Sensory System Prof. K. Sivapalan
Receptor Mechanism. Receptor is a transducer: it converts various energies into action potential. Example- touch receptor: Pacinian corpuscle. Receptor potentials are proportional to energy of touch. When threshold is reached action potential is generated and transmitted in the nerve. When the first node of Ranvier is blocked by anaesthetics, action potential is not generated. 11/11/2018 Sensory system
Receptor Mechanism ctd. Receptor can be part of a neuron or a specialized cell. One receptor responds to one form of energy at low strength. Other forms of energy will require very high intensity to stimulate the same receptor. Adequate stimulus- particular form of energy to which a receptor is most sensitive. 11/11/2018 Sensory system
Receptive field One axon is connected to several receptors. Receptors of one axon will be spread in the receptive field. Receptive fields of axons can overlap. The size of the receptive field varies – small in highly sensitive areas and large in less sensitive areas. Mild stimulus excites few receptors and axons and strong stimulus excites more axons. 11/11/2018 Sensory system
Stimulus and response. Larger the stimulus higher the receptor potential and frequency of action potentials. 11/11/2018 Sensory system
Some sensory modalities and receptors. Sensation Receptor Organ. Vision-light Rods and cons Eye Hearing-sound Hair cells Ear Taste-chemical Taste receptors Taste bud Acceleration-mechanical Hair cell Internal ear Pressure-Mechanical. Nerve endings Skin Cold- thermal Blood pressure- mechanical Baroreceptor 11/11/2018 Sensory system
Tonic and phasic receptors. All receptors generate action potential on application of stimulus. Some continue to discharge as long as the stimulus is applied- tonic receptors. Others adapt and stop action potentials- phasic receptors. 11/11/2018 Sensory system
Somatic sensation. Skin has several different receptors. Encapsulated and expanded tips sense touch or pressure. Meissner’s and Pacinian corpuscles are rapidly adapting. Merkel’s disks and Ruffini endings are slowly adapting. Naked nerve endings can sense all cutaneous sensations. 11/11/2018 Sensory system
Sensory modalities. Cutaneous senses are: Crude touch. Fine touch. Pressure. Cold. Warmth. Sharp pain. Vague pain. Vibration. Proprioception (sense of position and movement). Itch and tickle. Sexual sensation. 11/11/2018 Sensory system
Dermatome. Axons from a dermatome enter the spinal cord through the respective dorsal root. Lesions in the spinal cord result in loss of sensation of the dermatomes of the respective segments. Lesions of nerves out side the spinal cord will result in loss of sensation in areas supplied by the damaged nerve. 11/11/2018 Sensory system
Transmission to Spinal Cord Somatic sensory nerves have the cell body in the dorsal root ganglion. Axons enter through dorsal root and synapse in the dorsal gray. This results in local reflexes and transmission to higher centers. 11/11/2018 Sensory system
Sensory pathway. Pain, temperature and crude touch- cross in the same segment and ascend in the spinothalamic tract. Fine touch and proprioception ascend in the dorsal columns in the same side without synapse. They synapse in the nuclei gracilis and cuneatus in Medulla and cross over and ascend in medial leminiscus. All second order neurons synapse in thalamus. Third order neuron synapses in the sensory cortex- sensory area I. Projections from SI and thalamus go to Sensory area II in the superior wall of the Sylvian fissure- head in inferior end and leg in the bottom. 11/11/2018 Sensory system
Transmission in the spinal cord. Sensory fibers in the spinal cord are arranges in layers. Spinothalamic fibers from lower parts are outer most and the upper fibers are placed inner layer as they cross over in the anterior commisure. In the dorsal columns, fibers from lower parts are most medial 11/11/2018 Sensory system
Sensory pathway in Spinal Cord Sacral segments carrry sensation of only a small area. Lumbar segments conduct more. The upper cord shows the dorsal tract divided in to two to accommodate fibers coming in. The gray matter also shows variation according to the amount of muscles innervated 11/11/2018 Sensory system
Sensory Decussation in Medulla Nucleus gracillis and cuneatus contain the second order neurons of the dorsal columns. Fibers arising in these nuclei pass anteriorly, cross over and form the medial leminiscus. Fibers in spinothalamic fibers accompany the medial lemisiscus laterally. 11/11/2018 Sensory system
Higher Medulla. 11/11/2018 Sensory system
Upper Pons 11/11/2018 Sensory system
Lower Mid Brain 11/11/2018 Sensory system
Upper Mid Brain 11/11/2018 Sensory system
Thalamus All sensory fibers synapse in the thalamus. Third order fibers pass from the thalamus to the cerebral cortex in the corona radiata. Fibers ascending to and descending from the cortex pass lateral to thalamus and the caudate nucleus medial to lentiform nucleus. This is known as internal capsule. Sensory fibers occupy the genu of the internal capsule 11/11/2018 Sensory system
11/11/2018 Sensory system
Cerebrum 11/11/2018 Sensory system
Representation in cortex 11/11/2018 Sensory system
Cortical representation. 11/11/2018 Sensory system
Neural basis of somatic sensation. Localization: depends on the area stimulated in the cortex. Stimulation of points in post central gyrus projects specific sensation to particular cutaneous area. Stimulation by anything anywhere in the pathway provokes the same sensation as that comes from the receptor site. Coding sensory information depends on specificity of receptor and the cells stimulated in cortex. Gate control theory- limiting sensory input by gates in spinal cord. Transmission of sensation in spinal cord is limited by: Collaterals from afferents. Descending tracts from brain stem. Circulating chemicals- endorphins Cortical activity can modify perception of sensation. 11/11/2018 Sensory system
Lesions of somato-sensory cortex Critical tactile sensation is lost. Slight crude touch returns- thalamus has slight discrimination of touch. Little effect on sense of pain and moderate effect on temperature. Thalamus and brain stem have significant role in perception of these sensations. Ablation of SII causes defects in learning based on tactile discrimination. SII is concerned with further elaboration of sensory data. 11/11/2018 Sensory system
Characteristics of sensation. Sensations conducted in the dorsal columns and antero-lateral systems differ in many aspects. Dorsal columns: Velocity of transmission- very fast. Degree of spatial localization- best. Gradation of intensity- good. Ability to transmit rapidly changing and repetitive signals- good. 11/11/2018 Sensory system
Pain. Sensory modalities generate emotional responses in addition to several other functions. Pain has ‘built-in’ unpleasant effect But it is important for protective function. Several different signals are perceived as pain. 11/11/2018 Sensory system
Pain Receptors. Naked nerve endings, found in almost every tissue – superficial layers of skin, periosteum, arterial walls and joint surfaces. Not in brain tissue. Fast pain: bright, sharp, localized sensation. It is conducted by Small Aδ fibers at 12-30 m/s. Slow pain: dull, intense, diffuse, and unpleasant sensation. It is conducted by unmyelinated C fibers at 0.5-2 m/s. Adequate stimulus- mechanical, thermal [45 °C, the temperature of tissue damage] and chemical energy. Fast pain is elicited by mechanical and thermal stimuli and slow pain by all three. 11/11/2018 Sensory system
About pain sensation. Receptors don't adapt- especially for slow pain. Pain impulses can be inhibited in dorsal horn by collaterals from touch fibers. Brain’s opiate system – endophin and encephalins inhibit at spinal and central levels. At the spinal level pain evokes withdrawal and at cortical level avoidance reactions. Pain also activates mediofrontal cortex, insular cortex, cerebellum, and reticular system in brain stem. Effects of these are complex- could be emotional and autonomic. 11/11/2018 Sensory system
Post injury and neuropathic pain After moderate or severe injury, pain persists even after the healing of the injury. Stimuli to injured area that usually cause mild pain evoke severe pain - hyperalgesia. Other stimuli such as touch can cause pain [alldynia]. If the nerves to that area are damaged, the pain may persist and may become excruciating- neuropathic pain. 11/11/2018 Sensory system
Deep pain. Poorly localized, nauseating and associated with sweating and changes in blood pressure. In periosteum, ligaments, muscles, etc. This initiates reflex contraction of nearby muscles. Muscle spasm causes ischaemia, pain and more spasm- vicious circle. Ischaemic pain - caused by substance P in skeletal and cardiac muscles. [uteress] Colicky pain in intestines, biliary tract and urinary tract is associated with strong contractions. 11/11/2018 Sensory system
Visceral Pain. Poorly localized, unpleasant, associated with nausea and autonomic symptoms and signs. Viscera has few temperature and touch receptors. Pain fibers are also less than in skin. They reach CNS by autonomic afferents. In the spinal cord, they travel in spinothalamic tract. Cortical areas are inter mixed with somatic representational areas. Stimulation: hollow viscera are sensitive to distension. When inflamed, miner stimuli causes severe pain. Visceral pain stimulates contraction of nearby skeletal muscle reflexly- rigidity of abdominal wall in peritonitis. 11/11/2018 Sensory system
Referred pain: Pain sensation originating in viscera is felt in somatic structure that may be in considerable distance away. Such pain is referred to the somatic structure. Pain seems to spread [radiate] from local to distant site when it is both local and referred. Visceral pain stimulates contraction of nearby skeletal muscle reflexly- rigidity of abdominal wall in peritonitis. 11/11/2018 Sensory system
Surface areas of referred pain. 11/11/2018 Sensory system
Dermatomal rule. Convergence: cutaneous and visaral afferents converge on same second order neuron. Facilitation: visceral afferents send colateral that facilitates transmission of somatic sensation. Somatic anesthesia- convergence no effect, facilitation reduces. Usually pain is referred to structures from the same dermatomal segment. Role of experience: pain can be referred to sits where pain was experienced- as in surgical scars. 11/11/2018 Sensory system