CL 4 Sensory System PDF

SENSORY SYSTEM ASSOCIATE PROF. DR. AZIZAH UGUSMAN DEPARTMENT OF PHYSIOLOGY FACULTY OF MEDICINE UKM  OBJECTIVES Explain the organization and features of the sensory nervous system. Explain the classification, characteristics and functions of various receptors. Disc...

SENSORY SYSTEM ASSOCIATE PROF. DR. AZIZAH UGUSMAN DEPARTMENT OF PHYSIOLOGY FACULTY OF MEDICINE UKM  OBJECTIVES Explain the organization and features of the sensory nervous system. Explain the classification, characteristics and functions of various receptors. Discuss the brain perception of intensity and quality of different stimuli. Discuss the sensory homunculus, the somatic sensory pathways and their functions. Discuss the role of sensory receptors in reflexes and homeostasis.  BASIC COMPONENTS OF THE SENSORY SYSTEM STIMULUS AFFERENT Energy form that acts upon NERVE and activates a receptor. Eg: heat, touch, light, Sensory nerve that carries sensory sound etc. SENSORY impulses from CNS RECEPTOR receptors toward the Integrating centre that CNS, forms interprets the sensory Sensor that transduces/ ascending tract impulses for sensation transforms the stimulus and perception of the energy into electrical signals stimulus  SENSORY RECEPTORS Specialised nerve cells that transduce energy into neural signals. Each type of receptor is highly sensitive to one type of stimulus and almost non-responsive to others. Sensory unit: a single afferent nerve and all the sensory receptors which transmit information to it. CLASSIFICATION OF RECEPTORS BASED ON: Location Function Structure  LOCATION EXTERORECEPTORS Sensitive to stimuli occurring outside or on the body surface. Example: receptors for touch, pain, temperature, vision, hearing, smell, and taste sensations. INTERORECEPTORS Sensitive to stimuli inside the body from the visceral organs, blood vessels, and the musculoskeletal system. Examples: chemoreceptor, baroreceptor, proprioceptors  FUNCTION Respond to Respond to tissue damage temperature changes  STRUCTURE FREE NERVE ENDINGS Dendrites whose terminal ends have little or no physical specialization. Nociceptor, thermoreceptor ENCAPSULATED NERVE ENDINGS Dendrites whose terminal ends are enclosed in a capsule of connective tissue. Mechanoreceptor SENSE ORGANS Specialized hair cells for the special senses (vision, hearing, smell, taste, and equilibrium)  Mechanoreceptors in the skin o Respond to physical changes including touch, pressure, vibration, and stretch. o Four main types: (i) Meissner’s corpuscles - respond to fine (discriminative) touch, stroking movements (ii) Pacinian corpuscles - detect vibration and heavy pressure (iii) Ruffini’s corpuscles - respond to skin stretch (iv) Merkel’s disks - sense steady pressure, fine touch, understanding of structure and texture of objects  SENSORY TRANSDUCTION o The process of converting energy forms into electrical signals via a receptor/ generator potential which triggers an action potential if it reaches threshold. o A receptor potential is a type of graded potential like an excitatory postsynaptic potential (EPSP).  SENSORY CODING Each stimulus is “coded” by sensory system based on its: Type/Modality (WHAT) Intensity (HOW BIG) Location (WHERE) Duration (WHEN)  TYPE OF STIMULUS o Stimuli can be divided into different types or modalities. o Sensory receptors are specific: normally respond to only one type of stimulus, known as adequate stimulus. o Adequate stimulus: type of energy that has the lowest threshold for receptor activation/the form of energy to which a specific receptor is most sensitive (eg: light is an adequate stimulus for photoreceptors) o Non-adequate stimulus: type of energy that has a higher threshold for receptor activation (eg: intense pressure on the eyes can stimulate photoreceptors → perceived as "seeing stars").  INTENSITY OF STIMULUS Intensity is coded by: ⮚ frequency of action potentials generated ⮚ number of receptors stimulated Stronger stimuli → ↑ magnitude of receptor Stronger stimuli → ↑ number of receptors potentials → ↑ frequency of action stimulated in a single or multiple sensory units potentials generated. → more action potentials sent to the CNS.  LOCATION OF STIMULUS How accurate a stimulus can be located (acuity) depends on: o Size of receptive field ⮚ Receptive field= the area where application of a stimulus causes a sensory neuron to respond o Receptor density ⮚ Density of the receptor is inversely related to the size of the receptive field ⮚ Smaller receptive field has higher receptor density o Lateral inhibition o Touch acuity can be measured by two-point discrimination test. o Refers to the ability to perceive two fine points pressed against the skin as two separate points and not as one. o The smaller the receptive fields, the smaller the distance between two points of stimulation can be discriminated. o Areas with smaller receptive fields (lips, fingertips) have better two-point discrimination than those with larger fields (back, shoulder). o Smaller receptive field and higher receptor density → greater acuity o Lateral inhibition: the ability of the stimulated neurons to inhibit the activity of neighboring neurons, thus preventing the spread of neuronal activity laterally. o Leads to ↑ contrast of neuronal activities between neighboring neurons → better sensory acuity.  DURATION OF STIMULUS Sensory adaptation: refers to a declining response to unchanging or repetitive stimuli. o Slowly adapting (tonic) receptors ⮚ Do not adapt or adapt slowly. ⮚ Continue to transmit impulses to the brain as long as the stimulus is present. ⮚ Important to maintain information about the stimulus. ⮚ Example: proprioceptors, nociceptors o Rapidly adapting (phasic) receptors ⮚ Adapt rapidly by no longer responding to unchanging stimuli. ⮚ Important to signal a change in stimulus intensity rather than to relay information. ⮚ Example: touch receptors SENSORY NEURON FIBER TYPES Axon Myelin  SENSORY/ASCENDING PATHWAYS FOR TRANSMISSION OF SOMATOSENSORY SIGNALS TO CNS Posterior/dorsal column (medial lemniscal) pathway: fine touch, vibration, proprioception ⮚ Fasciculus gracilis: input from lower part of the body ⮚ Fasciculus cuneatus: input from upper part of the body Spinothalamic (anterolateral) pathway: ⮚ Anterior spinothalamic tract: crude touch, pressure ⮚ Lateral spinothalamic tract: pain, temperature  Sensory pathway: 3 neurons o Sensory pathways are composed of first, second and third order neurons. o 10 neuron is the primary afferent: ⮚ conveys information from the receptors to the spinal cord/brainstem. ⮚ cell body in dorsal or cranial root ganglion. o 20 neuron conduct impulses from the spinal cord/brainstem to the thalamus. o 30 neuron conducts impulses from the thalamus to the somatosensory cortex. A B A - DORSAL COLUMN PATHWAY B – SPINOTHALAMIC (ANTEROLATERAL) PATHWAY Unilateral damage to the dorsal column pathway: Below the level of Unilateral damage to medulla – ipsilateral the spinothalamic loss of fine touch, pathway: vibration and Contralateral loss of proprioception pain, temperature and sensation crude touch sensation Above the level of medulla – contralateral loss of fine touch, vibration and proprioception sensation Fine touch, vibration and Pain, temperature and proprioception crude touch Somatosensory cortex Consist of primary somatosensory cortex (SI) and secondary somatosensory cortex (SII). SI: ⮚ Located in the postcentral gyrus of the brain’s parietal lobe. ⮚ Responsible for processing of sensory information for interpretation, localization and perception of the stimuli. ⮚ Receives input from contralateral side of the body because ascending pathway crosses over in the spinal cord/brainstem. SII : ⮚ Located posterior to SI in the parietal lobe. ⮚ Function not well understood ⮚ Believed to be involved in tactile object recognition and memory.  SENSORY HOMONCULUS o Homunculus = little man o Parts of the body are represented as a sensory homunculus map in the somatosensory cortex. o A particular location in the somatosensory cortex receives information from a particular part of the body. o The amount of space in the sensory cortex dedicated to each body part correlates with the sensitivity and not the size of that body part. o The hands, lips and face are very sensitive to sensations, so more area of the somatosensory cortex represents these body parts.  Role of sensory receptors in reflexes and homeostasis o Reflexes are automatic, subconscious response to changes within or outside the body. o Reflex arc: a neural pathway that controls a reflex. o Sensory receptor is a component of the reflex arc. o Reflexes maintain homeostasis (eg. baroreceptor reflex regulates the blood pressure). o Reflexes maintain balance and posture (eg. stretch reflex). o Reflexes carry out the automatic action of swallowing, sneezing, coughing and vomiting. THANK YOU