SENSATIONS SENSORY RECEPTORS OBJECTIVES 1 Define sensory systems

ﺑﺴﻢ ﺍﻟﻠﻪ ﺍﻟﺮﺣﻤﻦ ﺍﻟﺮﺣﻴﻢ SENSATIONS & SENSORY RECEPTORS OBJECTIVES: 1. Define sensory systems, sensory units, receptive fields, sensory information, sensation, & perception 2. Describe the neural pathways in somato-sensory systems 3. Describe sensory receptors: their functions, classification & properties 4. Define sensory coding 5. Understand the roles played by the various sensory components in sensory coding 6. List the various types of sensaions 7. Describe in detail the types & properties of somatic sensations Human Physiology, Vander, Sherman & Luciano, 8 th ed. , Chapter 9 Human Anatomy & Physiology, Elaine Marieb, 4 th ed. , Chapter 13, pp. 456 -461; Chapter 15; pp. 518 -522; Chapter 16, pp. 436

WHAT IS A SENSORY SYSTEM? § A Sensory System is apart of the nervous system that consists of sensory receptors that recieve stimuli from the external or internal environment, the neural pathways that conduct information from the receptors to the brain, and those parts of the brain that deal with processing the information § Information processed by a sensory system may or may not lead to conscious awareness of the stimulus § Regardless of whether the information reaches consciousness, it is called Sensory Information § If the information does reach consciousness, it is called a Sensation § The person’s understanding of the sensation’s meaning is called Perception § Sensations & perceptions occur in the brain § An analogy is how the telephone works: The telephone changes sound waves into electric impulses, which are then transmitted along wires to the reciever. The reciever then changes the coded electric impulses back into sound waves. The only difference is that our brain does not physically translate the code into sound, but the coded information itself or is what we percieve as sound § A Sensory Unit consists of a single afferent neuron with all its receptor endings § The portion of the body that, when stimulated leads to activity in a particular afferent neuron, is called the Receptive Field for that neuron

NEURAL PATHWAYS IN SOMATO-SENSORY NEURONS Ascending sensory pathways exhibit a chain of three neurons: 1) the cell body of the afferent (or first-order neuron in the dorsal root ganglion) 2) the second-order neuron in the dorsal horn of the spinal cord or medulla oblongata 3) the third-order neuron in the thalamus § Collateral synapses occur all along the pathway

SENSORY RECEPTORS § Sensory Receptors are specialised structures located at the peripheral end of the sensory neuron § Activation of a sensory receptor by an adequate stimulus results in local depolarisations or graded potentials that, in turn, trigger nerve impulses along the afferent fibres going to the CNS § The functions of sensory receptors can be sumarised as follows: (1) Detection: detect energy changes in the external & internal environments (2) Trancduction: transforming detectied changes into elecrical impulses & action potentials (APs) (3) Inform the CNS about changes occuring inside & outside the body The properties of sensory receptors are: (1) Specificity: each receptor responds to a specific form of energy or stimulus. Examples: a) Cold & Warm receptor are sensitive to thermal form of energy b) Touch & Pressure receptors are sensitive to mechanical form of energy c) Carotid & Aortic receptors are sensitive to chemical form of enenrgy d) Rods & Cones of the retina are sensitive to electromagnetic form of energy (2) Excitability: This is the ability of the receptors to respond to various stimuli by generating graded potentials: Properties of receptor potentials: a) sub-threshold stimuli produce local excitatory state which cannot initiate a nerve impulse b) if the potentials are successively repeated they can summate to produce an AP & produce a nerve impulse

sensory receptors-continued (3) Rate of discharge: if the stimulus intensity is increased the frequency of APs of sensory nerves increases. A 1000 fold increase in the stimulus intensity would result in a 3 fold increase in the frequency of discharge (i. e. , log 1000). This is called the Compression Function of the Receptor. The magnitude of the APs is not influenced by the magnitude of the graded potentials (4) Adaptation: It means that there is decline in the frequency of discharge of APs from the receptors with continuous stimulation Causes of adaptation: a) gradual decrease of excitability at the first node of Ranvier b) loss of stimulus energy into the surrounding tissues c) accomodation of the afferent nerve fibre to the generator potential d) gradual closure of the sodium channels decreases the magnitude of the receptor potential which decreases the rate of discharge Degree of adaptaion: a) rapidly adapting: the rate of discharge declines rapidly inspite of maintained stimulation (e. g. , touch receptors) b) slowly adapting: continue discharging as long as they get stimulated (e. g. , pain) c) intermediately adapting: falls between (a) & (b). Examples are the smell & taste as well as thermoreceptors § Recovery from adaptation is fast

sensory receptors-continued Fatigue differs from daptation: Adaptation Fatigue Cause Maintained stimulation Previous activity Mechanism Closure of Na channels, Accomodation …. )(see previous page Lactic acid accumulation Lack of O 2 No effect Accelerate fatigue Onset & Recovery Rapid Slow Site of Occurance Many tissues + sensory receptors Some tissues but not Sensory receptors

CLASSIFICATION OF SENSORY RECEPTORS Classification according to type of stimulation: 1 - Mechanoreceptors: These are stimulated by mechanical forms of energy & include: a- Tactile receptors: present in the skin (e. g. , free nerve endings, Merkel’s disks, Meissner’s corpuscles & Pacinian corpuscles) b- Baroreceptors: present in the carotid sinus and aortic arch c- Proprioceptors: present in the muscles, tendons, ligaments, & joints (e. g. , the muscle spindle & Golgi tendon organs) d- Sound & Vestibular receptors: present in the internal ear e- Pressure receptors: present in the skin & deep tissues (e. g. , Pacinian corpuscles) f- Stretch receptors: present in the walls of the right atrium, urinary bladder & lung alveoli 2 - Thermoreceptors: These are stimulated by thermal forms of energy. Those in the skin are specialised free dentritic nerve endings. Krause’s end bulbs and Ruffini corpuscles are also receptors of cold and warmth, respectively. 3 - Chemoreceptors: These are stimulated by chemical forms of energy. They include the following: a- Peripheral chemoreceptors in the carotid & aortic bodies b- Central chemoreceptors in the brain stem c- Taste receptors (gustatory or taste cells) in epithelium of tongue d- Smell receptors (olfactory receptor cells) in the olfactory mucous e- Osmoreceptors & glucoreceptors in the hypothalamus

classification of sensory receptors-continued 4 - Nociceptors: These are nerve endings that percieve pain sensations. They are stimulated by all forms of energy and respond to stimuli potent enough to cause tissue damage 5 - Electromagnetic receptors or Photoreceptors: They include Rods & Cones which are stimulated by the electromagnetic waves of light Classification according to location: 1 - Exteroceptors: sensitive to stimuli outside the body 2 - Interoceptors or visceroceptors: sensitive to stimuli in the viscera & blood vessels 3 - Proprioceptors: they are located in skeletal muscles, tendons, joints, ligaments, and connective tissues covering bones & muscles

SENSORY CODING Sensory Coding: it is the ability of the NS to discriminate (know) the Modality (type of sensation e. g. , heat, cold , sound etc. ), Locality, & Intensity of various sensations, athough all types of sensations are transmitted from their receptors to the higher centres via specific sensory pathways in the same form of APs 1 -Modality discrimination: § This depends on the property and the type of the specific sensory receptors 2 -Locality discrimination: § The various sensory pathways are discrete (i. e. , separate from each others). Cortical stimulation of conscious persons shows that there is fine representation of the locality of various receptors in the sensory areas of the cerebral cortex. This is known as low projection which states that stimulation of a certain sensory pathway anywhere along its course to the sensory cortex causes a sensation that is referred to the location of the receptor § The main factor coding for stimulus location is the site of the stimulated receptor § The precision, or acuity, with which one stimulus can be located and differentiated from an adjacent one depends upon the amount of Convergence of neuronal input in the specific ascending pathway. The greater the convergence the less the acuity and vice versa § Another factor determining acuity is the size of the receptive field covered by a single sensory unit and the amount of overlap of nearby receptive fields

Sensory coding-continued 3 -Intensity discrimination: Discrimination of the intensity of stimulation depends on the following: A- The number of activated receptors: this increases as the stimulus intensity increases B- The discharge frequency from the activated receptor C- The state of nerve centres: for example, when O 2 or glucose decreases, the sensation becomes dull & their intensities decreases § In this process the APs amplitude does not change only the frequency of APs generated §The area being stimulated increases also as the intensity of the stimulus is increased. This will generate more APs. This process is called Recruitment

TYPE OF SENSATIONS 1 - Somatic sensations: skin & deep tissues (e. g. , muscles, joints & bones) 2 - Visceral sensations: GIT & urinary tracts 3 - Special sensations: vision, hearing, vestibular, taste & smell 4 - Organic sensations: hunger, thirst & sexual desire

SOMATIC SENSATIONS 1 -Mechanoreceptive sensations: 1 - Touch sensations: A-Crude touch: poorly localised sensations -Receptors: free nerve endings -Afferent nerves: A-delta nerve fibres -Central pathway: Ventral spinothalamic tract -Test: Stroke (hit or rub) the skin lightly with a piece of cotton B- Fine touch: -Receptors: Merkel’s disks & Meissner’s corpuscles -Afferent nerves: A-beta nerve fibres -Central pathway: Gracile & cuneate tracts It involves: a- Tactile localization: the ability to localise a touched skin point (without visual help b- Tactile discrimination: the ability to distinguish 2 touch stimuli applied simulatneously as 2 separate points of touch (without visual help) c- Stereogenesis: the ability to recognize the nature of objects by handling them (their size, shape & weight) (without visual help) d- Texture of material sensation: the ability to recognise the nature of materials (e. g. , cloth) by touching it (without visual help)

somatic sensations-cotinued 2 -Pressure sensations: the ability to differentiate between various weights without lifting them - Receptors: found in deeper layers of the skin (slowly adapting) - Stimuli: stronger than those producing touch sensations - Central pathway: gracile & cuneate tracts 3 - Vibration sensations: repetitive tactile sensation as a result of stimulating rapidly adapting mechanoreceptors -Receptors: e. g. , Meissner’s corpuscles, Pacinian corpuscles etc. -Central pathway: gracile & cuneate tracts 4 - Tickle & Itch sensations: -Receptors: slowly adapting free nerve endings -Afferent nerves: Unmyelinated C nerve fibres -Central pathway: Ventral spinothalamic tract 5 - Proprioceptive sensations: sensations arising from deep structures (e. g. , muscles & joints) and give rise to conscious perception of orientation of the various parts of the body in space as well as the movement of each part - Central pathway: gracile & cuneate tracts a- Sense of positsion (static proprioception): it is the sensation of the position of different parts of the body with respect to each other -Receptors: are slowly adapting and include the muscle spindles & Glogi tendon organs b- Sense of movement (dynamic proprioception): it is the sensation of movement of joints. -Receptors: located around the joints & are rapidly adapting

somatic sensations-cotinued 2 -Thermoreceptive sensations: There are two types of thermoreceptors: 1 - Internal: located in the anterior hypothalamus in the spinal cord 2 - External: located under the skin and include cold & warmth receptors - There are discrete cold & warmth sensive spots in the skin - Cold spots are 4 -10 times more in number - In between these spots are thermally insensitive areas - Warmth receptors are specialised free nerve endings (& Ruffini corpuscles) that transmit sensation by thin C nerve fibres. They respond to temperatures between 25 & 50 o. C (optimal 37 to 40 o. C) - Cold receptors are of two types: a- Krause’s end bulbs that transmit cold sensation by A-delta nerve fibres b- Free nerve fibres that respond to temperatures between 10 & 35 o. C (optimal at 25 o. C) Paradoxical cold sensation: this happens when the skin is suddenly exposed to temperatures of about 45 to 50 o. C where a temporary sensation of cold is first felt. This is due to the or sharp rapid stimulation of the cold receptors at this temperature as well as to their greater number than the warmth receptors

somatic sensations-cotinued 3 -Pain sensations: § Include intense mechanical deformation, excessive heat, many chemicals such as neuropeptide transmitters, bradykinin, histamine, cytokines, & prostaglandins, several of which are released by damages cells. The receptors for pain are called nociceptors (free nerve endings)