Thalamus

Question 1

where is the thalamus located?

Answer 1

diencephalon, located just underneath the telencephalon. in adult brain the thalamus is located between the cerebral cortex and the mesncephalon

Question 2

structure of the thalamus

Answer 2

a midline symmetrical structure, w. 2 hemispheres (left and right) surrounding the 3rd ventricle . the 2 parts are connected by the interthalamic adhesion.

Question 3

what is the function of the thalamus?

Answer 3

• the main function of the thalamus is receiving sensory input and relay it to the cortex.
• every sensory modality goes through thalamus (except for olfactory);
• each sensory info (taste, audio, visual, temp., etc.) is processed by a specific thalamic nucleus –>the signal is projected from each nucleus to is associated cortical taget

Question 4

types of nuclei

Answer 4

a. first order nuclei- input from the periphery and other, lower brain areas, sends output to neocortex
b. higher order nuclei- input from neocortex projected to other cortical areas for further processing.

Question 5

first order nuclei (sensory/motor)

Answer 5

sensory:
a. somatosensory- VP, receives afferents from medial lemniscus (limb position, tactile…) and from the anterolateral pathway (nociceptive and thermal afferents)

b. auditory- MGNv, receives auditory afferents from inferior colliculus and sends efferents to auditory cortex.
c. visual- LGN, afferents from retina–> efferents to visual cortex.

motor:
- VA & VL- info from deep cerebral nuclei–> efferents to motor and premotor cortex

Question 6

higher order nuclei (general function + the nuclei)

Answer 6

function: connecting different cortical area
- Mediodorsal nucleus (MD)- connects olfactory, frontal amygdala & frontal cortex
- Laterodorsal nucleus (LD)- connects different parts of cingulate corex
- Pulviar (PU)- connects occipital and temporal lobes

Question 7

is there any location processing within the thalamus or is it “just a relay”?

Answer 7

Yes, there is local processing within the thalamus, namely in the thalamic reticular nucleus (TRN).

• -> reticular cells of the TRN send axons to other thalamic nuclei only. they provide inhibitory, GABAergic input to thalamic relay cells (feedback inhibitory loop).
• -> the thalamic nuclei (apart from TRN) act as a relay between different cortical areas. They do not communicate with each other, however, TRN is involved in the local processing of information and communication between thalamic areas (“not just a relay”).

Question 8

explain the concept of somatotopy with an example from the thalamus.

Answer 8

• somatotopic arrangement: point for point correspondence of an area of the body to a specific point in the CNS.
• Example: whisker system of rodents:
  rodents are nocturnal animals and have to navigate with little vis. information. They have 24 whiskers on each side that provide them info about their immediate environment.
  the info from the whiskers is somatotopically mapped onto somatosensory cortex S1 (primary);
• each whisker connects to a specific group of sells in the brainstem (barellette)–> signals from specific barellete projects to a specific group of cells in thalamus (barreloid) –> barreloid projects to a target group of cells in layer 4 of S1 called “barrel cortex”

Question 9

what 2 parallel pathways does the signal from whiskers follow?

Answer 9

a. lemniscal pathway- sensory information is processed in the trigeminal nucleus of the brainstem–>projected to VPM (first order nucleus)–> to primary somatosensory cortex S1
b. paralemniscal pathway- sensory info comes from rostral part of the spinal trigeminal nucleus “pars interpolaris”–> to PO in the thalamus (higher order) –> to primary & secondary somatosensory cortex and motor cortex

Question 10

why is the barrel cortex called like that?

Answer 10

The barrel cortex appears as a barrel, because the centre of the structure has no soma (contains only dendrites) and therefore appears lighter

Question 11

where do VPM and PO project to?

Answer 11

VPM-projects to L4 and 5 of cortex (barrel cortex)

PO- projects everywhere from L1 to L5 (no distinct pattern)

Question 12

what is the thalamo-cortico-thalamic loop?

Answer 12

• the first order thalamic relay represents 1st relay of peripheral or subcortical info to a type of primary cortical area.
• the higher order thalamic relay relays info from L5 from one cortical are to another (as a loop 1 cort. area->higher order thalamic relay–> L4 of next cortical area).
• difference- the driver input which is subcortical for 1st order and from L5 of cortex for higher order.
• modulator input goes always from L6 of cortex to the thalamic relay, from which the signal came from (as a feedback inhibition)

Question 13

percentage of excitatory sensory synapses in the thalamus

Answer 13

10 % (receive sensory input from periphery); 
the rest (90%) of synapses are from other sources- e.g. modulating input from excitatory cortical area (L5-6) and inhibitory feedback from TRN (ca. 25%).

Question 14

What are X & Y cells in the cat’s visual system?

Answer 14

X cells-

• have tufted, elongated dendritic branches
• complex dendritic appendages that look like knots and occur next to primary branch points
• participate in triads

Y cells-

• cruciate arbours (cross like), spread out
• mostly devoid of appendages

Question 15

what are glomeruli?

Answer 15

• complex synaptic zones in thalamic nuclei
• barely any astrocytic cytoplasm, but instead- entire glomerulus is enwrapped by glial sheath
• function unknown
• within glomerulus synapses are arranged in triadic way (3 synapses work closely together)
• exist only at dendrites of X cells

Question 16

give an example of triadic arrangement

Answer 16

Triadic arrangement in the retina:

• retinal axon from the eye excites dendrite of X cell and at the same time F2 (inhibitory neuron)–> when F2 is excited it inhibits the X-cell.
• another modulatory excitatory input comes from parabranchial input (highly active when animal is fully aroused and absent in SWS).
• retinal triad- direct excitation and indirect inhibition

Question 17

Vantages of triadic arrangement

Answer 17

• Interneurons can multiplex–> independent computational routs to affect relay cells.
• the influence on x-cells lasts longer–> controlling overall responsiveness of relay, regulating voltage gated conductance in relay cell.

Question 18

what ion channels exist in the thalamus?

Answer 18

1. Na+ channels:
   a. voltage gated, fast Na+ channels (ascending AP)
   b. one persistent Na+ channel (always open)
2. Ca2+ channels:
   a. high threshold Ca2+ channels- open at depol. membrane (L-type)
   b. low threshold channels- open when cell is hyperpol. (T-type)
3. K+ channels:
   a. delayed rectifying K+ channels - repolarise the cell after AP
   b. Ih- together w. low threshold Ca2+ channel (hyperpol.-activated Ca2+ dependent K+ channel)

Question 19

what is the difference between tonic and burst firing?

Answer 19

Tonic firing: at relatively depolarised membrane (ca. -60 mV) the cell fires tonically, low threshold Ca2+ channel is inactivated; non rhythmic firing, no pacemaker.

Burst firing: at semi- hyperpolarised membrane (ca. -70 mV) and cell is injected with current –> burst of AP; low threshold Ca2+ channel is de-inactivated–> current activates low threshold spike;