SOMATIC SENSATION Definition: Sensation from the skin, muscles, bones, tendons and joints. Initiated due to activation of a number of distinct somatic receptors that respond specifically to changes in heat, cold, touch, pressure, limb position, limb movement, or pain. Receptors for visceral sensations (i.e. from internal organs) are of same type as those for somatic sensation. Some organs, e.g. liver have no sensory receptors. Skin receptors Information from somatic receptors enters the CNS and synapse on neurons that ascend and go primarily to the somatosensory cortex via the brainstem and thalamus. 1
The Anterolateral Tract Pathways cross over to opposite side of spinal cord to ascend. e.g. from pain or temperature receptors. The Dorsal Tract Pathways ascend ipsilateral and then cross over in brainstem. e.g. from vibration or joint position receptor. Thus, somatic receptors on left side of body go to the right cerebral hemisphere and vice-versa, both for anterolateral and dorsal tracts. 2
Suppression of Pain Transmission 3
Pathway terminations in Somatosensory cortex Somatosensory cortex In general, densely innervated areas of the body (e.g. fingers) are represented by the largest areas of the somatosensory cortex. Although the areas in this topographical map appear to be well defined, there is some anatomical overlap. Interpretation of different stimuli 1) Touch-pressure Interpretation of a mulititude of different stimuli (e.g. deep pressure, vibration) occurs due to the processing of information from multiple receptor types located in the skin. 4
Receptor types Pacinian corpuscles: found deep in the dermal layers of both hairy and glabrous skin, and have relatively large endings that are widely spaced. Thus, relatively large receptive field per cell. Upon stimulation pacinian corpuscles rapidly evoke action potentials, they adapt and stop firing throughout the stimulus, and then fire action potentials at the end of the stimulus. This rapid adaptation allows rate and pressure detection of the stimulus. Ruffini s endings: found deep in the dermal layers of both hairy and glabrous skin, and have relatively large endings that are widely spaced. Thus, relatively large receptive field per cell. Continue to respond during stimulation, i.e. Slowly adapting. The differences in fast and slow adaptation between Pacinian and Ruffini relate to the absence of a capsule on the end organ of the Ruffini, not to differences in the electrophysiological properties of the cells. Meissner s corpuscles: approx. 1/10 the size of Pacinian corpuscles. Located in ridges of glabrous skin, e.g. raised parts of fingerprints. Small receptive field compared with Pacinian and Ruffini. Rapidly adapting. Merkel s disk: located in the epidermis. Consist of a nerve terminal and a flattened non-neural epithelial cell. Small receptive field. Slowly adapting. Hair follicle receptors: hairy skin only. Can be either rapidly adapting or slowly adapting. 5
2) Pain Receptors, termed nociceptors, respond to stimuli that is about to cause tissue damage, such as excessive mechanical strain, excessive heat, and chemicals released from nearby cells and damaged tissue, such as neurotransmitters or prostaglandins. These factors activate receptors on the nociceptor membrane. Nociceptors are divided in to 4 classes: mechanoreceptors, thermal receptors, chemoreceptors, and polymodal receptors (these respond to all 3 stimuli). Nociceptors are similar to other receptor types but generally respond to higher levels of stimulus. e.g. general thermal receptors respond to temperatures < 45 o C, whereas nociceptive thermal receptors respond to temperatures > 45 o C. Nociceptors have either free nerve endings or nonencapsulated end organs. They are not found in bone or brain tissue. Following tissue damage and nociceptor stimulation, the threshold for subsequent stimulation is decreased and thus, sensitivity to that stimulus is increased. This process is termed hyperalgesia. In some cases, hyperalgesia can induce a change in nociceptor sensitivity from unimodal to bimodal, e.g. a mechanoreceptor can become sensitive to thermal stimuli. Because hyperalgesia can modify the pain sensation, the pain felt shortly after an injury can be very different to that felt initially. 3) Posture and movement Proprioceptors provide information about body position. Generally, this information is not sent to the higher centers in the brain, but is processed in the spinal cord via reflexes. There are 2 classes of muscle proprioceptors: i) Muscle spindles, that monitor length and rate of stretch of muscles. ii) Golgi tendon organs, that adjust force by monitoring muscle tension. Additional information is provided by mechanoreceptors that are located in the connective tissue of joints. NOTE *Skeletal motor control is covered in detail later in this series of lectures* 4) Temperature 2 types of receptors in skin: i) Warmth receptors: respond to temperatures between 30 o C and 43 o C. Action potential discharge rate increases with elevation of temperature. ii) Cold receptors: action potentials are stimulated by small decreases in temperature. 6
HEARING Principles of Sound Sound energy is transmitted via vibration of molecules. Disturbance of air molecules causes a zone of compression, where molecules are closer together and pressure increases, and a zone of rarefaction, where molecules are further apart, and pressure becomes lower (Fig. a, b, and c). A sound wave consists of rapidly alternating high and low pressures (Fig. d). Sound waves/vibrating tuning fork Amplitude: difference between low and high pressures. Related to the loudness of the sound. Frequency: speed at which the high and low pressures interchange. Determines pitch. Human ears hear sound frequencies between 20 20,000 Hz, but are most sensitive to those between 1000 Hz 4000 Hz. 1 Hz = 1 per second. Humans can distinguish 400,000 different sounds, and can filter unwanted noise in order to distinguish important sounds. 7
Transmission of sound to the inner ear (cochlea) The human ear Complex shapes within the pinna help to amplify and direct sound waves into the external auditory canal. Air molecules push against the tympanic membrane, which is stretched across the end of the external auditory canal, causing it to vibrate at the same frequency as the sound wave. A zone of compression (i.e. high pressure) pushes the membrane inwards, and vice versa. The distance the membrane moves is related to the loudness of the sound. The tympanic membrane separates the external auditory canal from the middle ear cavity, which is maintained at atmospheric pressure through the eustachian tube, which ends in the pharynx. 3 small bones in the middle ear: malleus, incus and stapes, amplify sound waves to vibrate the oval window of the fluid filled inner ear, or cochlea. Amplification (~ 15 20 times) occurs primarily because the oval window is smaller than the tympanic membrane, but also due to the lever action of the bones. 2 small skeletal muscles can modify amplification by: i) adjusting the tension of the tympanic membrane and, ii) moving the position of the stapes. 8
The cochlea The middle ear bones and the cochlea Receptor cells are located in the cochlea, which is a fluid filled, spiral-shaped passage. It is almost completely divided lengthwise by the cochlear duct. The oval window separates the middle ear from the scala vestibuli. The scala tympani is separated from the middle ear by a second membrane called the round window. The scala vestubuli and the scala tympani meet at the helicotrema, which is at the end of the cochlear duct. Movement of the oval window by the stapes creates waves of pressure in the fluid of the scala vestibuli, vibrating the cochlear duct. Transmission of sound vibrations 9
Membranes and compartments of inner ear On one side of the basilar membrane of the cochlear duct is the organ of Corti, which contains the receptor cells for the ear. The basilar membrane vibrates with pressure changes across the cochlear duct. Sounds of a particular pitches vibrate the basilar membrane at different distances from the oval window. High frequency sounds vibrate the basilar membrane nearest to the oval window, low frequency sounds vibrate the membrane furthest away from the oval window. 10
Hair cells of the organ of corti The receptor cells are called hair cells, and they transform pressure waves into membrane potentials. Hairlike stereocilia protruding from one end of the receptor cell are in contact with the tectorial membrane. Movement of the basilar membrane in response to a sound, bends the sterocilia which activates ion channels in the plasma membrane, inducing membrane depolarization of the hair cell. Depolarization releases glutamate onto adjacent neurons, whose axons form the cochlear nerve. Glutamate activates action potentials in these cells. The louder the sound, the higher the frequency of action potentials generated by the nerve fibers. Efferent fibers from the brainstem can dampen the hair cells to protect them from loud sounds. 11
Neural pathways in hearing The cochlear nerve is a component of cranial nerve VIII. Neurons of the cochlear nerve enter the brainstem and synapse with interneurons. Different arrival times and intensities of inputs from each ear determine the direction of the sound source. Information from the brainstem goes to the thalamus and auditory cortex for processing. 12