107 B Section 7 TA: Flavia Filimon

SOMATOSENSORY SYSTEM AND AUDITORY SYSTEM

I. Somatosensory system

2 Pathways to cortex:

1) Dorsal column pathway

- Touch information

- From dorsal ganglion cells à dorsal column nuclei (cuneate and gracile nuclei) à contralateral VB (ventrobasal nucleus of the dorsal thalamus)

- Hand, arm, upper body, etc. : to cuneate nucleus (ipsi)

- Foot, leg: to gracile nucleus (ipsi)

2) Spinothalamic pathway

- Pain + temperature information

- Fibers from dorsal ganglions synapse in ipsilateral spinal cord and cross immediately (to contralateral side), connect to:

· intralaminar nucleus

· ventrobasal nucleus

· posterior nucleus (all in dorsal thalamus)

If fibers on the Right are damaged below 4^th ventricle à

cannot feel touch on ipsilateral (right) side of body; cannot feel temperature and pain on opposite (left) side of body.

* POINT-TO-LINE projection pattern in the somatosensory system

a point on the skin (2-D map) projects to a column in the VB

Somatosensory Cortex

( see map)

· from VB à SS ctx.

- there are multiple copies of the body surface in somatosensory cortex.

- The maps are somewhat plastic, not fixed

Know about discontinuities in somatosensory system versus visual system:

· SS ctx: you can move a short distance in cortex and end up on completely different parts of your body, jumping a long distance on skin. And vice-versa.

· Example: area 3b à underside vs. dorsal side of fingers represented discontinuously

· Also: The cortical representations of different fingers are separated by a sharp boundary à there is a discontinuity between fingers, and receptive fields corresponding to different fingers. This discontinuity is not seen in the visual system; different fingers can be stimulated in isolation, whereas neighboring receptive fields in the retina are usually co-activated in a correlated manner.

· Visual ctx: you can never move a short distance in visual ctx and jump to a totally different part of your visual field. However, you can move a short distance in visual field and jump between cortical areas

· Example: upper vs. lower visual field; left versus right visual field (in V1: completely different hemispheres).

However, most of the maps in Somato-Sensory cortex can be explained by correlated sensory input.

Plasticity experiments:

Know the different plasticity experiments and their epxplanations.

1) if you de-innervate 1 finger (e.g. 2), the cortical representations for the adjacent fingers (1, 3) will expand and fill in the area formerly representing finger 2.

2) à Silverspring monkey experiment: whole arm de-innervated, face representation expanded and filled in the former arm representation.

3) syndactyly: sewing two fingers together (in monkeys): fingers that have been sewn together and that are stimulated together will have receptive fields that cross the former boundary between the two fingers à no more discontinuity between both receptive fields from the two fingers and their cortical representations.

· also: repetitive stimulation (e.g. touching) of a finger will lead to an enlarged representation in cortex.

4) Skin (+ nerve) transplant:

- transplant patch of little finger skin to de-innervated thumb:

- after a few days stimulating the thumb will feel like the little finger still – in cortex, the area representing the little finger will be activated

- after 2 weeks: there is an expansion of the thumb area in cortex – stimulating the thumb now feels like a thumb and activates the representation of the thumb, rather than that of the little finger

Two mechanisms are possible (explanations for the plasticity):

1) regrowth of axons.

- Structural change/ rewiring of cortex: in cases where the shift of areas involves several cm, the change has to be due to a rewiring: new axons and synapses are probably sent over to the area no longer activated by sensory input (e.g. the arm area in example 2).

2) change of existing synapses (e.g. turning up or down of existing synapses; removing lateral inhibition between different finger areas). These changes are possible since they involve only a few mm which are covered by the synapses. This mechanism is likely to explain the syndactily and 1st examples.

II. Auditory system

- receptors of the auditory system: hair cells.

- How sound gets transduced: The basilar membrane vibrates up and down, causing the cilia of the hair cells to shear sideways against the tectorial membrane à neurotransmitters released

- Basilar membrane has a gradient of stiffness à different hair cells are tuned to particular frequencies

TINNITUS

1) top-down modulation from the CNS to the cochlea might cause the cochlea to vibrate, and hence patients to hear sounds

2) electrical tuning of hair cells: if an incoming sound contains a certain frequency that a given hair cell is tuned to, that frequency will get amplified by the hair cell

Frequency selectivity of hair cells:

1) MECHANICAL: different parts of the basilar membrane vibrate at different frequencies (due to stiffness gradient)

2) ELECTRICAL TUNING: frequency/ sound will get amplified if it is the one the cell is selective for.

OWL AUDITORY SYSTEM

NA = nucleus angularis

NM = nucleus magnocellularis

NL = nucleus laminaris

ICc medial = Inferior Colliculus central nucleus, medial part

ICc lateral = Inferior Colliculus central nucleus, lateral part

ICx = Inferior colliculus, external part

Know the different properties of the cochlear nuclei (NA + NM)

Both NA and NM are tonotopic (selective for different frequencies)

· NA – responds for amplitude

· NM – codes for phase : regardless of amplitude, its neurons spike with a fixed delay relative to each wavefront of sound.

Problem:

Since we’re breaking up sound into several frequencies, how do we figure out where the sound is coming from?

à NL doesn’t solve the problem, because it is just a coincidence detector.