Diencephalon and Basal Ganglia

Question 1

What is the Telencephalon (cerebrum) responsible for?

Answer 1

-Conscious thought processes, intellectual functions
-memory storage and processing
-conscious and subconscious regulation of skeletal muscle contractions

Question 2

What is the Metencephalon (cerebellum) respsonsible for?

Answer 2

• Coordinates complex somatic motor patterns
• Adjusts output of other somatic motor centres in brain and spinal cord

Question 3

What is the medulla oblongata (myelencephalon) responsible for?

Answer 3

• Relays sensory information to thalamus
  -Autonomic centres for regulation of visceral functions such as cardiovascular, respirator, and digestive activities

Question 4

What is the Metencephalon (pons) responsible for?

Answer 4

-Relays sensory information to cerebellum and thalamus
-Subconscious somatic and visceral motor centres

Question 5

What is the Mesencephalon (midbrain) responsible for?

Answer 5

• Processing of visual and auditory data
  -Generation of reflexive somatic motor responses
• Maintenance of consciousness

Question 6

In the Diencephalon, what is the role of the Thalamus?

Answer 6

Relay and processing centres for sensory information

Question 7

In the Diencephalon, what is the role of the Hypothalamus?

Answer 7

Centres controlling emotions, autonomic functions and hormone production

Question 8

What is the Diencephalon?

Answer 8

Part of cerebrum
Largest part of the brain
Third ventricle lies between halves of diencephalon

Includes:
Thalamus and its geniculate bodies
Hypothalamus
Subthalamus
Epithalamus

Generally, it is the main processing centre for information destined to reach cerebral cortex from all ascending sensory pathways

Question 9

What is the largest division of the Diencephalon?

Answer 9

Thalamus

Question 10

What is the Thalamus?

Answer 10

Each half of brain contains a thalamus
Largest division of the diencephalon

Large ovoid mass of nuclei involved in motor, sensory and limbic functions

Thalamic output neurons  cerebral cortex

Very little information reaches cerebral cortex without first being processed by thalamic neurons
Thalamus regarded as functional “gatekeeper” to cerebral cortex

Nearly all regions of cerebral cortex have reciprocal projections  thalamic region from which they originally received input

Thalamic radiations are fibre bundles (white matter) emerging from lateral surface of thalamus  cerebral cortex

Question 11

What are the 3 parts that the Thalamus is subdivided into?

Answer 11

Gray matter of thalamus is divided by a vertical sheet of white matter, internal medullary lamina, into medial and lateral halves
Anterosuperiorly, internal medullary lamina splits to become Y-shaped

Anterior part, between limbs of Y

Medial part, on medial side of stem of Y

Lateral part, on lateral side of stem of Y

Each part contains a group of thalamic nuclei

Question 12

What is the anterior part of the Thalamus?

Answer 12

Contains anterior thalamic nuclei
Receive mammillothalamic tract from mammillary nuclei
Receive connections from cingulate gyrus and hypothalamus

Function is associated with those of the limbic system and include emotional tone (Papez circuit of emotion) and mechanisms of recent memory and Spatial Memory

Question 13

What is the medial part of the Thalamus?

Answer 13

Contains the large medial dorsal nucleus

Two way connections with whole prefrontal cortex of frontal lobe
Connections with hypothalamic nuclei
Interconnected with other thalamic nuclei

Responsible for the integration of sensory information including somatic, visceral and olfactory information, and the relation of this information to ones emotional feelings and subjective states

Question 14

What parts are the lateral part of the Thalamus divided into?

Answer 14

Subdivided into dorsal & ventral tiers
Includes lateral & medial geniculate nuclei

Question 15

What is the dorsal tier of the lateral part of Thalamus?

Answer 15

Contains lateral dorsal, lateral posterior and pulvinar nuclei

Lateral dorsal and lateral posterior nuclei have connections with cingulate gyrus and parietal lobes respectively
Pulvinar receives input from tectum - superior colliculus  visual association cortex
Other parts of pulvinar  temporal, parietal and frontal lobes that are especially concerned with visual function and eye movements and integration of visual and auditory input

Lateral and Medial Geniculate Nuclei (Bodies)
Considered with lateral part of thalamus

Medial Geniculate Nucleus
Receives auditory input via inferior colliculus  primary auditory cortex in temporal lobe

Lateral Geniculate Nucleus
Receives visual input from retina via optic tract  primary visual cortex on medial surface of occipital lobe

Question 16

What is the ventral tier of the lateral part of Thalamus?

Answer 16

Contains ventral anterior nucleus (VA) and ventral lateral nucleus (VL), which are important motor related nuclei

Ventral anterior nucleus (VA) receives input from globus pallidus
Ventral lateral nucleus (VL) receives input from reticular portion of substantia nigra

Also contains ventral posterior nucleus which is subdivided into ventral posterolateral (VPL) and ventral posteromedial (VPM) nuclei, which conveys somatosensory information to the cerebral cortex

Question 17

What are the inputs of the VPL - Ventral posterolateral nuclei?

Answer 17

Medial Lemniscus
Spinalthalamic tract

Question 18

What is the output of the VPL - Ventral posterolateral nuclei?

Answer 18

Somatosensory cortex

Question 19

What are the functions of the VPL - Ventral posterolateral nuclei?

Answer 19

Relay somatosensory spinal inputs to cortex

Question 20

What are the inputs of VPM - Ventral posteromedial nuclei?

Answer 20

Trigeminal Lemniscus
Trigeminothalamic tract
Taste

Question 21

What are the outputs of VPM - Ventral posteromedial nuclei?

Answer 21

Somatosensory cortex

Question 22

What are the functions of VPM - Ventral posteromedial nuclei?

Answer 22

Relay somatosensory cranial nerve and taste inputs to cortex

Question 23

What are the functions of the the hypothalamus?

Answer 23

Mainly involved in visceromotor, viscerosensory and endocrine activities (unlike thalamus, which is primarily related to somatic functions) as well as autonomic function
Serves as a link between nervous and endocrine systems

Receives many afferent fibres from viscera, olfactory mucous membrane, cerebral cortex and limbic system

Efferents include descending fibres to brainstem and spinal cord, mammillary bodies and limbic system

Connected to pituitary gland

Autonomic Function
Sympathetic and parasympathetic activating regions

Body Temperature
Evokes autonomic responses that results in loss (sweating and cutaneous vasodilation), conservation (vasoconstriction) or production of body heat (shivering)
Higher temperature, or fever, is result of a change in set point/thermostat of hypothalamus by e.g. pyrogens in blood

Water Balance
Hypothalamic influence on vasopressin secretion by posterior pituitary is activated by osmoreceptors that are stimulated by any increase in blood osmolarity
Pain, stress and emotional states also stimulate vasopressin secretion
Lack of secretion caused by hypothalamic or pituitary lesions can result in polyuria and polydipsia

Anterior Pituitary Function
Direct influence on secretions of anterior pituitary and an indirect influence on secretions of other endocrine glands by releasing or inhibiting hormones carried by pituitary portal vessels
Regulates many endocrine functions, including reproduction, sexual behaviour, thyroid and adrenal cortex secretions and growth

Circadian Rhythm
Many body functions including temperature, corticosteroid levels, oxygen consumption are cyclically influenced by light intensity changes that have a day to day rhythm
Suprachiasmatic/preoptic nucleus in preoptic area functions as an independent clock with a period of about 25 hours per cycle
Lesions in this nucleus results in the loss of all circadian cycles

Expressions of Emotion
Involved in expression of rage, fear, aversion, sexual behaviour & pleasure
Patterns of expression and behaviour are subject to the limbic system

Question 24

What is the location and relations of the Hypothalamus?

Answer 24

Lies below the thalamus
Extends from optic chiasma to caudal border of mammillary bodies
Forms floor and inferior part of lateral walls of third ventricle
Caudally, it merges into tegmentum of midbrain
Lateral boundary is formed by internal capsule

Question 25

What is the subthalamus?

Answer 25

Between dorsal thalamus and tegmentum of midbrain
Medial to internal capsule

Stimulates globus pallidus which has an inhibitory effect on the thalamus

Unilateral destruction or disruption of the subthalamic nucleus — which can commonly occur via a small vessel stroke in patients with diabetes, hypertension, or a history of smoking – produceshemiballismus.

Question 26

What is the basal nuclei?

Answer 26

Gray matter nuclei deep within the white matter of cerebral hemispheres
Corpus Striatum: caudate nucleus, putamen & globus pallidus
(striped appearance)

Striatum: caudate nucleus & putamen
(similarity in histology, connections & functions)

Globus Pallidus
(or pallidum)

Lentiform Nucleus: putamen & globus pallidus
(lens shaped)

Caudate Nucleus
Pear-shaped head is continuous with putamen, adjacent to inferior border of anterior horn of lateral ventricle
Curves backward and downward as the tail

Amygdala: Found at end of tail (part of limbic system)

Question 27

What is the direct motor loop of the basal nuclei?

Answer 27

Facilitates flow of information through the thalamus

Corticostriate neurones excite neurones in caudate and putamen
Inhibitory neurons project from striatum to globus pallidus (internal)
Projections from globus pallidus to thalamus (VA and VL) are inhibitory
Thalamic neurons excite motor cortex
Fibers from caudate nucleus also travel with the oculomotor nerve to the frontal eye fields
Activity to motor cortex is suppressed by striatum and globus pallidus
The amount of this suppression determines how much motor activity (movement) occurs

Question 28

What is the motor loop (indirect pathway ) of basal nuclei?

Answer 28

Inhibits flow of information through the thalamus

Cortex > striatum > globus pallidus (external zone) > subthalamic nucleus > globus pallidus (internal zone) > subthalamic nucleus > globus pallidus (internal zone) >thalamus > motor cortex

Subthalamic nucleus is excitatory to globus pallidus & thus suppresses movement

Question 29

What is the direct pathway of movement?

Answer 29

The direct pathway is involved in facilitation of wanted movements. The projections of the caudate, putamen, internal segment of the globus pallidus, and the SNc create an excitatory effect on the thalamus by decreasing the GABA released by the striatum. This inhibitory effect decreases inhibitory projections from the thalamus, creating an overall net excitatory effect.

Question 30

What is the indirect pathway of movement?

Answer 30

The indirect pathway is involved in preventing unwanted movement. The projections of the caudate, putamen, external segment of the globus pallidus, and substantia nigra pars reticulata create an inhibitory effect on the thalamus through the release of GABA. This inhibitory effect decreases the excitatory projections from the thalamus, creating an overall net inhibitory effect on movement.

Question 31

What is the blood supply of the corpus striatum and the internal capsule?

Answer 31

Clinically important as it is involved in cerebrovascular accidents (e.g. cerebral thrombosis, embolism or haemorrhage)

As cortical projection fibres are close together in internal capsule even localised lesions can have devastating clinical consequences

Vessels are small striate arteries, branches from middle cerebral artery (often anterior cerebral/internal carotid)

In addition, some of the blood supply to posterior part of internal capsule is from anterior choroidal artery (from internal carotid)

Question 32

What are clinical correlations of basal ganglia?

Answer 32

Parkinson’s Disease (Substantia Nigra)
Idiopathic type MC (unknown cause)

80% minimum loss of dopaminergic neurones in nigrostriatal pathway

Dopaminergic neurones are inhibitory and their loss  increased activity of inhibitory neurones in globus pallidus

Net result is an increased inhibition of motor function, leading to the characteristic symptoms of Parkinson’s disease

This disease is complex and as yet not fully understood and this mechanism does not account for all the symptoms of Parkinson’s

Question 33

What are the signs and symptoms of Parkinson’s disease?

Answer 33

“TRAP”
Tremor (at rest)
Involuntary rest tremor in distal parts of limbs, head or entire body (pairs of antagonistic muscles contracting alternately)
“Pill Roll” tremor, as the sufferer appears to be rolling an imaginary pill-sized object between their thumb and fingers

Rigidity (cogwheel)
Muscular stiffness and increased muscle tone
Akinesia/Bradykinesia
Difficulty in initiating and carrying out certain movements
Expressionless or ‘mask-like’ appearance of face
Slowness of movement and slurred speech
Decrease in: eye blinking, eating and chewing, walking speed

Postural Instability
Results in a “shuffling” or festinating gait

Question 34

What are the clinical correlations of Ballismus, Choreiform and Athetoid Movements?

Answer 34

Damage to subthalamic nucleus > unexpected ballistic movements on contralateral side
Hemiballismus (damage to one side) > uncontrolled flinging of an upper or lower extremity (e.g. punching/kicking)

Degeneration of neurones in the striatum (Huntington disease), particularly those associated with the indirect pathway, cause jerky dance-like movements of the limbs and facial musculature

Tourette’s syndrome-uncontrolled movements and vocalization
Wilson’s Disease, poor copper excretion leads to progressive degeneration of basal ganglia with rigidity, dystonia, tremor and choreathetotic movements. May have dark ring around iris-Keyser-Fleisher rings

Disruption of indirect pathway > athetoid movements, which are slow sinuous writhing movements of limbs & face