13 Mechanoreceptors and Chemoreceptors Notes

Learning Outcomes

• At the end of this lecture, the learner will be able to:
  • Define the five functional types of sensory receptor
  • Describe three types of encapsulated end organ and one free nerve mechanoreceptors: (Pacinian corpuscles, Meissner's corpuscles, Ruffini's end organs and Merkel's discs)
  • Discuss muscle spindles and Golgi tendon organs as proprioceptors
  • Recall the structure of the taste bud chemoreceptors and describe their function
  • Discuss the olfactory sense and describe the structure and function of the olfactory membrane

Sensory Receptors

• Input into the nervous system is provided by the sensory receptors.
• These are nerve endings or specialised cells which convert stimuli from the external and internal environment into afferent nerve impulses which pass into the CNS.
• SENSE organs of the body include the EYE [optic] and the EAR [auditory].
• Though important, the major senses only provide us with a part of our sensory information.
• The other two special senses, TASTE and SMELL together with the GENERAL SENSES such as light touch, pressure, temperature, pain and a sense of body and limb position are of major importance in maintaining homeostasis

Sensory Receptor Classification

• Sensory receptors may be classified into five functional categories:
  • However there is no entirely satisfactory system of classification which incorporates both functional and morphological features.
  • MECHANORECEPTORS - these detect mechanical stimulation
  • THERMORECEPTORS - these detect changes in temperature
  • NOCICEPTORS (Pain) - these are all free nerve endings (pinching, tearing, burning)
  • PHOTORECEPTORS - the rods and cones of the retina
  • CHEMORECEPTORS - responsible for the senses of taste and smell

Mechanoreceptors

• Of all the receptors, the mechanoreceptors are the most varied;
  • They include free nerve endings, tactile hair cells and complicated ENCAPSULATED END-ORGANS.
  • Free nerve endings:
    • are the ends of dendrites of sensory neurons in tissues detect pain, temperature, and light touch
  • Encapsulated nerve endings:
    • Encapsulated receptors consist of nerve endings surrounded by one or more layers of cells
  • Separate cells
    • Hair cells in the ear
• Mechanoreceptors may be found in many parts of the body such as the skin, muscle, gut.

Mechanical/Tactile Receptors

• Ruffini endings (pressure)
• Meissner corpuscle (touch, vibration)
• Free nerve endings (pain)
• Hair Merkel cells (light touch)
• Pacinian corpuscle (deep pressure)
• Hair receptor (hair movement)

Pacinian Corpuscles

• Pacinian corpuscles (PC) have been compared to a small onion pierced by a thin wire.
• PC are the largest, encapsulated nerve ending: they consist of a central non-myelinated tip of a nerve fibre surrounded by up to 30 concentric layers of connective tissue.
• PC respond to strong or rapid movement within the tissues, they are good detectors of vibration.
• Tissue movement acts on the fluid layer between the concentric layers of connective tissue in the corpuscle.
• PC are the largest of the encapsulated nerve endings reaching lengths of 1 to 2 mm, and 0.5 mm in diameter.

Pacinian Corpuscles Location and Function

• Pacinian corpuscles are found quite deep in the SUBDERMAL skin layers, abdominal mesenteries, external genitalia, tendons, ligaments and joints.
• Pressure transmitted through the connective tissue layers, to the naked nerve ending is thought to cause $Na^+$ ion channels to open.
• This causes depolarisation in the nerve fibre.

Meissner’s Corpuscles

• A common encapsulated sensory receptor, smaller than Pacinian corpuscles. Sensitive to fluttering, light touch. Located in the superficial part of the dermis.
• Oval receptors that contain one or two non-myelinated, spiralling dendritic endings surrounded, by a thin cellular capsule.
• These smaller corpuscles are about 150mm long and 75mm wide. Loose stacks of glial cell lamellae separate 1 or 2 nerve endings.
• They allow the body to recognize what exact point has been touched.

Meissner’s Corpuscles Location

• MC are concentrated in the DERMAL PAPILLAE of the fingertips, lips, palms of the hands, soles of the feet and other sensitive areas of skin.
• Where light touch sensation is important.

Ruffini’s End Organs

• Described by Ruffini in 1894 as loosely encapsulated nerve endings intermediate in size between Pacinian corpuscles and Krause’s end bulbs.
• In a loose connective tissue capsule, the myelinated nerve fibres lose their sheaths and break into a set of branches with terminal knobs.
• There has been disagreement as to their exact role, Ruffini thought they had a tactile function intermediate between Meissner’s (light touch) and Pacinian corpuscles (deep pressure).
• Later they were said to be thermoreceptors.
• Now considered to be slowly adapting mechanical receptors responsive to continuous deformation and distortion (stretch). In joint capsules they indicate the degree of rotation.
• Found in both the DERMIS and deeper organs such as joints.

Merkel’s Discs

• Merkel cells are special cells found in the EPIDERMIS near the stratum basale.
• Also called expanded tip tactile receptors, they are found in smooth skin but less frequently in hairy skin.
• Thought to be concerned with sustained, light touch sensations. E.g. reading Braille.
• Expanded discs at the end of non-myelinated, naked nerve fibres contact the base of the cup- shaped Merkel cells.
• The Merkel cell cytoplasm contains vesicles similar to those found at synapses, but no neurotransmitter has yet been found.

Proprioceptors

• Sense of position- no adaptation (or very slow adapting receptors).
• These receptors continually respond to tension and movement in striated muscles and in tendons.
• There are two main types of proprioceptors,
  • Muscle spindles (detect muscle length) and
  • Golgi tendon organs (detect muscle tension).
• The signals from the two receptors operate entirely at a subconscious level causing no sensory perception at all.
• Despite this, the information they provide allows the CNS to exactly control and coordinate skeletal muscle contraction.

Muscle Spindle

• Each MUSCLE SPINDLE is about 2mm long and 0.5mm wide consisting of 3 to 10 small and specialised muscle fibres surrounded by a tangle of sensory nerve endings enclosed in a connective tissue sheath.
• They appear to emit sensory nerve impulses all the time and are probably more continually active than any other sensory receptors.
• The spindles detect relative muscle length so stretching the muscle increases the rate of firing, thus muscle spindles continually monitor muscle tone.

Golgi Tendon Organ

• The GOLGI TENDON ORGAN resembles the Ruffini endings and lies within the muscle tendon immediately beyond its attachment to the muscle fibres.
• Composed of connective tissue fibers surrounded by dendrites and encased in a capsule.
• On average 10 - 15 muscle fibres are connected to each Golgi organ.
• The Golgi tendon organs detect muscle tension.
• It provides the nervous system with almost instantaneous information on the degree of tension on each small segment of each muscle.

Proprioception and Coordination

• Different amounts of tension and stretch in different tendons/ muscles dependent on position of the joint.
• Proprioception permits coordination and balance.

Sensory Adaptation

• Rapid and slow adaptation of different types of receptor.
• All receptors adapt either partially or completely to a constant stimulus.

Chemoreceptors

• There are two major chemoreceptors those involved in taste and those involved in smell.

Taste

• This is sensed by small organs the taste buds of the tongue.
• Taste buds:
  • In humans there are about 3,000 located in tiny elevations of the tongue, the lingual papilla.
  • Found in smaller numbers on the roof of the oral cavity, the pharynx, and the larynx.
  • Composed of about 50 modified epithelial cells which are arranged around a taste pore
  • Stimulated by chemicals in the food we eat and are therefore referred to as chemoreceptors.
  • Classically two types of cell are described from the taste bud.

Chemoreceptors - Taste

• Gustatory cells and darker sustentacular/ supporting cells with a third basal cell now recognised which may give rise to both.
• Gustatory and sustentacular cells have long microvilli/taste hairs protrude into the pore and are the taste receptors.
• The plasma membrane of taste hairs contain clusters of protein molecules = receptors. These receptors bind to food molecules dissolved in water (saliva).
• Stimulation of the receptor cells causes the stimulates the dendrites of the sensory nerves wrapped around the receptor cells and impulses from the taste bud are transmitted to the brain.
• Zinc- stimulates division of the cells in taste buds.
• Although the gustatory cells are thought to be the taste receptors, the sustentacular cells may serve some receptor function.

Taste bud innervation

• The taste buds are innervated by up to 50 non-myelinated nerve fibres which innervate both cell-types.
• But they seem to have closer relationship with the gustatory cells.

Sweetness sensation

• The taste buds on the tongue are continuously bathed in saliva and the taste receptors detect chemicals dissolved in the saliva.
• Certain molecules, such as for example sugar molecules, activate a signal transduction process involving a G protein.
• Once the chemical interacts with the receptor molecule a chain of events occurs, involving the G protein plus adenylyl cyclase which increases cyclic AMP levels.
• This in turn activates a protein kinase which phosphorylates and closes $K^+$ ion channels.
• Decreased $K^+$ ion permeability sets up a depolarizing receptor potential which generates an action potential in the sensory neuron.
• Other tastes result in changes in different ion permeabilities.

Taste Sensation Distribution

• Humans are said to have four primary sensations of taste - sour, salty, sweet and bitter which are assigned to different areas of the tongue.
• Each area is said to be occupied by a particular type of taste bud, but it has been shown taste buds are more generally distributed.
• A fifth taste, glutamate, (umami) was reported in 2000 but is still somewhat controversial.
• Smell affects flavour because odour pass from the nasal chamber to the mouth.
• Persons with cold or allergies often complain of loss of taste, however it is their olfactory sense that is affected.

Olfactory Sensation- Smell

• The olfactory epithelium is a patch of receptor cells in the roof of the nasal cavity that detects odours.
• The epithelium lies in the upper part of each nostril where it is folded over the turbinal bones giving a surface area of 2.4 $cm^2$ in each nostril.
• In many tetrapods and particularly mammals the sense of smell is the main source of information concerning the outside world.
• We tend not to be conscious of this as the human sense of smell is very poorly developed.

Chemoreceptors - Olfaction

• The olfactory membrane contains about 100 million olfactory receptor cells.
• These cells are quite simple structures contained within the columnar epithelium of the mucus membrane.
• Once more there are three cell types: olfactory receptor cells, sustentacular/supporting cells and basal epithelial cells.

Olfactory Cell Types

• The role of the SUSTENTACULAR CELLS is poorly understood, they have long microvilli but are probably mostly concerned with mechanical and physiological support.
• The olfactory receptor cells are true BIPOLAR NEURONS whose cell body is located in the middle stratum of the mucosa.
• The axons of the sensory neurons pass through about 20 small holes on each side of the cribiform plate of the ethmoid bone and enter the olfactory bulb in the brain.

Olfactory Mucosa

• A single dendritic process extends from the soma to the free surface where it terminates as a small swelling the olfactory knob.
• From the olfactory knob a dozen or so cilia or olfactory hairs (50 - 150 mm long) project outward.
• Each cell gives rise to single non- myelinated axon.
• These join up with others to pass through the bone to reach the olfactory bulbs of the Forebrain.

Olfactory Process

• The cilia are the CHEMORECEPTORS and react to odours in the air stimulating the olfactory cells.
• This process may be similar to the process in the taste buds with a molecule receptor, a G protein, adenylyl cyclase and cyclic AMP.
• cyclic AMP opens gated channels in the receptor membrane.
• The opening of $Na^+$ ion channels allows $Na^+$ ions and other cations to enter the cell causing depolarization and an action potential.

Olfactory Sense

• Classically humans are said to detect seven main classes of odours.
  • Camphor, musk, floral, peppermint, ethereal, pungent and putrid.
• About 1000 genes code for about 1000 types of olfactory receptor.
• Odours are usually composed of several chemical components and each type of receptor may bind with a particular component.
• The combination of receptors activated determines the odour perceived, as a consequence we can distinguish between up to 10,000 different scents.

Olfactory Sensitivity and Adaptation

• The olfactory receptors respond to very small amounts of substance.
• For example, the threshold for olfactory perception for ionone, the synthetic odour of violets, is 1 part in 30 billion.
• Adaptation is a feature of all sensory receptors, and the olfactory receptors adapt most rapidly.
• 50% in the first second or so after stimulation, as a consequence, even offensive odours seem odourless after a few minutes.