Basic Principles of SensationTouch and Pressure SensesWarmth and Cold Senses

Amelyn R. Rafael, MD

Sensory Receptor

A specialized structure which can be stimulated by environmental changes as well as by changes within the body.It transforms different types of energy into nerve impulses which travel through sensory nerve fibers toward the CNS.

The information provided by the sensory nerve impulses, when decoded and analyzed, results in sensation.

General System

Feedback system in maintaining homeostasis:

Information about external/internal environment

CNS

External / Internal environment (Corrective Action)

Sensors of the internal state of the body:

Interoceptors – internal environmentProprioceptors – position in space

Sensors of external environment

Teleceptors – distant environmentExteroceptors – immediate environment

Kinesthetic Sensations

Kinesthesia Conscious recognition of the orientation of the different parts of the body with respect to each other as well as of rates of movement of the different parts of the body.

Information Processing

How information is relayed from receptors to CNSEnvironmental change Transducer

ConverterTransmission line

Processing Center

Transducer changes one form of energy to electricityConverter changes output of a transducer to a form best carried by the transmission line (information coding)Transmission line transmits the frequency code from the periphery to the CNS

Two types of Coding Mechanisms

Amplitude modulation A force is transformed to an electrical signal of proportional magnitude There is linear relationship between the input force and the amplitude of electrical pulse

Frequency Modulation A force is transformed to electrical signals of a proportional frequency, all of the same amplitude There is a linear relationship between the input force and the frequency of electrical impulses

Since the nervous system codes sensory information in AP There can be no amplitude modulation in the nervous system All peripheral neurosensory information is coded as a frequency modulation

Specific Irritability of Receptors

Discrimination between different types of stimuli begins at the level of receptors Each receptor has a low threshold for only one particular type of stimulus A receptor maybe stimulated by a much stronger stimulus of any kind but the resulting sensation will be the one normally engendered by that receptor

Quality of Sensation

Modality of sensation The specific quality of the sensation associated with a particular receptor

Determined by where in the brain the sensory pathway from the receptor ends

Localization of Sensation

For any quality of sensation, the intensity can vary

Variation in the frequency of action potential discharge Variation in the number of sensory receptors activated

Projection of Sensation

Sensory input from a receptor reaches the level of consciousness or is perceived, in the cerebral cortex, but the sensation is projected back to the site of stimulation

Generator Potential

If a sensory receptor is stimulated, one may record electrical activity originating at the nerve endings, which in turn is responsible for the initiation of the all-or-none discharges in the nerve fiber Initial electrical activity Originate at the nerve endings and remain localized without being propagated

Adaptation of Receptors

When a maintained stimulus of constant strength is applied to a receptor, the frequency of APs in its sensory nerve declines over time They adapt either partially (tonic) or completely (phasic) to the same specific stimuli after sometime.

Mechanism by which Receptors Adapt

Results from the readjustments in the structure of the receptor itself. Results from accomodation in the terminal nerve fibril.

Difference Between Tonic and PhasicReceptors

Tonic Receptors • show a steady depolarization with time •steady rate of AP firing •gives constant information about the absolute status of some parameters in the periphery

Phasic Receptors

Phasic Receptors show decreasing depolarization with time only fire during changes in stimulation and therefore detects rate of change of the stimulus

Types of Sensory Receptors

Mechanoreceptors – detect mechanical deformation Thermoreceptors – detect changes in temperatures Nociceptors – detect tissue damage Electromagnetic receptors – detect light changes Chemoreceptors – detect changes in taste, smell and blood concentrations

Mechanoreceptors

Pacinian Corpuscle An oval capsule about 1 mm long which surrounds the primary afferent neuron Found in the mesentery, viscera, near bones, and in the dermis esp in the fingertips Rapidly adapting

Mechanoreceptors

Ruffini Endings Thin capsule surrounds the nerve terminal The nerve terminal breaks up into a tuft of fibers which are connected to collagen fibrils of the skin Slowly adapting Functions as a pressure receptor

Mechanoreceptor

Merkel’s Disk Receptors are found in touch spots about 100-400 U in diameter Modified epithelial cells serve as receptors and are connected to the primary afferent neurons by synaptic transmission Slowly adapting, but activity will cease during a maintained stimulus

Mechanoreceptors

Free Nerve Ending Most numerous of endings in the skin Formed by the termination of both myelinated and unmyelinated fibers Adapts relatively quickly

Mechanoreceptors

Meissner’s corpuscle a touch receptor of special sensitivity an elongated encapsulated nerve ending that excites a large (type Aβ) myelinated nerve fiber present in nonhairy parts of the skin and are particularly abundant in the fingertips, lips, and other areas of the skin where one's ability to discern spatial characteristics of touch is highly developed particularly sensitive to movement of very light objects over the surface of the skin and also to low frequency vibration.

Mechanoreceptors

Hair end-organ a touch receptor ( the hair and its basal nervefiber) slight movement on any hair stimulates the nerve fiber entwining its base adapts readily detects mainly movement of objects on the surface of the body or initial contact with the body

Temperature

Peripheral thermoreceptors contribute to temperature regulation Thermoreceptors respond either to an increase or decrease in skin temperature and are distributed throughout the skin The temperature receptors cannot give information about absolute temperature only about a change in temperature in their immediate vicinity Morphologically distinct receptors for temperature have not been found. Both cold and warm receptors are served by free nerve endings

Olfaction

Olfactory receptors are found on the specialized ends of the olfactory nerve fibers

The distal process enlarges to form an olfactory knobThe receptors are found on the cilia which extend from the olfactory knobs

Olfactory neurons can either increase or decrease their activity both when the stimulus is presented and when it is removed

Pain

Pain receptors are found on free nerve terminals of unmyelinated C fibers and myelinated A delta fibers. They are sensitive to:

Strong mechanical stimulation such as pressure, pinprick or squeezing of the skin

Thermal sensation above 45 deg C or below 15 deg CChemical stimulation with such agents as serotonin, bradykinin, histamine, prostaglandin, and hydrogen ion

Specialized Sensory Receptors

that transmit their signals in type Aβ nerve fibers that have transmission velocities ranging from 30 to 70 m/sec:

Meissner's corpuscle Iggo dome receptors hair receptors Pacinian corpuscles Ruffini's endings

Free nerve ending tactile receptors -small type Aδ myelinatedfibers (5 to 30 m/sec) The more critical types of sensory signals ( precise localization on the skin, minute gradations of intensity, or rapid changes insensory signal intensity) -more rapidly conducting types of sensory nerve fiber Crude pressure, poorly localized touch, tickle –slower, very small nerve fibers that require much less space in the nerve bundle than the faster fibers.

The Two Sensory Pathways

for transmission of somatic signals into the CNS

The dorsal column-medial lemniscal system The anterolateral system

The Dorsal Column-Medial LemniscalSystem

carries signals mainly in the dorsal columns of the cord, and then after synapsing and crossing to the opposite side in the medulla, upward through the brain stem to the thalamus by way of the medial lemniscuscomposed of large, myelinated nerve fibers that transmit signals to the brain at velocities up to 30 to 110 m/sec

The Dorsal Column-Medial LemniscalSystem

has a very high degree of spatial orientation of the nerve fibers with respect to their origin sensory information that must be transmitted rapidly and with temporal and spatial fidelity is transmitted here Limited to the more discrete types of mechanoreceptive sensations

The Dorsal Column-Medial LemniscalSystem

Types of sensations transmitted: Touch sensations requiring high degree of localization of stimulus. Touch sensations requiring transmission of fine gradations of intensity. Phasic sensations (vibratory) Sensations that signal movement against the skin Position sensations Pressure sensations having to do with fine degrees of judgment of pressure intensity.

Widespread bilateral excision of SSA I causes: 1. Inability to localized discretely the different

sensations in the different parts of the body. 2. Inability to judge critical degrees of pressure

against the body 3. Inability to judge the weights of objects. 4. Astereognosis – inability to judge shapes or

forms of objects 5. Inability to judge texture of materials

Somatic Sensory Association Areas

Brodmann areas 5 and 7Important roles in deciphering the sensory information that enters the SSAsReceives signals from:

SSA IVentrobasal nuclei of thalamusOther areas of thalamusVisual cortexAuditory cortex

Amorphosynthesis

Effect of removing the somatic association area Lost of ability to recognize complex objects and complex forms by the process of feeling them on the opposite side of the body He loses most of the sense of form of his own body on the opposite side

The Anterolateral System

signals, after originating in the dorsal horns of the spinal gray matter, cross to the opposite side of the cord and ascend through the anterior and lateral white columns of the cord to terminate at all levels of the brain stem and also in the thalamus. composed of much smaller myelinated fibers ( ave 4um diameter) that transmit signals at velocities from few meters/second to 40m/sec has a smaller degree of spatial orientation sensory information which does not need to be transmitted rapidly nor with great spatial fidelity is transmitted here has the ability to transmit a broad spectrum of sensory modalities: pain, warmth, cold, and crude tactile sensations

Origin of Fibers

Laminae I, IV, V, VI in the dorsal horns of the SC

The upper terminus is twofold: throughout the reticular nuclei of the brain stem in two different nuclear complexes of the thalamus, the ventrobasal complex and the intralaminar nuclei

The Anterolateral System

Types of sensations transmitted: Pain Thermal sensations ( warm and cold) Crude touch and pressure sensations capable of only crude localizing ability on the surface of the body Tickle and itch sensations Sexual sensations

Characteristics of Transmission in the Anterolateral Pathway

The velocities of transmission are only 1/3 to ½ those in the dorsal column-medial lemniscal system (8 and 40 m/sec) The degree of spatial localization of signals is poor (pain pathways)The gradations of intensities are also far less accurate, most of the sensations being recognized in 10 to 20 gradations of strength (as many as 100 gradations for the dorsal column system)The ability to transmit rapidly repetitive signals is poor

Thermal Sensation

Different gradations of cold and heat: freezing cold, cold, cool, indifferent, warm, hot, burning hotThermal gradations are discriminated by 3 different types of sensory receptors:

Cold receptorsWarmth receptorsPain receptors

Pain receptors

Stimulated only by extreme degrees of heat or coldAre responsible for “freezing cold” and “burning hot” sensations

Cold and Warmth receptors

located immediately under the skin at discrete but separate points there are 3 to 10 times as many cold receptors as warmth receptors; fewer numbers of warmth points

Cold Receptor

special, small, type A delta myelinated nerve ending that branches a number of times, the tips of which protrude into the bottom surfaces of basal epidermal cells signals are transmitted via delta nerve fibers at velocities of up to 20m/sec some cold sensations via type C nerve fibers

Mechanism of stimulation of thermal receptors

Changes in metabolic ratesTemperature alters the rates of intracellular chemical reactions more than twofold for each 10 degree centigrade change

Thermal detection results from chemical stimulation of the endings as modified by the temperature

Spatial Summation of Thermal Sensations

It is difficult to judge gradations of temperature when small areas are stimulated When a large area of the body is stimulated all at once, the thermal signals from the entire area summate

Transmission of Thermal Signals in the Nervous System

Transmitted in almost parallel pathways as pain signalsOn entering the SC few segments upward or downward then terminate in Laminae I, II, III of the dorsal horns enter long, ascending thermal fibers that cross to the opposite anterolateral sensory tract and terminate in:

The reticular areas of the brainstemThe ventrobasal complex of the thalamus