Pacinian corpuscle

A Pacinian corpuscle is a sensory receptor that detects mechanical deformation, such as pressure. It contains the ending of a sensory neurone surrounded by layers of connective tissue called lamellae. The membrane of the neurone ending contains stretch-mediated sodium ion channels.

Pacinian corpuscle response to pressure

Pressure deforms the corpuscle and stretches the membrane of the sensory neurone ending. This opens the stretch-mediated channels, allowing sodium ions ($$\ce{Na+}$$) to enter the neurone. The entry of positive ions makes the inside of the membrane less negative in that local region: this is depolarisation.

The local depolarisation is called a generator potential. If it is sufficiently large, it can initiate action potentials in the sensory neurone.

The Pacinian corpuscle illustrates receptor specificity: it is sensitive to mechanical deformation rather than responding equally to every type of stimulus.

Rod and cone photoreceptor comparison

Rods and cones are light-detecting cells, or photoreceptors, in the retina. Each contains a light-sensitive optical pigment. When light stimulates a photoreceptor, signals pass through retinal neurones to ganglion cells. The axons of the ganglion cells form the optic nerve.

Rods and cones differ in their pigments and in how their signals are connected to ganglion cells. These differences help explain their different light sensitivities, their roles in colour vision and their visual acuity: the ability to distinguish fine detail.

Rods: dim-light sensitivity

Rods contain the pigment rhodopsin and are sensitive to low light intensities. Signals from several rods can converge on the same pathway to a ganglion cell. Their effects can therefore be combined, making a signal more likely to reach the ganglion cell in dim light.

This convergence has a cost: the signal does not identify as precisely which rod was stimulated. The position of the light is therefore harder to distinguish, giving rods lower visual acuity. Rods have only one type of optical pigment and do not distinguish colours.

Cones: colour and visual acuity

Cones function best at higher light intensities. There are three types of cone, each containing a pigment with a different sensitivity to wavelengths of light. Comparing the responses of the three types enables colour vision.

Cones generally show less convergence than rods. In the fovea, a cone may connect through a bipolar cell to an individual ganglion cell. This separate pathway allows the position of a stimulus to be distinguished more precisely, giving high visual acuity.