PATHWAYS IN THE BRAIN

Kaan YüceM.D., Ph.D. 12. May.2014 Monday

Ascending tractsSensoryDescending tractsMotor

General arrangement of both tracts1st order neuron2nd order neuron3rd 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.

1st order neuronstarts at the cerebral cortex in the primary motor cortex

2nd order neuronaxon 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 neuronThe 3rd order neuron is located in the ventral horn of the spinal cord, which will exit with the spinal nerve to supply the muscle.

DESCENDING TRACTS

Lateral corticospinal tractAnterior 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); lies in front of the central sulcus

Types of descending tracts:

MOTOR Homunculus arrangement: arranged upside down; the finer the movement, the more the cortical representationfingers, face, tongue – moretrunk, lower limbs – lessmedial surface: lower limbssuperolateral surface: everything else

Motor homunculusSensory homunculus

Motor homunculusSensory homunculus

1st order neuronFibres of the 1st order neuron arise from the precentral gyrusThese fibres converge and enter a small areainternal capsuleALL the fibers (from ascending & descending tracts) converge herebounded medially by the thalamus and caudate nucleusbounded laterally by the lenticular nucleusThe descending fibres passes through the LATERAL half of the posterior limb of internal capsule 2nd order neuronFibres of the 1st order neuron ends when it enters the brain stem and synapse with the 2nd order neuronThe fibres pass through the brainstem1st – through the crus cerebri of midbrain2nd – through the anterior part of the pons3rd – in the medulla oblongata80-85% of the fibres cross to the opposite side: Motor decussationEnters the spinal cord

2nd order neuronFibres of the 1st order neuron ends when it enters the brain stem and synapse with the 2nd order neuronThe fibres pass through the brainstem1st – through the crus cerebri of midbrain2nd – through the anterior part of the pons3rd – in the medulla oblongata80-85% of the fibres cross to the opposite side: Motor decussationEnters the spinal cord

3rd order neuron2nd order neuron fibres in the medulla oblongata enters the spinal cord and synapse with the 3rd order neuronMotor decussation in the spinal tract, the crossed tract descend as the lateral corticospinal tractTherefore, 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 affectedbelow the motor decussation same side of body affected ipsilateral sideUncrossed fibres: in the spinal tract, the uncrossed tract descent as the anterior corticospinal tractits fibres cross at spinal level?

Spinothalamic tractsLateral spinothalamic tractpain & temperatureAnterior spinothalamic tractlight touch & pressure

Dorsal column tractdeep touch & pressureproprioceptionvibration sensation

Spinocerrebellar tractposture & coordination

ASCENDING TRACTS

SPINOTHALAMIC TRACTS1st order neuronArise from sensory receptors of the body; The fibres enter the white mater from the tip of posterior gray horn2nd 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 sidePain & temperature fibres enters the lateral spinothalamic tractLight touch & pressure fibres enters the anterior spinothalamic tractThese tracts ascends to brainstemtracts flattened in the brainstem: spinal lemniscusEnds @ the ventral posterolateral nucleus of the thalamus 3rd order neuronArise from the thalamus and pass through the internal capsulethalamocortical fibres pass through the medial part of the posterior limb of the internal capsuleEnters the postcentral gyrus

1st order neuronArise from the sensory receptors of the bodyFibres enter the dorsal column of the SAME side (post column of spinal cord)

ascends to the medulla oblongata(does not synapse and end here like spinothalamic tract)Ends in the gracile and cuneate nucleus

2nd order neuron Starts at the gracile & cuneate nucleus of the medulla oblongataThese fibres crosses to the opposite side of the medulla oblongata.Ascends through the brain stem as flattened bundle medial lemniscusEnds 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.Ends @the postcentral gyrus

DORSAL COLUMN TRACT

1st order neurons:Arise from the sensory receptors of the bodyEnters the spinal cordEnds in the Clarke’s Column of the posterior grey hornSynapse

2nd order neurons:Arise from the Clarke’s Columnsynapse with 1st order neuronsAscends in the spinocerebellar tracts, enters the cerebellum through the interior and superior cerebellar pedunclesthe only tract that enters the cerebellum

These tracts decussate 2 times; therefore cerebellum controls same side of bodyİpsilateral; eg. right spinocerebellar tract controls the right side vice versa

SPINOCEREBELLAR TRACT

What is limbic system?

YOUR MISSION IN LIFE?

survival

Two main functions

Emotional processing

Motivation

The Anatomy of Behaviour

Medial Temporal Lobe Hippocampus Parahippocampal gyrus Amygdala

Emotion

Memory

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

James Papez

A list of structures in the brain and a closed circuit related to emotions

Papez Circuit

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.

Paul D. MacLean

Limbic Systemadded Amygdala Septum Pre-frontal cortex to the Papez circuit

Klüver-Bucy Syndrome

What happens if we remove the medial temporal lobe of an animal, a monkey? bilateral removal of amygdala and hippocampal formation

Became docile;”good monkeys”.

A tendency towards oral behaviour such as attempting to ingest inedible objects.

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 guys of the limbic system

hippocampus & amygdala

3.1. Hippocampal formation/Hippocampus

HC

Temporal horn of lateral ventricle

TERMINOLOGY

HIPPOCAMPAL FORMATION VS.HIPPOCAMPUS

1. Hippocampus (proper) Cornu Ammonis (CA) CA1-CA4

2. Dentate gyrus

3. Subicular complex

Fornix [Arch]The road/white matter tract from the hippocampusPrecommissural fibers 25% Septal areaPostcommmissural fibers originate from the subicular complexMamillary bodies

Fornix

3.4.1. Anterior cingulate cortex (ACC)1=BA25 (subcallosal gyrus) 2=BA24sg (SGPFC) 3=BA32 (paracingulate gyrus)

3.5. Limbic structures in the Prefrontal cortex (PFC)• DLPFC, dorsolateral prefrontal cortex• VLPFC, ventrolateral prefrontal cortex• FP, frontopolar cortex• OFC, orbitofrontal cortex• DMPFC dorsomedial prefrontal cortex• VMPFC, ventromedial prefrontal cortex

3.6. Hypothalamus

Glaser R, Kiecolt-Glaser JK. Stress-induced immune dysfunction: implications for healthNat Rev Immunol. 2005 Mar;5(3):243-51.

3.7. Thalamus

Limbic thalamic nuclei Anterior thalamic nuclei

Mediodorsal thalamic nuclei

3. 8. Striatum4 major nuclei (1) corpus striatumCaudate nucleusPutamenVentral striatum –nuc.accumbens (2) globus pallidus internal and external segments

(3) substantia nigra pars compacta pars reticulata (4) subthalamic nucleus

Anterior hippocampusrostral hypothalamus and amygdalaHPA-axis controlStress

Posterior hippocampusspatial memory

Multiple functions of the hippocampus

Learning and memoryMood regulation - Affect - Emotional BehaviorRegulation of HPA axisPainErectile functionAttention

When you meet someone you know at the street

Hippocampus......Context

Amygdala.....Emotions

Emotional Memory

Fear

Amygdala

4.4. Anatomy of cognition & emotion

Two circuits & Crossing roadscognition means thinking and emotion means feeling

TWO MAIN CIRCUITS IN THE BRAIN COGNITIVE CIRCUIT EMOTION CIRCUIT DORSAL CIRCUIT VENTRAL CIRCUIT

The cognitive networks inhibit the ventral circuit.

Dorsal (cognitive) circuit o Hippocampuso Dorsolateral prefrontal cortex (DLPFC)o Dorsal regions of the anterior cingulate cortex (ACC)o Parietal cortexo Posterior insular region

Modulates selective attention, planning and effortful regulation of affective state.

Ventral (limbic) circuit structures:o Amygdalao Insula (Particularly, anterior insula)o Ventral striatum o Ventral regions of the anterior cingulate cortex (ACC) o 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. 2003