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12. 05. 2014
Kaan Yücel
M.D., Ph.D.
Dr.Kaan Yücel
Pathways in the brain & Limbic system
Ascending tracts
Descending tracts
General arrangement of both tracts
1st order neuron
2nd order neuron
3rd order neuron
The only difference is the different locations where each order of neuron ends.
Decussation is the cross-over of the tract from one side to the other. Therefore, there are instances where the
left side of the body is controlled by the right brain hemisphere. Decussation occurs at different locations for
each tracts.
General arrangement of descending tracts
1st order neuron
starts at the cerebral cortex in the primary motor cortex
2nd order neuron
axon of the 1st order neuron will synapse with the 2nd order neuron at the level of the brain stem, which
commonly decussate (crosses over) to the opposite side.
3rd order neuron
The 3rd order neuron is located in the ventral horn of the spinal cord, which will exit with the spinal nerve to
supply the muscle.
Types of descending tracts:
Lateral corticospinal tract
Anterior corticospinal tract
Therefore, the descending tract is also known as corticospinal tract.
Corticospinal tract arise from long axons of the pyramidal cells of the precentral gyrus (primary motor centre of
the cerebral cortex) which lies in front of the central sulcus
Homunculus arrangement: arranged upside down; the finer the movement, the more the cortical
fingers, face, tongue – more
trunk, lower limbs – less
medial surface: lower limbs
superolateral surface: everything else
1st order neuron
Fibres of the 1st order neuron arise from the precentral gyrus
These fibres converge and enter a small area
internal capsule
ALL the fibers (from ascending & descending tracts) converge here
Function: separates the caudate nucleus and the thalamus from the lenticular nucleus (putamen+ globus
internal capsule: bounded medially by the thalamus and caudate nucleus and bounded laterally by the lenticular
Parts of internal capsule (not homunculus arrangement, normal head to toe)
anterior limb: head & neck fibres most anterior
Dr.Kaan Yücel
Pathways in the Brain- Limbic System
posterior limb: lower limb fibres most posterior
The descending fibres passes through the LATERAL half of the posterior limb of internal capsule
After the internal capsule, the fibres enter the brain stem; midbrain, pons and medulla.
2nd order neuron
Fibres of the 1st order neuron ends when it enters the brain stem and synapse with the 2nd order neuron
The fibres pass through the brainstem
1st – through the (mid 5th) crus cerebri of midbrain
2nd – through the anterior part of the pons
3rd – in the medulla oblongata
80-85% of the fibres cross to the opposite side: Motor decussation
Enters the spinal cord
3rd order neuron
2nd order neuron fibres in the medulla oblongata enters the spinal cord and synapse with the 3rd order neuron
Motor decussation in the spinal tract, the crossed tract descend as the lateral corticospinal tract
Therefore, the motor cortex of the cerebral hemisphere controls the opposite side of the body (L – R, R – L)
contra-lateral side.
In upper motor neuron lesions: above the motor decussation (above medulla), opposite side of body affected
below the motor decussation same side of body affected ipsilateral side
Uncrossed fibres: in the spinal tract, the uncrossed tract descent as the anterior corticospinal tract
its fibres cross at spinal level?
Types of ascending tracts:
Spinothalamic tracts
Lateral spinothalamic tract
pain & temperature
Anterior spinothalamic tract
light touch & pressure
Dorsal column tract
deep touch & pressure
vibration sensation
Spinocerrebellar tract
posture & coordination
1st order neuron: Arise from sensory receptors of the body
The fibres enter the white mater from the tip of posterior gray horn
2nd order neuron:The fibres of 1st order neuron synapse with the 2nd order neuron at the substantia
gelatinosa. These fibres then cross to the opposite side
Pain & temperature fibres enters the lateral spinothalamic tract
Light touch & pressure fibres enters the anterior spinothalamic tract
These tracts ascends to brainstem to medulla oblongata, pons and midbrain
tracts flattened in the brainstem: spinal lemniscus
Reaches the ventral posterolateral nucleus of the thalamus and ends here.
3rd order neuron: The 3rd order neurons arise from the thalamus and pass through the internal capsule
thalamocortical fibres pass through the medial part of the posterior limb of the internal capsule
Enters the postcentral gyrus - sensory cortex of the cerebrum, behind the central sulcus.
Same homunculus arrangement; more sensitive areas in the body have a greater representation.
1st order neuron:
 Arise from the sensory receptors of the body
 Fibres enter the dorsal column of the SAME side (post column of spinal cord)
Dr.Kaan Yücel
Pathways in the brain & Limbic system
ascends to the medulla oblongata
(does not synapse and end here like spinothalamic tract)
 Enters medulla oblongata
 ends in the gracile and cuneate nucleus
2nd order neuron: Starts at the gracile & cuneate nucleus of the medulla oblongata
These fibres crosses to the opposite side of the medulla oblongata.
Ascends through the brain stem as flattened bundle medial lemniscus
Ends in the ventral posterolateral nucleus of the thalamus.
3rd order of nucleus: Arise from the thalamus Pass through the internal capsule; medial aspect of the
posterior limb of internal capsule.
Reaches the postcentral gyrus and ends here.
1st order neurons:
Arise from the sensory receptors of the body
Enters the spinal cord
Ends in the Clarke’s Column of the posterior grey horn
2nd order neurons:
Arise from the Clarke’s Column
synapse with 1st order neurons
Ascends in the spinocerebellar tracts, enters the cerebellum through the interior and superior cerebellar
the only tract that enters the cerebellum
These tracts decussate 2 times; therefore cerebellum controls same side of body
eg. right spinocerebellar tract controls the right side vice versa
The limbic system has two main functions:
Emotional processing
Another function of the system; short-term memory (also emotional memory) is also important for “survival”.
The limbic system works to process our emotions and is related to motivation and with its connections with the
cognitive parts of the brain helps us to “use our mind” a.k.a. accomplish mental processes.
The limbic system structures are telencephalic & subcortical structures.
The complex network for the process of emotions and is also related to memory and learning in addition to
hippocampus, amygdala and parahippocampus includes:
 Cingulate gyrus
 Hypothalamus
 Major areas in the prefrontal cortex
 Striatum
 Some thalamic nuclei
 Orbitofrontal cortex
 Septal area
 Some medial components of the midbrain (e.g. VTA)
 Habenula …
 + white matter tracts
Dr.Kaan Yücel
Pathways in the Brain- Limbic System
In 1937 James Papez proposed the Papez circuit: A list of structures in the brain and a closed circuit related to
Hippocampal formation (Subiculum) → fornix → mammillary bodies
Mammillary bodies → mammillothalamic tract → anterior thalamic nucleus
Anterior thalamic nucleus → genu of the internal capsule → cingulate gyrus
Cingulate gyrus → cingulum → parahippocampal gyrus
Parahippocampal gyrus → entorhinal cortex → perforant pathway → hippocampus.
In 1952 Paul D. McLean added
 Amygdala
 Septum
 Pre-frontal cortex
to the Papez circuit and came up with the idea of a system: Limbic System.
In 2014, we now know that the system is more complex than it was first proposed and discussed in mid20th century.
Klüver-Bucy Syndrome
bilateral removal of amygdala and hippocampal formation
What happens if we remove the medial temporal lobe of an animal, a monkey?
 Became docile;”good monkeys”.
 A tendency towards oral behaviour such as attempting to ingest inedible objects.
Dr.Kaan Yücel
Pathways in the brain & Limbic system
 Hypersexualized behaviour by mounting females of the same and different species.
 A compulsion to attend and react to every visual stimulus
 No fear.
 Change in dietary habits
The most famous two members of the limbic system are hippocampus & amygdala.
Hippocampus (sea horse; hippocampal formation) is located in the medial temporal lobe under the inferior
(temporal) horn of the lateral ventricle. Amygdala (almond) resides at the tip of the temporal lobe anteriorly,
and is posterior to anterior part of hippocampus.
Hippocampus is the site of short-term memory. It is also an important structure in mood regulation with its
connections with the hypothalamus.
Amygdala is important in emotion processing with ventromedial prefrontal cortex and acts as an emotional
memory box.
Anterior cingulate cortex (ACC), medial part of prefrontal cortex, the basal ganglia (particularly caudate,
putamen, and nucleus accumbens; the site of pleasure), anterior and dorsomedial thalamic nuclei are some of
the important limbic system structures.
The cognitive networks inhibit the ventral circuit.
Dorsal (cognitive) circuit
Dorsolateral prefrontal cortex (DLPFC)
Dorsal regions of the anterior cingulate cortex (ACC)
Parietal cortex
Posterior insular region
Modulates selective attention, planning and effortful regulation of affective state.
Ventral (limbic) circuit structures:
Insula (Particularly, anterior insula)
Ventral striatum
Ventral regions of the anterior cingulate cortex (ACC)
Orbitofrontal cortex (OFC) and medial PFC
It is possible that the altered emotional regulation or cognition found in all of these syndromes involves
aberrant function of these circuits, but perhaps with different patterns on a molecular level. (Phillips et al.