BIO254:ReceptiveField
Receptive fields are known to exist in the somatosensory, visual and auditory system. The sensory information has a specific topographic location in the receptive fields. A stimulus that affects an area larger than the receptive field will also affect receptive fields in adjacent receptors. A dense population of receptors have small receptive fields and as a result finer resolution of spatial detail. The spatial resolution of a sensory system is not uniform everywhere.
Receptors project to first order neurons, which then project to second order neurons and higher order neurons. Neurons in each sensory relay nuclues have a receptive field that depends on the cells that converge on it. The receptive fields of higher order nucleus are larger and more complex. The receptive field of higher-order sensory neurons in the somatosensory and visual system has both excitory and inhibitory regions. The inhibitory regions increase the contrast between stimuli and result in a finer resolution.
Somatosensory System
A patch of skin has many overlapping receptive fields. When a region is touched, spikes are initiated at the node of Ranvier closest to the nerve terminals in the skin, conducted past the cell body, located in the dorsal root ganglion to the synaptic terminals in the spinal cord.
There are three relay sites between mechanoreceptors in the skin and the cerebral cortex.
Cortical neurons response are processed in the dorsal column nuclei, the thalamus and in the cortex. Cortical neurons are studied using extracellular recording.
Every point on the skin is represented in the cortex by cortical cells connected to the afferent fibers that are activated when that point on the skin is touched. Cortical neurons are grouped by function, with receptive fields arranged in an orderly topographic sequence that forms a map of the body. Receptive fields in higher cortical regions are larger. In the relay nuclei, properties of cortical receptive fields are due to divergent presynaptic connections and convergent postsynaptic connections.
When the skin is touched at two or more points simultaneously, regions of skin surrounding the excitatory region of a receptive field of a cortical neuron will suppress excitation to another stimulus because the afferent inputs surrounding the excitatory region are inhibitory. This is known as the inhibitory surround.
Visual System
Ganglion cells transmit information from the photoreceptors in the retina to higher order neurons with trains of action potentials. Bipolar, horizontal and amacrine cells lie between the photoreceptors and the ganglion cells. Receptive fields of photoreceptors are inputs to the ganglion cells.
The receptive areas of ganglion cells are roughly circular, and divided into two parts, the receptive field center (circular zone at the center) and the surround (the remaining area of the field). On-center ganglion cells are excited when light is at the center of the receptive field, and are inhibited when light is applied to the surround. Off-center ganglion cells are inhibited by light applied to the center of their receptive field, and are excited when light is applied to the surround. Ganglion cells have two parallel pathways from on-center cells and off-center cells for processing information. The receptive field of ganglion cells vary in size. Receptive fields are small in the foveal region of the retina and the fields are large at the periphery of the retina. The firing rate of a ganglion cell determine constrast in light between the center and surround.
Ganglion cells have two functional classes, M and P. M cells have large receptive fields, respond to large objects and have rapid changes in stimulus. P cells have small receptive fiels, are in larger numbers, respond to specific wavelengths and are involved in the perception of form and color.
In the primary visual cortex, the receptive fields of cells are different from those in retina. Cells respond to stimuli with linear properties. These are known as simple and complex cells. Simple cells have excitatory and inhibitory regions in their receptive fields, which are usually rectilinear. Complex cells have larger receptive fields than simple cells, with no clear on and off areas.
Photoreceptors in the retina are connected to bipolar cells that have connections to ganglion cells, which then project to geniculate cells. Geniculate cells project to simple cells that are connected to complex cells.
Auditory System
Receptors in the auditory system are spatially distributed according to the sound frequencies that they respond to. High frequencies are located at the base of the cochlea and low frequencies at the apex.
