Medical neuroscience

1. SENSORY
SYSTEMS
Domina Petric, MD

2. NEURONAL
PATHWAYS
I.

3. 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.

4. NEURONAL PATHWAY
It is a set of neurons that
are connected in a serial
fashion.

5. CONTRALATERAL
REPRESENTATION
II.

6. 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.

7. SENSORY
TRANSDUCTION
III.

8. 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.

9. INFORMATION
CODING
IV.

10. 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.

11. 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.

12. 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.

13. 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.

14. INFORMATION CODING AND RECEPTIVE FIELDS

15. 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.

16. 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.

17. 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.

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

19. LITERATURE
https://www.coursera.org/lear
n/medical-neuroscience:
Leonard E. White, PhD, Duke
University