Basic Neuroanatomy:Tegmentum, Substantia Nigra, MID BRAIN areas

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Neurological Basis of Behavior (PSY - 610)

Lesson28

Basic Neuroanatomy

Objectives:

The main objective of this lesson is to study

The Brain and the Peripheral systems:

Brain: Forebrain, Mid brain, Hind Brain functioning of each anatomical location in the CNS.

Starting from the posterior located areas upto the anterior-most.

Cerebral Cortex: its layers, Corpus Callosum and the two hemispheres

Main purpose:

Students will become familiarized with the location of various brain areas, their functions and their

relationship to other areas. The students would also understand how control and modulation of

behaviour related to each neuroanatomical site takes place

In lesson 27 we discussed some parts of the Mesencephalon, and now we continue with the remaining

areas which fall under Mesencephalon.

Tegmentum: This lies between the Tectum and Substantia Nigra. It contains nuclei and relay fibres

of ascending and descending tracts. It also contains the motor cranial nerves for eye movements

(oculomotor) and the trochlear cranial nerves. The three major fibre bundles are the Medial

Leminiscus ( ML)and the Trigeminal Leminiscus (TL) and the Spino-thalamic tract ( STT)

The ML lies above the Substantia Nigra (SN) and conveys kinesthetic and discriminative touch

sensory information to the Thalamus.

The TL also are tracts which travel upto the Thalamus

The STT conveys the pain and temperature sensations from the contralateral hemisphere ( e.g from

the left arm to the right side of the brain).

PAG: Peri-Aquaductal Gray. This is the grey matter which surrounds the cerebral aquaduct: This

has the neural circuitry for sequence of movements for species specific behaviors (fight, flight, and

mating). Research ahs also shown this to be an important area for pain sensations. If opiates are

injected in this area they reduce the sensitivity to pain ( raise the tolerance threshold)

In the basal portion of tegmentum we have the cerebral peduncles which are large fibre bundles

which are placed in the ventral region of the mesencephalon. These carry fibres of the cortico-

spinal tract, and the cortico-pontine tract. There are also large projections such as the pareito-

occipito-tempero-pontine projections which carry projection from cerebral cortex down through

pontine area into the spinal cord.

Substantia Nigra: dark pigmented mass of neurons, between cerebral peduncles and tegmentum,

zona compacta rich in Dopamine and brain opiate receptors. This is part of the brains motor control

and modulation sys tem, and is involved in movement and balance. If we create an imbalance in the

bilaterally located SN areas by a unilateral lesion, we see asymmetric body posture. The bdoy turns

from the high to the low region (lopsided posture). If the Dopamienrgic neurons degenerate in this

area it leads to Parkinson's disease. This is characterized by tremor rigidity, slowness of motor

activity, stiffness in muscles, pin rolling movement, loss of adaptation (facial expression and gait).

Inputs come from the Neostriatum, cerebral cortex, Globus Pallidus, and other parts of the

Tegmentum

Neurological Basis of Behavior (PSY - 610)

Red Nucleus: neurons with a pinkish hue are an important is importna area in Tegmentum. Inputs

come in from deep cerebellar nuclei and the cerebral motor cortex. If the deep cerebellar input is

damaged, tremor is manifested when the hand or foot is in motion (reaching out). The gross

movements of body are controlled by this system. If there is unilateral electrical stimulation, it leads

to circulatory motion, and lesions result in disturbances of gait (walking).

The Medulla, Pons, and the lower areas had evolved earlier on the revolutionary scale and are similar

from fish to man. The fish don't have a cerebral cortex, but have large inferior and superior colliculi, as

this is what fishes would need for determining direction while swimming in water. Bats are nocturnal

creatures which fly and hunt at night. For bats audition becomes more important for survival more than

vision, therefore the bats have bigger inferior than superior coliculli. The brain stem areas serves as a

connection between upper and lower areas, that is they connect the Telencephalon and Diencephalon to

the Spinal Cord

MID BRAIN areas: These comprise of the Corpus Striatum: Striated bodies( corpus) This group of

structures includes the Basal Ganglia the Caudate ( Tail Like) Nucleus, Globus Pallidus ( Pale Globe),

Putamen . These areas lie underneath the cerebral cortex

The Corpus Striatum receives input from and sends output to the cerebral cortices, especially the

frontal areas where the motor lobe is located. They also send fibres to the Thalamus (output). There are

numerous interconnections between different areas of the corpus striatum. These areas appear to be

involved in the modulation of motor movement, especially the movement/modulation of contralateral

aspect of the body. This also has the controls of initiation of movement, contralateral head turning,

circling, licking, chewing, gnawing etc. This is also important part of the Dopaminergic pathway

involved in Parkinson's disease. Bilateral lesions of pallidus lead to hypoactive and sleepy animal.

Thus, this appears to be the basis of decreased movement and hypokinesia of Parkinsons' disease. On

the hand, the Caudate promotes arousal of motor system. So they balance the controls of motor

movement.

If we look at the hierarchy of movement starting from the highest control, the cerebral cortex, we

see that the Motor cortex-sends commands to the striatum, from there the commands go through the

nigral system and then the action takes place. As motor skills are learned the basal ganglia takes

over role of executing learned strategy. So the commands go from the striatum to the nigral for

preparation of action to take place. This becomes more automated and programmed. Think about

how a child learns to walk, slow deliberate balancing takes place initially and then gradually

locomotion becomes a routine. However, when basal ganglia are damaged the individual reverts to

slower less automatic cortical response. Basically the preparation of execution of movement is the

main function: We could call it a Gating function: This means gating of sensori-motor processing

(controlling the gates of channels which provide information). Thus, it limits the access of

information to motor systems.

Huntington's chorea is due to degeneration of Basal ganglia. This degeneration results in

reduction of inhibitory output of BG- which leads to increased access of sensory information, which

leads to increased activity (hyperkinesis, sudden jerks, tics, and jerky movements of head, trunk and

extremities, facial grimaces, repetitive dancing movement.

On the other hand in Parkinson's disease, there is decreased Dopaminergic input which inhibits

action in Basal Ganglia. As you have seen earlier, if the basal ganglia is not working the cortex

would take over. Therefore the reduced Dopamine in Basal Ganglia then allows cortical areas to

stimulate the motor system. There is an increased inhibition of inhibitory BG output, (stimulates the

inhibitory hypokinesia). The damage to Basal Ganglia also results in deficits in Cognitive functions

such as deficits in spatial memory, and inability to switch to appropriate behaviour.

Neurological Basis of Behavior (PSY - 610)

Limbic system

Limbic system: ref www.healing arts. Org/n-r-limbic.htm

Limbic system: It borders the Telencephalon, and Diencephalon, in appearance looks like a ring

around the Thalamus.It was first described by Broca in 1878. This is also known as the Circuit of

Papez, as Papez (1937) first identified it as a reverbrating circuitry which was important in

emotions. Limbic areas are spread into parts of the Frontal, Parietal and Temporal lobes. Therefore

it appears to have diverse connections and functions. These are mainly emotions but also memory,

homeostatic and survival functions (fight, flight, feed, and mating). The areas which are part of the

limbic system are: Cingulate Gyrus, Septal Nuclei, Hippocampus, Amygdala, Hypothalamus,

Anterior Thalamus, Mammilliary Bodies

Hippocampus like a sea horse lesion- is important in th eforamtion of memeories especially long

term. If lesioned bilaterally there is damage to learned emotional response and memory is severly

affected (both recent and long term). Hippocampus is thus involved in Emotions, memory,

homeostatic responses fight/flight, motivational states

Fornix: This is a large fibre bundle shaped in the form of an arch which connects the hippocampal

formation to subcortical areas such as thalamus, hypothalamus, and septum. It runs directly under

the runs under the corpus callosum( bands of fibres which connect the two lobes)

Amgdala: (almonds: greek) major part limbic system: lcoated at the tip of temporal lobe beneath

the cortex and rostrally to hippocampus. Have connections with hippocampus, septum, emdaildorsal

thalamus and the prefrontal areas. It is because of these connections that the amygdale is important

in emotional responses love, friendship fear, and rage aggression. This is involved in physiological

response of emotions: heart rate pulse etc. orienting to novel stimuli, déjà vu. If stimulated of

olfactory and gustatory hallucination (temporal lobe epilepsy). Bilateral lesions of amygdalas lead

to the Kluver-Bucy syndrome: hyperorailty, hypersexuality (animate or inanimate objects) docility,

learned fears (and agression) such as fear of snakes gone as the animals put snakes in their mouth.

This accompanied by lack of affect, apathy and blunted expression. This syndrome was first

described by Kluver Bucy in 1939 after bilateral lesions in monkeys

On the other hand electrical stimulation of AMygdala lead to a rage reaction the amygdale involved

in identification of danger therefore is important for self preservation. When triggered, it gives rise

to fear and anxiety which lead the animal into a stage of alertness, getting ready to flight or fight.

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Neurological Basis of Behavior (PSY - 610)

Septum: lesion in the septum leads to intense rage reaction (called the Septal Rage) as does

stimulation of the amygdale. Septum leads to increased activity in a novel situation, increase reward

feeling with stimulation. This also plays am important role in motivational states such as feeding

and drinking etc.

Cingulate Gyrus: ( Cingulate: encircling) located between the cingulate sulcus and the corpus callosum

The anterior area gives rise to déjà vu that is smells and sights with pleasant memories of previous

emotions., also important role in emotional reaction to pain and agression cingulectomy tames unruly

and wild animals. If a single bundle of this gyrus is cut (cingulotomy) it interrupts the limbic areas

communicationswith each other (reverbratiung circuitry affected) thus leading to reduced depression

and anxiety levels (which preexisted in the patients). (Points to ponder- looks like a good option for

treatment--but who is to decide?)

Refernces

1. Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

2. Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

3. Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd

edition) Worth Publishers New York

4. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York

5. Brown,T.S. and Wallace.(1980) P.M Physiological Psychology

Academic Press New York

Note: References 1, 2, 3, 5 more closely followed in addition to the references cited in text.

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