Basic Neuroanatomy:Telencephalon, Mesencephalon. Myelencephalon

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Neurological Basis of Behavior (PSY - 610)

Lesson27

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

Basic Neuroanatomy is based on the following assumptions:

· The cortex is newly evolved as compared to the other structures, and it controls all major

functions (especially in the higher animals)

· The higher areas cover the lower areas as they have grown over these areas. The lower brain

areas have to send information to the higher areas and then the commands for actions come

back from these areas ( the thalamus send all messages it receives to the relevant cortices and

receives commands)

· The lower underlying areas are more primitive in terms of functions, their functions are more

survival functions ( the brain stem areas)

· These lower areas are similar in locations, site and functions in lower and higher order animals

(e.g. the hypothalamus, the thalamus).

Phylogenetically, encephalization (control of the higher brain areas) has occurred leading to increased

ability to interact and respond to the environment (greater adaptability).

The greater the encephalization, the more the functions are controlled by the higher brain areas,

therefore if these areas are damaged the animals cannot function at a normal level (Bridgeman, 1988,

Thompson 1980).

The following would elaborate how the brain is located with reference to the spinal cord and how other

areas within the brain and the top of the spinal cord

Brain------------------------------------------------------------------------------spinal cord

There are three major divisions of the brain: The Forebrain, the midbrain and the hind brain.

The Forebrain comprises of the Telencephalon (coritcal and some subcortical structures) and

Diencephalon (the Thalamus and the hypothalamus).

The Mid brain consists of older structures, the Mesencephalon and

The Hind brain The posterior most part of the brain is called the Hind brain, which has two

sections the Metencephalon and the Myelencephalon

We will elaborate these areas further moving from the frontal and dorsal areas to the more caudal

ventral areas. The anterior most and the dorsal area is the Telencephalon, with Myelencephalon at the

caudal most end

Neurological Basis of Behavior (PSY - 610)

Telencephalon: comprises of the Cerebral Cortex, the Corpus Striatum, Cingulate Gyrus, Septum,

Amygdala, Hippocampus (these last three are also part of the limbic system). Therefore this is cerebral

cortex which is a phylogenetically a newer structure, and some older primitive structures which underlie

the cerebral cortex

Diencephalon: Comprises of the Thalamus which is the junction and the gateway for all sensory-motor

and association pathways and the Hypothalamus which is the primitive survival centre

Mesencephalon: has the Tectum which comprises of the Inferior (for auditory systems) and the

Superior colliculi (for visual system) Tegmentum, cerebral peduncles Metencephalon: comprises of the

Cerebellum which lies dorsally over the Pons, and the Pons

Myelencephalon comprises of the Medulla Oblangata

Brain stem: includes the Mesencephalon, Metencephalon and the Myelencephalon i.e. technically

everything between spinal cord and cerebral cortex

Myelencephalon: Medulla Oblongata ( Oblong and narrow structure) is continuation of spinal cord and

the caudal part of the brain stem .Another important structure which is located in the Brain stem are is

the reticular formation.

Medulla contains all the acending and descending fibre pathways connecting the brain and the spinal

cord. It also has nuclei for the cranial nerves especially related to the control of respiration; heart and

digestive activity enter the brain at the level of medulla. Cranial nerves for tongue, larynx, pharynx, ear,

vestibular, involved in control of breathing while sleeping (sleep apnea), sneezing, swallowing,

vomiting are also found here.

Cranial nerves:

Hypoglossal nerve (tongue) -

Cranial Nerve XII

Accessory (larynx, voice, mouth mastoid) Cranial nerve XI

Vagus (taste, larynx, pharynx, ear)

Cranial Nerve X

Glossopharyngeal (Taste, posterior part of the Tongue) Cranial IX

Vestibulocochlear related to ear and vestibular

Cranial VIII

Sneezing: when nasal mucosa is irritated. The sneezing center is located in the medial part of medulla (it

also involves the trigeminal nerve).

Vomiting is found in the Medullar Reticular Formation

Neurological Basis of Behavior (PSY - 610)

Reticular Formation: this is a criss cross of nerve fibres , nuclei and cell bodies ( like a net ( reticulum

means: the net). It extends from the spinal cord to the Thalamus. It is a phylogenetically older structure.

It is a well organized are with both efferent and afferent ascending and descending fibres:

Ascending fibres go to thalamus, cortex, and are involved in sleep and awakening. Raphe Nuclei

produces serotonin and extends from the lower pons, andmedulla. Analgesia is produced by the

electrical stimulation of the Periaquaductal greay (PAG), is through the Raphe nuclei. It gets input from

cerebral cortex, the cerebellum and output to cerebellum and the spinal cord. "The reticular activating

system" (RAS) which is important in arousal. Sleep, arousal, muscle tone, alertness was found to be

located here by experiments of Moruzzi and Magoun (1949) on cats. They stimulated the raphe Nuclei

of a sleeping cat which led to an arousal EEG response (low voltage, high frequency theta response), it

woke up, and when the awake cat was stimulated and it became more alert. However, when this area

was lessioned animals went into a comatose stuporous state.

Destruction of the Raphe Nuclei led to Insomniac cats thus it was found that the RAS is important in

sleep and attention.

Metencephalon. Has many tracts going up and down,and contains two important brain areas the

Cerebellum and the POns.

Cerebellum: lies over Pons, it is a phylogenetically older structure, for sensory motor coordination,

movement and balance. It has two lobes like the CC with a large number of lobules separated by

fissures ( vermis, smaller grooves) with a large number of convulations, but these are similar all over

(unlike the cerebral cortex in which there is a difference). The nerve cell layer is 2mm thick below

which lies the white matter under which lies the cerebellar nuclei

Cerebellum receives inputs from the vestibular system, auditory and visual system, reticular formation,

and various regions of the cerebral cortex. It sends out fibers to the reticular formation, thalamus and the

vestibular system

Damage to the cerebellum leads to disturbances of gait in all regions. If one lobe is damaged then there

is postural imbalance. If both cerebellar poles are damaged there is tendency to fall backwards because

of the inability to maintain upright postures and gait disturbance with damage to the posterior region

In Humans, Ataxia, characterized by unsteadiness of gait is seen, this is with anterior lobe syndrome:

with the following symptoms:

Abnormalities in force, rate, and direction of rapid movements a) overshooting while reaching

targets b) jerky not smooth movements. Damage to the cerebellum impairs standing, walking

and performance of coordinated movements

Birds and reptiles have large cerebellums? Why?? To maintain balance during flight and since

sensory motor fibers are coming and going out to various areas, coordination of smooth

movements takes place in this area.

The oculomotor signs of cerebellar damage are: Nystagamus (rythmic and involuntary oscillatory

movements of the eyes)

Cerebellectomy (removal of the cerebellum) lead to inability to maintain gaze, defective smooth pursuit

movements, difficulty of fixation

Neurological Basis of Behavior (PSY - 610)

Cerebellar neurons: Purkinje cells (long dendritic trees), Basket cells (short axons, dendrites cover

Purkinje cells), Stellate Cells (axons terminate on Purkinje cells), granule cells (smallest, go vertical

for communication)

Pons: Bridge between the medulla, midbrain, and the cerebellum. The medulla is caudally located

with refernce to POns and the midbrain is rostrally located. The Dorsal surface of PONS is covered

by the cerebellum. Large rounded pontine nuclei contain ascending and descending fibre bundles.

One such bundle connects the rbain stem and the cerebellum and contains the Pyramidal fibres from

Cerebral cortex to the spinal cord (part of the cortico-spinal tract). The Trigeminal cranial nerve

enters and leaves the brain at the level of POns. Further the Cranial nerves for feeding, facial

expression, respiratory nuclei, relay nerves for auditory systems are also located here.

The cortico-ponto-cerebellar tract is the largest group of fibres which originate form a wide area of

the cerebral cortex 9 has over 19 million fibres on each side). These rises from the primary cortical

areas are involved in rapid correction of movements.

PONTINE NUCLEI, THE GIANT PONTINE CELLS play an important role in rapid eye

movement sleep by inhibiting the movement of body muscles to prevent damage which could occur

due to activation of the body (as the brain is active)

Mesencephalon is a small portion between the hind brain and the diencephalons- the anterior

section of the brain stem, it is tubular in form. It has three main areas: tectum, tegmentum, and basis

pedunculi

Tectum comprises of two pairs of relay nuclei, which look like 4 little lumps on the surface of the

brain stem. These are the Superior Colliculi and the Inferior Colliculi

Superior Colliculi: laminated grey and white matter important in visual reflexes and eye movement,

well defined and organized in terms of receptive fields and maps of visual space. It is part of the

pathway coming from the optic tract to the visual cortex for eye movement and gross spatial

localization. This is important for vertical gaze and pupillary reflexes. It receives inputs from retina

of eyes, thalamus, and inferior colliculus and sends outputs to thalamus and frontal and visual

cortices.

Inferior colliculi: it is an oval mass of small and medium sized neurons and is major relay nuclei for

the auditory pathway. Fibres come in from the Thalamus (Medial Geniculate Nuclei), auditory

cortex and cerebellar cortex, and fibres from Inferior colliculi project to the MGN, superior

Colliculi and the cerebellum... turn to look at sound reflex and localizing the source of sound.

Therefore both the inferior and superior colliculi work together as part of the reflex system which

Neurological Basis of Behavior (PSY - 610)

take care of attending and turning to wards the direction of a sound ( This is why we see extensive

connection with the thalamus, the cerebellum and the relevant cortices)

We would be continuing discussion on location of various brain areas and their connections and

functions in the next class.

References:

Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

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

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

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

3rd edition) Worth Publishers New York

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

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

Academic Press New York

Note: References 2, 3, 4, 7 more closely followed in addition to the references cited in text.

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