lecture 18 sensory systems 2 skin and CNS

Human skin is about how big of a surface area loaded with several modalities of receptors

2m^2 embedded in layers of skin superficial or deep are sense organs Ruffini, Pacinian corpuscle, meissners corpuscle, merkel difficult to know which sense organ you are stimulating

intensity coding isn't linear, but has how many phases?

3 if receptors act like speedometers it would be a straight line but it is not like that, it is curvy at the bottom is the threshold where the cell can not respond to, broad range in middle called log linear, at the top is saturation where it does not make a difference in velocity is increased (response is saturated, maybe enough transmitter released in cleft as can be left or enough AP responding as fast as they can, etc)

learning objectives and key facts/concepts:

Be able to explain the difference and the adaptive significance between tonic and phasic responses. Be able to describe the mechanisms underlying the three phases of sensory response curves. Be able to explain the mechanism of and the adaptive significance for center-surround organization of neural processing. Be able to discuss the organization of cortical maps. 1. Sensory receptors are still ambiguous. 2. Sensory response curves have three phases. 3. Center-surround neural organization can sharpen signals in populations of neurons. 4. The sensory world is mapped across cells in parts of the cortex of the brain, but the maps are distorted and over represent areas of the external stimulus field that are especially important to the animal.

assigned readings:

Campbell Biology (11th Edition), Chapter 49, pp. 1094-1097; Chapter 50, pp. 1105-1108, 1112

what are dermatomes?

Dermatomes are all the RFs of sensory afferent neurons that enter the spinal cord through the same spinal nerve

Cortical magnification occurs in other sensory modalities

Human visual system Left side of the visual field projects to the right side of the primary visual cortex Representation is inverted and the foveal region is magnified Auditory system of mustache bat Area of auditory cortex that processes sounds around main call frequency of 60-63 KHz is magnified

summary:

In the last lecture we saw how a variety of sense organs transduce environmental energy into an electrical signal that the nervous system can work with. Receptor cells in several systems did not fire action an potential, but merely changed the amplitude of their membrane potential in a graded manner that encoded the magnitude of the stimulus. But graded potentials do not travel very far due to short length constants of cells, thus some cells trigger regenerative, all-or-none action potentials that can carry the signal long distances and encode the stimulus magnitude by the frequency of action potentials. One characteristic of all sensory receptors is that they are ambiguous for the energy they transduce and the information about the outside world only becomes more precise by having higher order cells compare the activity across several, or a population, of nerve cells. In today's lecture we will continue with the last of the five common human external senses, touch. It is also ambiguous, in that individual organs (and there are several types at different depths under the skin) respond to mechanical stimulation from a wide area called a receptive field. However, our perception of the location of touch is much finer due to a neural comparison mechanism called center-surround organization, which is a two dimensional form of lateral inhibition and also occurs in other modalities. We will discuss how this comparison takes place in the periphery and also in the ascending neural pathways to the brain where the body surface is mapped in the somatosensory cortex of the brain. We will discuss other sensory and motor maps.

Playing a musical instrument

Intention Voluntary motor commands Sensory feedback - auditory - touch send a voluntary motor command goes through motor cortex back through brain stem to limbs and fingers press, get touch info and auditory info which goes to ear and back to temporal lobe, make an adjustment to move finger slightly remember, fastest conduction velocity to move finger/limb

Receptors respond with different time courses - -(slow adapting) cells generate spikes as long stimulus continues - -(fast adapting) cells generate spikes when the stimulus changes

Tonically responding Phasically responding: meissner gives ap with burst of change and again with pacinian corpuscle

Somatic senses are also developed in other mammals Star nosed mole Lives in swampy wet soil and feeds on small subterranean invertebrates that it feels with its 22-fingered star

coded with tactile organs that catches creatures and moves them to the mouth mole unculus

Cerebral cortex is divided into what functional regions?

frontal lobe, temporal, parietal lobe, occipital lobe foldings/grooves called sulci and folds are gyri central sulcus between frontal and parietal lobes motor cortex has commands for body movement somatosensory cortex contains a map of your body

What can sharpen spatial representations in the brain stem?

lateral inhibition look at center X, 3 axons projecting up into spinal cord cells surround inhibit response of center. when red cell gets excited it excites following neuron and inhibits central axon by hyper polarizing when you touch black triangle, no difference in response but as you start stimulating red it starts inhibiting central cell and depressing from regular rate, central responds as big as it did before

Conduction velocities of skin/muscle sensory fibers differs limb position, touch, pain/temp, pain/itch/temp:

limb position: proprioceptors conveying info about tension or length of muscles limb position has very fat myelination, some pain not at all limb position is very fast velocity so motor commands can be interpreted properly

Somatosensory cortex maps the skin surface (homunculus) what is cortical magnification?

map of parts of your body, different parts are overrepresented like lips more than trunk of body More sensitive areas are over represented in the cortex (cortical magnification) this is left side of brain but representing right side of face "homunculus"

Cross section of a spinal nerve Some neurons are myelinated, some have no myelin CHART

neurons of different diameters; some are myelinated and faster, some are slower/nonmyelinated

Sensory information follows discrete tracts to the which side of the brain? Almost all sensory and motor information relays in the thalamus CHART

opposite

Receptive fields of neighboring cells overlap or not? how is spacial ambiguity resolved?

overlap say we have 3 receptive fields overlapping and touch 2 different points. response of cell A is big and second point is smaller bc it is on the edge of cell A. cell B is also responding. cell C does not respond. so you can not discriminate it is two points separate points a bit. A is the same and B it is close to the center and C it is off to the edge. still can not discriminate whether it is 2 points of or one larger object. not until you stimulate 2 receptive fields with middle getting no stimulation that nervous system compares activity across cells

Cortical somatosensory map is adjacent to motor map and in spatial register

pre-central gyrus: motor commands but also has map of body just like post-central gyrus stroke can kill these cells and impair neurons so they can not move side of face, can find where stroke is if you can feel but can not move it may be in somatosensory cortex

Receptors in the skin are organized spatially each neuron has a what?

receptive field (RF) is the area of skin on surface area you can touch and excite that cell some cells have smaller or larger and intensity of red cell indicates sensitivity (more sensitive in middle) chart: w a weak stimulus, cell responds best in middle of RF and still responds a couple mm away but fewer spikes, as stimulus gets stronger it gets more and more spikes in middle of RF and fewer at edges if 10 APs, could be strong stimulus or a little weaker stimulus