Receptors detect stimuli. Pacinian corpuscles are receptors that detect stimuli, such as pressure, in your skin and photoreceptors are receptors that detect light in your eye. Stimuli can be detected by baroreceptors and chemoreceptors, which causes a change in heart rate.

Receptors

Definition

Receptors are cells or proteins on cell surface membranes that detect a stimulus and pass on information about it. Each receptor is specific to a different type of stimulus.

Pacinian corpuscles

Pacinian corpuscles are pressure receptors found deep in your skin, they respond to mechanical stimuli such as pressure and vibrations which is why they are known as mechanoreceptors.

Biology; Stimuli; KS5 Year 12; Mammalian sensory receptors — 1. Layers of connective tissue with viscous gel between, 2. Neurone ending, 3. Capsule, 4. Sensory neurone, 5. Blood capillary.

The centre of a Pacinian corpuscle contains a sensory nerve ending which a sodium ion channel known as a stretch-mediated sodium ion channel. When these ion channels are deformed by stretching or another mechanical stimulus, they open and sodium ions are able to diffuse into the cell.

Pacinian corpuscle mechanism

1.	In its resting state, the stretch mediated sodium ion channels are too narrow to allow sodium ions through. The Pacinian corpuscle neurone has a resting potential.
2.	If pressure is applied to the Pacinian corpuscle, it becomes deformed and the membrane surrounding its neurone is stretched. This causes the stretch-mediated sodium ion channels to widen and allows sodium ions to diffuse into the membrane.
3.	The influx of sodium ions causes the membrane to become depolarised as the potential of the membrane has changed and a generator potential is produced. The generator potential creates an action potential which passes along the neurone to other neurones to the central nervous system.

Photoreceptors

Definition

Photoreceptors are light receptors that are found in the eye.

1.	Iris	The muscles of the iris control the amount of light that enters the eye.
2.	Pupil	This is where light enters the eye.
3.	Lens	Light rays are focused by the lens.
4.	Retina	Light rays are focused onto the retina which contains photoreceptor cells.
5.	Fovea	This is an area of the retina that contains lots of photoreceptors.
6.	Optic nerve	This is a bundle of neurones that carries nerve impulses from photoreceptor cells to the brain.
7.	Blind spot	This is where the optic nerve leaves the eye, there are no photoreceptor cells and it is therefore insensitive to light.

Rod and cone cells

There are two main types of photoreceptor, these are rod and cone cells.

Rod cells	Cone cells

Rod-shaped.	Cone-shaped.
Much more abundant than cone cells.	Fewer numbers than rod cells.
Found in the peripheral parts of the retina, they are absent in the fovea.	Less are found in the periphery, they are more concentrated in the fovea.
Low visual acuity because many rods join the same neurone so light from two points close together cannot be differentiated.	High visual acuity because cones are close together and one cone is attached to one neurone.
Sensitive to light so work well in dim light because many rods join one neurone, meaning many weak generator potentials combine to reach the threshold and trigger an action potential.	Less sensitive to light so work better in bright light as one cone is attached to one neurone so more light is required to reach the threshold and trigger an action potential.
Contains the pigment rhodopsin.	Contains the pigment iodopsin.
One type; only give information in black and white.	Three types; each contain a different optical pigment: red-sensitive, green-sensitive and blue-sensitive. They are stimulated in different colours which is what allows you to see in different colours.

Structure of the retina

Heart rate

Stimuli that are detected by receptors can cause heart rate to speed up or slow down.

Stimulus	Receptor	Neurone and transmitter	Effector	Response
High blood oxygen, low carbon dioxide or high pH.	Chemoreceptors	This sends impulses to the medulla, which in turn, sends impulses along parasympathetic neurones. They secrete acetylcholine which binds to receptors on the sinoatrial node.	Cardiac muscles	Heart rate decreases to return oxygen, carbon dioxide and pH levels back to normal.
Low blood oxygen, high carbon dioxide or low pH.	Chemoreceptors	This sends impulses to the medulla, which in turn, sends impulses along sympathetic neurones. They secrete noradrenaline which binds to receptors on the sinoatrial node.	Cardiac muscles	Heart rate increases to return oxygen, carbon dioxide and pH levels back to normal.
High blood pressure.	Baroreceptors	This sends impulses to the medulla, which in turn, sends impulses along parasympathetic neurones. They secrete acetylcholine which binds to receptors on the sinoatrial node.	Cardiac muscles	Heart rate decreases to reduce blood pressure back to normal.
Low blood pressure.	Baroreceptors	This sends impulses to the medulla, which in turn, sends impulses along sympathetic neurones. They secrete noradrenaline which binds to receptors on the sinoatrial node.	Cardiac muscles	Heart rate increases to increase blood pressure back to normal.

Learn with Basics

Length:

Unit 1

The nervous system and the reflex arc

Unit 2

Responding to stimuli: the nervous system and reflexes

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Unit 3