بسم الله الرحمن الرحيم

RECEPTORS AND SENSORY PATHWAYS

LEARNING OBJECTIVES Classify the sensory receptors Enumerate the receptors found in the skin for different sensory modalities Explain the terms receptor potentials and sensory transduction Describe the sensory pathways that mediates touch, proprioception and vibration sense Describe the organization of sensory cortex

Peripheral Nervous System Consists of nerve fibers that carry information between the CNS and other parts of the body Afferent division Detects, encodes, and transmits peripheral signals to the CNS (sends information from internal and external environment to CNS) To maintain homeostasis Visceral afferent Incoming pathway for information from internal viscera (organs in body cavities) Sensory afferent Somatic (body sense) sensation Sensation arising from body surface and proprioception Special senses Vision, hearing, taste, smell

Receptors Structures at peripheral endings of afferent neurons Detect stimuli (change detectable by the body), exists in various forms e.g. heat, light, sound, pressure & chemical change Each receptor is specialized to one type of stimulus ,its adequate stimulus e.g. eyes- light, ears- sound, warmth- heat Convert forms of energy into electrical signals (action potentials) Process is called transduction

Types of Receptors Photoreceptors Responsive to visible wavelengths of light Mechanoreceptors Sensitive to mechanical energy Thermoreceptors Sensitive to heat and cold Osmoreceptors Detect changes in concentration of solutes in body fluids and resultant changes in osmotic activity Chemoreceptors Sensitive to specific chemicals Include receptors for smell and taste and receptors that detect O2 and CO2 concentrations in blood and chemical content of digestive tract Nociceptors Pain receptors that are sensitive to tissue damage or distortion of tissue

Tactile Receptors Free nerve endings: Detect pain, touch and pressure found everywhere in the skin and viscera Meissner’s corpuscles: rapidly adapting (within a fraction of a second) and detect movement of light objects over skin, position sense ,dynamic fine touch (manipulation) -found on nonhairy skin, fingertips and lips Merkel’s discs: respond rapidly at first and then slowly adapt, detect the “steady state”, position, static touch(shape, edges, texture) -found on hairy as well a non hairy skin

Tactile Receptors Hair end organ Adapts rapidly and detects movement over the body Ruffini’s end organ Slowly adapting and respond to continual deformation of the skin and joint rotation Pacinian corpuscle Very rapidly adapting and is stimulated only by rapid movement detects vibration and other rapid changes in the skin

Receptors May be: -Specialized ending of an afferent neuron -Separate cell closely associated with peripheral ending of a neuron -Stimulus alters receptor’s permeability which leads to graded receptor potential -Usually causes nonselective opening of all small ion channels -This change in membrane permeability can lead to the influx of sodium ions. This produces receptor (generator) potentials. -The magnitude of the receptor potential represents the intensity of the stimulus. -A receptor potential of sufficient magnitude can produce an action potential. This action potential is propagated along an afferent fiber to the CNS.

Conversion of Receptor and Generator Potentials into Action Potentials

Receptors May adapt slowly or rapidly to sustained stimulation Types of receptors according to their speed of adaptation Tonic receptors Do not adapt at all or adapt slowly Ex: Muscle stretch receptors, joint proprioceptors CNS must continually get info. about degree of muscle length & joint position So, these receptors must continue to generate APs to relay information to CNS Phasic receptors Rapidly adapting receptors No longer responds to maintained stimulus Important to signal a change in stimulus intensity, rather than relay status quo information Tactile (touch) receptors in skin

Adaptation of receptors When a continuous stimulus is applied, receptors respond rapidly at first, but response declines until all receptors stop firing.

Transmission of Receptor Information to the Brain -The larger the nerve fiber diameter the faster the rate of transmission of the signal -Velocity of transmission can be as fast as 120 m/sec or as slow as 0.5 m/sec -Nerve fiber classification type A - myelinated fibers of varying sizes, generally fast transmission speed subdivided into a, b, d, g type C - unmyelinated fibers, small with slow transmission speed

Classification of Somatic Sensations Mechanoreceptive - stimulated by mechanical displacement Tactile touch pressure vibration tickle and itch Position or proprioceptive static position rate of change

Classification of Somatic Sensations Thermoreceptive: Detect heat and cold Nociceptive: Detect pain and are activated by any factor that damages tissue

Pathways for the Transmission of Sensory Information Almost all sensory information enters the spinal cord through the dorsal roots of the spinal nerves Two pathways for sensory information: -Dorsal column-medial lemniscal system - Anterolateral system

Dorsal Column System Contains large myelinated nerve fibers for fast transmission (30-110 m/sec). High degree of spatial orientation maintained throughout the tract. Transmits information rapidly and with a high degree of spatial fidelity (i.e., discrete types of mechanoreceptor information). Touch, vibration, position, fine pressure

Anterolateral System Smaller myelinated and unmyelinated fibers for slow transmission (0.5-40 m/sec) Low degree of spatial orientation Transmits a broad spectrum of modalities Pain, thermal sensations, crude touch and pressure, tickle and itch, sexual sensations.

Somatosensory Pathways Pathways conveying conscious somatic sensation- Consists of chains of neurons, or labeled lines, synaptically interconnected in particular sequence to accomplish processing of sensory information First-order sensory neuron Afferent neuron with its peripheral receptor that first detects stimulus Synapses on Second-order neuron Second-order sensory neuron Either in spinal cord or medulla Synapses with third-order neuron Third-order sensory neuron Located in thalamus Terminate in somatosensory cortex

Somatic Sensory Cortex Located in the postcentral gyrus Highly organized distinct spatial orientation Each side of the cortex receives information from the opposite side of the body Unequal representation of the body lips have greatest area of representation followed by the face and the thumb trunk and lower body have the least area

Acuity Refers to discriminative ability Influenced by receptive field size and lateral inhibition

RECEPTIVE FIELD -Receptor has its receptive field ie. It responds to stimulus within a restricted region of skin -Receptive field is inversely proportional to density of receptors -Smaller is the receptive field, greater is acuity or discriminative ability -More receptors on finger tips as compared to elbow

LATERAL INHIBITION

Perception Conscious interpretation of external world derived from sensory input from a pattern of nerve impulses delivered to brain Why sensory input does not give true reality perception Humans have receptors that detect only a limited number of existing energy forms Information channels in our brains are not high-fidelity recorders Cerebral cortex further manipulates the data

What Do You Perceive? Do you “see” a white square that is not really there? Variable perceptions from the same visual input. Do you see two faces in profile, or a wineglass?

Uses For Perceived Information Afferent input is essential for control of efferent output -Processing of sensory input by reticular activating system in brain stem is critical for cortical arousal and consciousness -Central processing of sensory information gives rise to our perceptions of the world around us -Selected information delivered to CNS may be stored for further reference -Sensory stimuli can have profound impact on our emotions

References Human physiology by Lauralee Sherwood, seventh/eighth edition Text book physiology by Guyton &Hall,12th edition Text book of physiology by Linda .s contanzo,third edition