Basic Principles of Sensation Touch and Pressure Senses Warmth 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 environment Proprioceptors – position in space
Sensors of external environment
Teleceptors – distant environment Exteroceptors – 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 CNS Environmental change Æ Transducer Converter Transmission line Processing Center
Transducer changes one form of energy to electricity Converter 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 Phasic Receptors 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 knob The 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 C Chemical 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 myelinated fibers (5 to 30 m/sec) The more critical types of sensory signals ( precise localization on the skin, minute gradations of intensity, or rapid changes in sensory 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 Lemniscal System
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 lemniscus composed of large, myelinated nerve fibers that transmit signals to the brain at velocities up to 30 to 110 m/sec
The Dorsal Column-Medial Lemniscal System
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 Lemniscal System
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 7 Important roles in deciphering the sensory information that enters the SSAs Receives signals from:
SSA I Ventrobasal nuclei of thalamus Other areas of thalamus Visual cortex Auditory 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 hot Thermal gradations are discriminated by 3 different types of sensory receptors:
Cold receptors Warmth receptors Pain receptors
Pain receptors
Stimulated only by extreme degrees of heat or cold Are 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 rates
Temperature 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 signals On entering the SC Æ few segments upward or downward then terminate in Laminae I, II, III of the dorsal horns Æ enter long, ascending thermal fibers that cross to the opposite anterolateral sensory tract and terminate in:
The reticular areas of the brainstem The ventrobasal complex of the thalamus
