Sensory

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In the name of Allah,In the name of Allah,the most Beneficent,the most Beneficent,

the most Mercifulthe most Merciful

Sensory receptors

• Sensory receptors respond to stimuli and transmit data about them to the brain.

• In the skin, receptors detect touch, pressure, vibration, temperature, and pain.

• Elsewhere in the body, more specialized receptors detect light (eye works), sound (mechanism of hearing), smell, and taste. Internal receptors called proprioceptors sense body position and the location of body parts in relation to each other.

Sensory Receptors

• In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism.

• In response to stimuli the sensory receptor initiates sensory transduction by creating action potentials in the same cell or in an adjacent one.

Receptor Category Name of Receptor Response to Stimulus

Mechano receptor Pacinian corpuscle Pressure

Meissner’s corpuscle Touch

Merkel cell Light touch

Cochlea (organ of Corti) Sound waves

Semicircular canals Positional changes – balance

Muscle spindles Proprioception

Golgi tendon organ Proprioception

Chemoreceptors

Taste buds Chemicals in saliva

Olfactory tract Chemicals in inhaled air

Carotid body

Oxygen in blood

Nociceptors

Pain due to trauma

Thermoreceptors Cold receptors in epidermis Cool temperatures(10 to 30 C)

Warm receptors in dermis Warm temperatures (32 to 48 C)

Nociceptors Very cold or very hot temp.

Photoreceptor Retina Light

 

Transduction

In physiology, transduction is the conversion of a stimulus from one form to another.

Transduction in the nervous system typically refers to synaptic events where in an electrical signal, known as an action potential, is converted into a chemical one via the release of neurotransmitters.

Sensory Receptors

Figure 7.1

Sensory Receptors

Classification of Sensory Receptors

Based on stimulus location Telereceptors – detect distant stimuli, e.g., vision

and hearing Exteroceptors – detect stimuli on the outside of

body, e.g., pressure and temperature Interoceptors – detect stimuli inside the body,

e.g., blood pressure and blood oxygen

Classification by receptor complexity:

Free nerve endings are dendrites whose terminal ends have little or no physical specialization.

Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue.

Classification of Sensory ReceptorsBased on type of stimuli the receptors can detect

Chemoreceptors – chemicals, e.g., smell and taste Mechanoreceptors – pressure and movement, e.g., touch,

hearing, balance, blood pressure Photoreceptors – light, e.g., vision; detect photons Electroreceptors – electrical fields Magnetoreceptors – magnetic fields Thermoreceptors - temperature

Classification by location

• Sensory receptors can be classified by location:

• Cutaneous receptors are sensory receptors found in the dermis or epidermis.

• Muscle spindles contain mechanoreceptors that detect stretch in muscles

Classification by morphology• Somatic sensory receptors near the surface of

the skin can usually be divided into to groups based on morphology:

• Free nerve endings characterize the nociceptors and thermoreceptors and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout the dermis and epidermis.

• Encapsulated receptors comprise of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning

Receptor (biochemistry)

• In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. Ligand-induced changes in the behavior of receptor proteins result in physiological changes that constitute the biological actions of the ligands

Binding and activation• Ligand binding to a receptor is an equilibrium process: Ligands

bind to an empty receptor and they dissociate from it (according to the law of mass action):

• (the brackets stand for concentrations)

• fits into a given receptor is the binding affinity which is measured as the dissociation constant Kd (good fit means high affinity and a low Kd). The activation of the second messenger cascade and the final biological response is achieved only when at a certain time point a significant number of receptors are activated by bound ligands.

Agonists versus antagonists

• Not every ligand that binds to a receptor also activates the receptor. The following classes of ligands exist:

Full agonists

• (Full) agonists are able to activate the receptor and result in a maximal biological response. Most natural ligands are full agonists

Partial agonists

• Partial agonists are not able to activate the receptor maximally, resulting in a partial biological response compared to a full agonist.

Antagonists

• Antagonists bind to the receptor but do not activate it. This results in a receptor blockade that inhibits the binding of agonists.

Inverse agonists

• Inverse agonists are antagonists that are able to further reduce the receptor activation by decreasing its basal activity

Mechanoreceptor• A mechanoreceptor is a sensory receptor that responds

to mechanical pressure or distortion. • There are four main types in the skin of humans:• Pacinian corpuscles, • Meissner's corpuscles, • Merkel's discs, • and Ruffini corpuscles.• There are also mechanoreceptors in the hairy skin, and

the hair cells in the cochlea are the most sensitive mechanoreceptors in tranducing air pressure waves into sound.

Stimulus Encoding

All stimuli are ultimately converted into action potentials in the primary afferent neurons

Sensory receptors encode four types of information Stimulus modality Stimulus location Stimulus intensity Stimulus duration

YouTube - How the Body Works _ The Sensory Cortex and Touch_1.flv

Pacinian corpuscles

• Pacinian corpuscles are one of the four major types of mechanoreceptor. They are nerve endings in the skin, responsible for sensitivity to pain and pressure.

• Nomenclature• The Pacinian corpuscle was named after its

discoverer, Italian anatomist Filippo Pacini.

Pacinian corpuscles

• Structure• Pacinian corpuscles are larger and

fewer in number than both Merkel cells and Meissner's corpuscles.

• The Pacinian corpuscle is oval shaped and approximately 1 mm in length.

• It has 20 to 60 concentric lamellae separated by gelatinous material. The lamellae are very thin, flat, modified Schwann cells.

• In the center of the corpuscle is the inner bulb, a fluid-filled cavity with a single afferent unmyelinated nerve ending.e

The tip of the central f iber inside the The tip of the central f iber inside the capsule is unmyelinated, but the f iber capsule is unmyelinated, but the f iber does become myelinated shor tly before does become myelinated shor tly before leaving the corpuscle to enter a leaving the corpuscle to enter a peripheral sensory nerve. peripheral sensory nerve.

• All sensory receptors have one feature in common. Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane electrical potential of the receptor. This change in potential is called a receptor potential

• by compression of the corpuscle, ion channels opened in the membrane, allowing positively charged sodium ions to diffuse to the interior of the fiber.

• This creates increased positivity inside the fiber, which is the "receptor potential."

• The receptor potential in turn induces a local circuit of current flow, that spreads along the nerve fiber.

• At the first node of Ranvier, the local current flow depolarizes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system.

(Increasing "Stimulus Strength") (Increasing "Stimulus Strength") applied to the central core of a applied to the central core of a pacinian corpuscle the amplitude pacinian corpuscle the amplitude increases rapidly at first but then increases rapidly at first but then progressively less rapidly at high progressively less rapidly at high stimulus strength. stimulus strength.

that very intense stimulation of the that very intense stimulation of the receptor causes progressively less receptor causes progressively less and less additional increase in and less additional increase in numbers of action potentials.numbers of action potentials.

This is an exceedingly important This is an exceedingly important principle that is applicable to principle that is applicable to almost all sensory receptors.almost all sensory receptors.

It allows the receptor to be It allows the receptor to be sensitive to very weak sensory sensitive to very weak sensory experience and yet not reach a experience and yet not reach a maximum firing rate until the maximum firing rate until the sensory experience is extreme. sensory experience is extreme.

This allows the receptor to have an This allows the receptor to have an extreme range of response, from extreme range of response, from very weak to very intense. very weak to very intense.

Relation Between Stimulus Intensity and the Receptor Potential.

Adaptation of Receptors• receptors adapt either partially or completely to any constant stimulus after a period of time.

• That is, when a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often to none at all.

Meissner's corpuscles

• Meissner's corpuscles (or tactile corpuscles) are a type of mechanoreceptor

• They are a type of nerve ending in the skin that is responsible for sensitivity to light touch.

• In particular, they have highest sensitivity (lowest threshold) when sensing vibrations lower than 50 Hertz. They are rapidly adaptive receptors.

• Meissner's corpuscles were discovered by the anatomist Georg Meissner (1829-1905).

Meissner's corpuscle

• Papilla of the hand, treated with acetic acid. Magnified 350 times.

A. Side view of a papilla of the hand.a. Cortical layer.b. Tactile corpuscle.c. Small nerve of the papilla, with neurolemma.d. Its two nervous fibers running with spiral coils around the tactile corpuscle.e. Apparent termination of one of these fibers.

Location

• They are distributed throughout the skin, but concentrated in areas especially sensitive to light touch, such as the fingertips, palms, soles, lips, tongue, face and the skin of the male and female genitals (non hari skin) .

• They are primarily located just beneath the epidermis within the dermal papillae

Function texture • Any physical deformation in the corpuscle will cause an

action potential in the nerve. Since they are rapidly adapting, the action potentials generated quickly decrease and eventually cease.

• If the stimulus is removed, the corpuscle regains its shape and while doing so (ie: while physically reforming) causes another volley of action potentials to be generated.

• (This is the reason one stops "feeling" one's clothes.) Because of their superficial location in the dermis, these corpuscles are particularly sensitive to touch and vibrations

http://science.tjc.edu/_vti_bin/shtml.dll/Course/BIOLOGY/bott/anp1_online/skin/nerves_skin.htm/map

Merkel nerve ending• Merkel nerve endings are

mechanoreceptors found in the skin and mucosa that provide touch information to the brain.

• Each ending consists of a Merkel cell in close apposition with an enlarged nerve terminal. This is sometimes referred a Merkel disk receptor.

• A single afferent nerve fibre branches to innervate up to 90 such endings. They are classified as slowly adapting type I mechanoreceptors.

Location

• In mammals, Merkel nerve endings have a wide distribution. Merkel nerve endings are found in the basal layer hairy skin, in hair follicles, and in oral and anal mucosa.

• In humans, Merkel cells (along with Meissner's corpuscles) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips that make up fingerprints.

Functions

• Their rigid structure, and the fact that they are not encapsulated, causes them to have a sustained response (in the form of action potentials or spikes) to mechanical deflection of the tissue. They are the most sensitive of the four main types of mechanoreceptors

• Because of their sustained response to pressure, Merkel nerve endings are classified as slowly adapting.

• Merkel nerve endings are extremely sensitive to tissue displacement, and may respond to displacements of less than 1 μm

Ruffini ending temperature • Named after Angelo Ruffini,

the Ruffini ending is a class of slowly adapting mechanoreceptor thought to exist only in the glabrous (without hair)dermis and subcutaneous tissue of humans.

• This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement

Free nerve ending

• A free nerve ending (FNE) is an specialized, afferent nerve ending, meaning it brings information from the body's periphery toward the brain.

• They function as cutaneous receptors and detect pain.

Structure

• Free nerve endings are unencapsulated and have no complex sensory structures, unlike those found in Meissner's or Pacinian corpuscles. They are the most common type of nerve ending, and are most frequently found in the skin. They penetrate the epidermis and end in the stratum granulosum.

Types

• Free nerve ending have different rate of adaptation, stimulus modalities and fiber types

Rate of adaption

• Different types of FNE can be rapidly adapting, intermediate adapting, or slowly adapting. A δ fibres are fast-adapting, while C fibers are slowly adapting.

Modality

• Free nerve endings can detect temperature, mechanical stimuli (touch, pressure, stretch) or pain (nociception). Thus, different free nerve endings work as thermoreceptors, cutaneous mechanoreceptors and nociceptors. In other words, they express polymodality

• nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain

Receptors and stimulus Location: Can distinguish the location of the

stimulus (touch, light or odour) Duration: Determine length of stimulus by

responding to the stimulus for the duration of the stimulus.

Intensity: Increase in action potential frequency or increase in neurotransmitter release.

Sensitivity to Multiple Modalities

• Adequate stimulus – preferred or most sensitive stimulus modality

• Many receptors can also be excited by other stimuli, if sufficiently large, e.g., pressure on eyelid perceive bright light

• Nociceptors – sensitive to strong stimuli, e.g., pain; many are polymodal receptors

Memory• Only a small fraction of the most important sensory

information usually causes immediate motor response. • Much of the information is stored for future control of

motor activities and for use in the thinking processes.• Most storage occurs in the cerebral cortex, but even the

basal regions of the brain and the spinal cord can store small amounts of information.

• The storage of information is the process we call memory, and this, too, is a function of the synapses. That is, each time certain types of sensory signals pass through sequences of synapses, these synapses become more capable of transmitting the same type of signal the next time, a process called facilitation.

• After the sensory signals passed through the synapses for multiple times, the synapses become so facilitated that signals generated within the brain itself can also cause transmission of impulses through the same sequences of synapses, even when the sensory input is not excited.

• This gives the person a perception of experiencing the original sensations, although the perceptions are only memories of the sensations.

"Second Messenger" System in the Postsynaptic Neuron.

• The process of memory-require prolonged changes in neurons for seconds to months after the initial transmitter substance is gone.

• The ion channels are not causing prolonged postsynaptic neuronal changes because these channels close within milliseconds after the transmitter substance is no longer present.

• prolonged postsynaptic neuronal excitation or inhibition is achieved by activating a "second messenger" chemical system inside the postsynaptic neuronal cell itself, and then it is the second messenger that causes the prolonged effect.

• The most common types of second messenger systems, use a group of proteins called G-proteins. • G-protein is attached to the portion of the receptor that protrudes into the interior of the cell. • The G-protein in turn consists of three components: an alpha (α) component that is the activator

portion of the G-protein, and beta (β) and gamma (γ) components that are attached to the alpha component and also to the inside of the cell membrane adjacent to the receptor protein. On activation by a nerve impulse, the alpha portion of the G-protein separates from the beta and gamma portions and then is free to move within the cytoplasm of the cell.

• Inside the cytoplasm, the separated alpha component performs four changes. They are as follows:

1 Opening specific ion channels through the postsynaptic cell membrane. a potassium channel that is opened in response to the G-protein; this channel often stays open for a prolonged time, in contrast to rapid closure of directly activated ion channels that do not use the second messenger system.

• Activation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) in the neuronal cell. Recall that either cyclic AMP or cyclic GMP can activate highly specific metabolic machinery in the neuron and, therefore, can initiate any one of many chemical results, including long-term changes in cell structure itself, which in turn alters long-term excitability of the neuron.

• Activation of one or more intracellular enzymes. The G-protein can directly activate one or more intracellular enzymes. In turn the enzymes can cause any one of many specific chemical functions in the cell.

• Activation of gene transcription. This is one of the most important effects of activation of the second messenger systems because gene transcription can cause formation of new proteins within the neuron, thereby changing its metabolic machinery or its structure. Indeed, it is well known that structural changes of appropriately activated neurons do occur, especially in long-term memory processes.

YouTube - G-Protein Signaling.flv

YouTube - cAMP Signaling.flv

YouTube - Zoya Maslak_ Yuri Rashkin - The G Protein Story.flv

Dr Alamzeb MBBS M.Phil