise the single bipolar cell. This makes rods highly effective in dim light.
• Disadvantage (Low Acuity): The brain cannot tell which specific rod out of the hundred was stimulated, meaning the image is blurred and lacks detail.

Cone Connections: Low Sensitivity, High Acuity

Cones show a very low degree of convergence, especially in the fovea:

• Often, one cone cell connects to one bipolar cell, which connects to one optical nerve fibre.
• Advantage (High Acuity): Because the signal path is direct, the brain can precisely identify the location of the light source, resulting in a sharp image (high visual acuity).
• Disadvantage (Low Sensitivity): Each cone needs a strong stimulus on its own to reach the threshold and generate an impulse. This is why you cannot see colours or fine detail in dim light.