Sensory systems Domina Petric, MD

Neuronal pathways I.

Example of neuronal pathway First order neuron recieves an input from some sensory organ with its peripheral axon. First order neuron has also a central process that enters the nervous system and synapses with second order neuron. Second order neuron projects some distance through the central nervous system to a third order neuron. Third order neuron provides input to a vast network of interconnected cells. Interconnected neurons are the processing station in the cerebral cortex.

It is a set of neurons that are connected in a serial fashion. Neuronal pathway It is a set of neurons that are connected in a serial fashion.

Contralateral representation II.

Contralateral representation The sensory input from one side of the body (for example right) will be interpreted in the contralateral brain hemisphere (left). Decussations: middline crossing.

Sensory transduction III.

Sensory transduction in sense of touch Pacinian corpuscle contains a nerve ending that has cations selective channels. These channels are sensitive to stretching forces applied to the membrane. When there is mechanical deformation (touch), channels open. Cations (sodium ions) enter the cell: depolarisation of the sensory ending of the axon. The stimulus has to be strong enough to make a depolarisation that is above receptor potential treshold: action potential.

Information coding IV.

Information coding and labeled lines Labeled lines: the response of an axon in a sensory nerve is going to be shaped or dictated by the kind of receptor that is elaborated at the end of that axon. That sensory receptor is specialised to encode a certain quality of information.

Labeled lines example Pacinian corpuscle in the subcutaneous layers of the skin is especially sensitive to vibration. Meissner corpuscle is especially sensitive to light touch. The selectivity of the different axons that terminate in these receptors is determined by the specializations present at the terminal endings of the axons.

Information coding and adaptation When a stimulus is applied to a sensory ending, there is a barrage of action potential at the onset of that stimulus. After some time there is no more action potentials. Some receptors adapt rapidly and some slowly. Slowly adapting receptors are usefull for detecting the persistent stimulus. Rapidly adapting receptors are usefull for detecting dynamic changes in the presence of a stimulus.

Information coding and treshold Treshold is different for different kinds of receptors. Mechanosensors are very sensitive with a low treshold for activation. Pain fibers have a higher treshold for activation.

Information coding and receptive fields Receptive field is specific area in the body where, when the stimulus is presented, there can be a modulation of activity in a specific nerve cell.

Information coding and receptive fields Receptive fields have center surround organisation. Center of receptive fields, when stimulated, gives rise to a robust modulation of activity (dramatic increase of activity) in a specific nerve cell. Sometimes it can be dramatic decrease of activity. There is also a surround of receptive field which is antagonistic to the center of receptive field. So if the stimulation of the center of receptive field leads to increase of activity, stimulation of the surround of receptive field leads to decrease of activity.

Rule of convergence For neurons in the cortex, large receptive fields are created by a high degree of convergence: many neurons at lower levels of the sensory pathway are converging onto the same cortical neuron. Smaller receptive fields are created by minimal patterns of convergence: fewer neurons at lower levels of the sensory pathway are converging onto the same cortical neuron.

Two point discrimination There must be a stimulation of at least two receptive fields on the skin surface. The receptive fields are smaller in fingertips (spatial acuity is high) compared to other parts of the body. Two point discrimination is a phenomenon of acuity.

Acuity Regions of high sensory acuity have small receptive fields and densely innervated sensory surfaces.

Literature https://www.coursera.org/learn/medical-neuroscience: Leonard E. White, PhD, Duke University