Pacinian corpuscle

• Receptors can be classified as either anatomical receptors or cellular receptors.
• Anatomical receptors are specialist structures located close to sensory nerve cells, which allow us to detect physical changes such as temperature or pressure in our external environment and in our bloodstream and body organs.
• Cellular receptors are specialised proteins found on the surface of cell membranes, which respond to the presence of certain chemicals, such as glucose or histamine in the bloodstream or body organs.
• The Pacinian corpuscle is just one of the many different types of anatomical receptor found in the human body.
• Electrical impulses are a fixed, all-or-nothing size, meaning the information about the strength of the stimulus detected by the Pacinian corpuscle can only be carried to the central nervous system by the frequency of nerve impulses.
• Anatomical receptors are the first part in a chain of events that allow us to respond to changes that may be beneficial or harmful to us.
• Each of the different types of anatomical receptor is specific to one particular stimulus.
• The Pacinian corpuscle responds to mechanical force, making it a type of mechanoreceptor.
• Pacinian corpuscles are found in the dermis of the skin and in the hypodermis, a layer of adipose tissue that lies beneath the dermis.
• The upper layer of skin, the epidermis, contains dead cells, relatively few nerve cells and receptors.
• Pacinian corpuscles are found in skin covered by hair and in non-hairy skin, such as that covering the fingertips and lips.
• Pacinian corpuscles are also found near bone joints and in the connective tissue that supports and protects the intestines.
• Pacinian corpuscles are widely distributed in the skin and bone joints, allowing us to detect vibrations and pressure charges that originate from sources outside the body, as well as those that result from movement by parts of the body.
• The different types of mechanoreceptor in the skin each respond to different combinations of pressures and vibrations, giving us our overall sense of touch.
• Pacinian corpuscles respond to rapidly applied, relatively large pressures and to high frequency vibrations.
• Pacinian corpuscles, like all anatomical receptors, act as transducers, changing one form of energy into another type of energy.
• Anatomical receptors convert energy from a specific stimulus into electrical energy, for example, thermoreceptors convert heat energy into electrical energy.
• The electrical energy then travels as a nerve impulse along the sensory nerve cell (neurone) closest to the receptor, carrying information about the stimulus to the coordinator - the brain or spinal cord.
• A Pacinian corpuscle is the largest of the skin's mechanoreceptors.
• These slow movements within the layers of lamellae are not enough to deform the nerve cell membrane and allow the stimulus to be detected.
• Slowly applied weak pressures or low-frequency vibrations just cause the gel within the Pacinian corpuscle to move slowly within the layers of lamellae, like faint ripples on the surface of a pond.
• The generator potential travels along the sensory nerve cell.
• At the centre of the corpuscle is a single sensory nerve cell ending, which is not surrounded by a myelin sheath.
• The stronger the stimulus, the greater the size of the generator potential.
• The nerve cell ending is surrounded by between 20 and 60 layers (lamellae), and an outer capsule layer.
• These lamellae are similar to Schwann cells, the cell type that forms the myelin sheath around many nerve cells.
• Pacinian corpuscles are often more than 1mm in length, allowing Filippo Pacini to spot them with the naked eye.
• If the stimulus is of sufficient strength to deform the sensory nerve cell membrane, special proteins in the membrane are also stretched out of shape.
• These stretch-sensitive proteins act as channels through the membrane; only allowing positively charged sodium ions to enter the nerve cell cytoplasm from the fluid gel surrounding the cell.
• The stronger the pressure stimulus, the greater the size of the generator potential.
• The lamellae and the gel filter out the stimulus, so that only rapidly applied high pressures or rapid, high frequency vibrations result in the membrane being squashed.
• The entry of these positively charged sodium ions makes the inside of the sensory nerve cell ending temporarily more positive compared with the outside, producing what is called a generator potential.
• These nerve impulses rapidly transmit information about the stimulus to the brain or spinal cord.
• This gel, together with the lamellae, determines how the Pacinian corpuscle detects and responds to stimuli.
• For the Pacinian corpuscle to detect a-stimulus, the sensory nerve cell membrane must be physically squashed and stretched (deformed).
• The number of lamellae appears to get larger as humans age, increasing the overall size of the Pacinian corpuscle to several millimetres.
• Around the nerve ending and between each of the lamellae Is a thick, gelatinous liquid.
• At the point where the myelin sheath starts to surround the nerve cell, nerve impulses are produced, due to the opening of a different type of sodium ion protein channel.
• Unlike many other types of receptor, the Pacinian corpuscle does not produce generator potentials all the time that a stimulus is applied to It, It only produces generator potentials at the beginning and end of a pressure or vibration stimulus, making it a so-called 'rapidly adapting' or 'phasic receptor. This adaptation can be explained by the behavlour of the luid gel and the lamellae, which effectively 'reset' the shape of the sensory nerve ending, as pressure is applied and removed.