Neurobiology Biomed 509 Sensory transduction References: Luo 4.1 4.8, (4.9 4.23), 6.22 6.24, M4.1, M6.2 I. Transduction The role of sensory systems is to convert external energy into electrical signals that can be utilized by the nervous system. This process is known as transduction and, for those specific modalities which specific receptors have evolved, it can exhibit a sensitivity approaching the theoretical limits of that form of energy. A. Transduction is the process changing one form of information into another form of energy. B. Sensitivity (zepto = 10-21 ) molecular thermal energy (kt) = 4 zj photon absorption = 400 zj odorant binding = 80 zj hair cell threshold = 4 zj II. Receptor potentials Receptor potentials (sometimes called generator potentials) are graded, non-propagated potentials whose amplitude is proportional to the intensity of the stimulus being transduced. These potentials are converted into a frequency of action potentials at second or third order sensory neurons. A. Receptor potential encoding weak stimulus strong stimulus nerve a.p. receptor potential stimulus
B. Permeability changes With the notable exception of those in photoreceptors, the ion channels responsible for transducing sensory information are non-selective and thus current through them reverses direction at about 0 mv. pa 1.0 +30 mv 0.5 mv -60-30 30-30 mv -0.5-1.0-60 mv -1.5-2.0 III Spatial and temporal modification Sensory information is distorted in manners that are useful to the organism. Lateral inhibition distorts spatial sensory information in a manner that emphasizes edges and adaptation distorts temporal information in a manner that emphasizes the beginning and ends of stimuli. A. Lateral inhibition mutual inhibition of adjacent receptors accentuates spatial boundaries Lateral inhibition No lateral inhibition Lateral inhibition increases 2-point discrimination by inhibiting receptors between the two points.
B. Adaptation time IV. Specific transduction mechanisms A. Chemoreception odorant n R n G S ATP A.C. cyclic nucleotide channel B. Visual photon rhodopsin Gs PDE INa Dark Light Rho G-protein pathway 5'GMP photon
C. Auditory system 1. Auditory place theory oval window basilar membrane base apex 33 mm High frequency Low frequency <300 nm Displacement of the basilar membrane is frequency dependent. High frequency waves damp out quickly causing maximum displacement near the base while low frequency waves build to cause maximum displacement near the apex. 2. Stretch operated channels at tip of hair cell cilia are opened when adjacent cilia bend and the tip link between adjacent cilia mechanically open the gate.
D. Mechanical sensation. Organisms depend on a wealth of information that is related to direct interactions with the physical environment. This includes proprioceptive information from stretch receptors in vertebrates and slit organs in spiders, touch information from Pacinian and Rafini corpuscles, information about tissue damage from certain nociceptors, and air pressure and fluid movement information from auditory and vestibular hair cells. In a sense these are the most straightforward sensory processes, but only recently has detailed information become available regarding the responsible ion channels. One candidate for some of these sensory transduction channels is the TRPV family of channels. V. Sensory processing A. Receptive Fields A receptive field is a property of any cell in a sensory system. It is defined as: the area of the external world that causes a response in that particular cell. Visual cortical neuron ganglion cell Off region On region Center-surround antagonism is a central feature of visual receptive fields. The receptive field includes both a central area that produces either excitation or inhibition of the neuron and a surrounding area that produces the opposite response.
Auditory auditory tuning curve Loudness required for response (db) Center Frequency Sound pitch (Hz) The area for an auditory receptive field is a specific center frequency (pitch). This originates from specific basilar membrane hair cells being activated as a result of the place code. Central projections produce an ordered map on the auditory cortex of neurons with specific center frequencies. VI Cortical mapping A. Columnar organization Sensory cortices are arranged in vertical columns of similar receptive field properties. B. Topic organization auditory arm trunk leg foot 1 5 15 khz 20 hand face lip somatosensory visual 5 o 40 o 10 o fovea
Sensory cortices for vision (retinotopic), audition (tonotopic) and somatosensation (somatotopic) map the external world onto a distorted cortical map. Areas with more important sensory information fovea for vision, speech pitches for audition, and face and fingers for somatosensory have larger cortical representations. VII. Cortical plasticity Auditory Following high frequency hearing loss, the tonotopic map is rearranged so that lower frequencies are represented in those portions of the tonotopic map. Somatosensory Following amputation, the homunculus is modified with adjacent areas taking over the missing body areas Visual There is a critical period of experience-based retinotopic plasticity. It is important that children born with cataracts have them removed before this period is over so that the normal retinotopic map develops. This is not a problem in adults with cataracts since the retinotopic map has little plasticity and will still be stable after the cataract is removed.