Neurones and Receptors

Photoreceptors:

• Detect changes in light

Mechanoreceptors:

• e.g. Pacinian Corpuscle detect mechanical stimuli in the form of pressure and vibration

Pacinian Corpuscle:

• Found in joints, tendons, and ligaments (also fingers, soles of feet and external genitalia)
• Contain a single sensory neurone located in lamellae (connective tissue) which forms layers separated by a gel.

Contains stretch mediated sodium channels in the cell surface membrane:

• When not under pressure, these channels are closed - Under pressure these become deformed
• They open and allow the rapid influx of sodium ions to occur.
• Positive charge on the sodium ions changes the membrane potential, causing the membrane to become depolarised
• Results in a generator potential being generated which goes on to create an action potential in the axon.

Photoreceptors:

• Light receptors in the eye
• Light enters through the pupil and amount of light entering is controlled by muscles in the iris.
• Lens of the eye focuses the light on the retina (this is where photoreceptors are located, more specifically the fovea)
• Nerve impulses then carried via the optic nerve to the brain

Blind Spot - Point where the optic nerve leaves the eye (no photoreceptor cells are located here)

Cone Cells:

• Involved in colour vision
• Present at the greatest density in the fovea
• Contain the pigment iodopsin
• Not sensitive to light so require quite bright light to work
• Contains three different primary pigments (red, blue and green)
• Provide good visual acuity as each cone cell has its own synapse via a bipolar neurone which connects to the optic sensory neurone

Rod Cells:

• Mainly concentrated in highest density outside of the fovea
• Very sensitive to light - are stimulated by low light conditions
• Provide low visual acuity because more than one rod cell shares the same synapse with a bipolar cell - multiple rods have to be stimulated in order for a generator potential to occur.

Stimulus --> Receptor --> Sensory neurone --> Intermediate (relay) Neurone --> Motor Neurone --> Effector --> Response

Sensory Neurone - Carries nerve impulse from the receptor to the spinal cord

Intermediate (relay) Neurone - Located entirely in spinal cord and relays the nerve impulse from sensory neurone to the motor neurone

Motor Neurone - Carried nerve impulse from the spinal cord to the effector (can be a muscle or gland)

Reflex that bypasses the brain