• Basal Ganglia •

• Basal Ganglia • The term basal ganglia(or basal nuclei) is applied to five interactive structures on each side of the brain. They are telencephalic in origin and form the extrapyramidal system • BG is an accessory motor system that functions in close association with the cerebral cortex. It receives most input signals from the cerebral cortex and also returns output signals to the cortex • BG plays an important role in coordination of skilled movts, regulation of automatic associated movts and control of muscle tone by sending output signals to motor cortex, reticular formation and spinal cord. • They are masses of gray matter submerged in subcortical substance of cerebral hemisphere • they are the caudate nucleus, putamen, globus pallidus, sub-thalamic nucleus of Luys and substantia nigra • the caudate nucleus and the putamen collectively form the striatum, the putamen and the globus pallidus collectively form the lenticular nucleus

Primary motor Areas

Basal Ganglia Interconnection

• The Corpus Striatum • The corpus striatum is situated at the base of the cerebral hemispheres close to the thalamus. It is divided into two functionally distinct parts by the internal capsule. These are the caudate nucleus and the lentiform nucleus • The caudate nucleus and the putamen (which are phylogenetically similar) form one unit called the striatum, while the globus pallidus forms another the pallidum • The Caudate Nucleus is an elongated arched gray mass that lies medial to the internal capsule. It has a head and a tail. It extends along the dorsolateral surface of the thalamus and ends in amygdaloid nucleus • The substantia nigra has a pars compacta region which uses dopamine as neurotransmitter and a pars reticulate region which uses GABA as a neurotransmitter.

ed • The Lenticular nucleus is wedged shape situated lateral to internal capsule. It is divided into the outer putamen and the inner globus pallidus by the external medullary lamina. • Substantia Nigra: • Situated below red nucleus. It consists of a large pigmented(rich in iron) and small non-pigmented cells. • Subthalamic Nucleus : • Situated lateral to red nucleus and dorsal substantia nigra.

Components

Afferent

Efferent

Corpus Straitum

Thalamic nuclei to caudate and putamen Cerebral cortex to caudate and putamen Sub Nigra to putamen Subthalamic to globus pallidus

Thalamic nuclei Subthalamic N Red N Sub Nigra Hypothalamus Reticular Formation

Subtantia Nigra

Putamen Frontal lobe Sup Colliculus Mamillary body of Hypothalamus Medial and lateral leminisci Red nucleus

Putamen

Sub-thalamic N of Luys

Globus pallidus

Globus pallidus Red N

eas

• Motor Functions of the BG • One of the principal roles of the BG in motor control is to function in association with the corticospinal system to control complex patterns of motor activity. For example writing. Damage to the BG can result in crude writing as though one is learning to write for the first time. • other patterns are cutting paper with scissors; hammering nails; shooting a basketball through a loop; passing a football, throwing a baseball; most types of vocalization; controlled eye movements; most skilled movements performed subconsciously. • There are two major neural circuits of BG • Putamen and Caudate circuits • Neuronal pathways of the Putamen circuit • They begin mainly in the premotor and supplementary areas of the motor cortex and in the somatosensory areas of the sensory cortex

BG connections

• Then they pass to the putamen (bypassing the caudate) then to the internal portion of the global pallidus, next to the ventroanterior and ventrolateral relay nuclei of the thalamus and finally return to the cerebral primary motor cortex and to portions of the premotor and supplementary cerebral areas closely associated with the primary cortex. • In close association with this primary putamen circuit are ancillary circuits that pass from the putamen thru the external globus pallidus, the subthalamus and the substantia nigra-finally returning to the motor cortex via the thalamus. • THE CAUDATE CIRCUIT • The neural connections between the caudate nucleus and the corticospinal motor control system are somewhat different from those of the putamen circuit. • This is partly so because the caudate nucleus extends into all lobes of the cerebrum • Furthermore the caudate nucleus receives large amounts of its inputs from the association areas of the cerebral cortex

• The caudate nucleus plays a key role in the cognitive control of motor activities e/g a person seeing a lion and responds instantenously and automatically by 1.turning away 2.beginning to run 3.attempting to climb over a tree. The cognitive control of motor activity determines these sub-conscious patterns of movt necessary for achieving a complex goal • EFFECT OF BG IN SCALING AND TIMING OF MOVT INTENSITY • The brain has the capability of 1.determining how rapid the movt is to be performed 2. controlling how large the movt will be For instance writing an alphabet “slowly” or “rapidly”; “small” or “large”

• General Functions of BG • Control of muscle tone: they exert inhibitory effects on muscle tone by inhibiting the gamma motor neurons. As such lesion of the BG increases muscle tone significantly • Control of Reflex motor activity: reflex muscular activities esp those for maintenance of posture (visual and labyrinthine reflexes). Lesions can result in abnormal reflexes • Control of Motor activity: BG is able to regulate voluntary, conscious (cognitive earlier described) and sub-conscious movts. Though voluntary movts are initiated by cerebral cortex the BG controls it. That is why movts are awkward and inaccurate during BG lesions due to loss of this control. The sub-conscious occur during trained motor activities e.g sewing, paper cutting, nail hammering etc

• Control of Automatic Associated Movts: these movts occur with other motor activities. For instance, swinging of the arms while walking, certain facial expressions while talking or working. Lesions can cause absence of automatic associated movts- statue-like body • Role in wakefulness: because of its connection with the reticular formation the BG (globus pallidus in particular) is involved in wakefulness. Therefore extensive lesion of the G.Pallidus can cause drowsiness • BG NEUROTRANSMITTERS Different neuronal pathways within the BG secrete different types of NTs 1. Dopamine:released by dopaminergic fibres from substantia nigra to corpus striatum aka doparminergic nigro-striatal fibres

2. GABA: secreted by intrinsic fibres of c.striatum and s.nigra 3. Acetylcholine:released by pathway running from the cortex to the caudate nucleus and putamen 4.Substance P: released by fibres from g.pallidus to s.nigra 5. Noradrenaline: secreted by fibres between BG and reticular formation 6. Glutamic acid: secreted by fibres from sub-thalamic nucleus to globus pallidus and s.nigra 7. Enkephalin, Norepinephrine ans serotonin: secreted by multiple general pathways from the brainstem 8. Glutamate: secreted by multiple glutamate pathways providing most excitatory signals. This balances out the large numbers of inhibitory transmitters like dopamine, GABA and serotonin.

APPLIED PHYSIOLOGY • Agnosia:inability to accurately perceive objects due to lesion of the posterior parietal cortex. This is so because the caudate circuit of the BG functions mainly with the association areas of the cerebral cortex such as the posterior parietal cortex. Also, in such lesions there is personal neglect syndrome a situation where such individual is almost oblivious of the existence of one side of his/her body. Lesion of the right cortex will affect the left side of the body. • Parkinson’s Disease: aka paralysis agitans. A slowly progressive degenerative dx of the nervous system associated with widespread destruction of that portion of the substantia nigra (pars compacta) that sends dopamine-secreting fibres to the caudate nucleus and putamen.

• Cause remains mainly unknown • The disease is characterized by : 1.Rigidity of muscles 2. Involuntary tremor at rest 3-6 cycles per seconds 3. Slowness of movt (bradykinesia); ultimately serious difficulty in initiating movt (akinesia) 4. Postural instability with impaired reflexes leading to poor balance and falls 5. Loss of normal gait i.e festinant gait. Quick short steps, bending forward as if trying to catch up the centre of gravity 6. Speech problems esp slurred, soft speech 7. Dyspahgia 8. Fatigue 9. Dementia may develop in later stages. Underlying Mechanism of the Disease : Dopamine secreted by the dopaminergic fibres is an inhibitory NT. The destruction of this pathway from any cause would result in the caudate and putamen to be overtly active and possibly cause continuous output of excitatory signals to the corticospinal motor control system. These could overexcite muscles of the body leading to rigidity. Oscillation from high feedbacks lead to tremors.

• Treatment of Parkinson’s Dx: • Use of L-Dopa: This drug is able to cross the BBB to relieve symptoms of the dx. Bcos of its susceptibility to hepatic conversion to dopamine it is co-administered with carbi dopa. • L-deprenyl:Inhibits the action of MAO on secreted dopamine in the brain • Fetal dopamine cells transplant • Surgical destruction of the feedback circuitry in BG: surgical lesions in the Ventrolat and ventroant nuclei of the thalamus can block feedback from the basal ganglia to cortex has been characterized by surprising improvement in symptoms.

• Wilson’s Disease: an inherited dx characterized by excessive copper deposition in tissues. This can damage the putamen. Symptoms of Parkinson’s dx are also seen in addition to liver failure • Chorea: abnormal involuntary jerky movts due to lesion in the caudate and putamen • Athetosis: Slow rhythmic twisting movt. Caudat and putamen lesions • Choreoathetosis: Aimless involuntary movts. Combination of above two.Lesion of caudate and putamen. • Huntington chorea: inherited progressive neural disorder due to degeneration of GABAergic neurons in corpus striatum and S.nigra. Hyponia, chorea and dementia are common. • Hemiballismus: Violent involuntary abnormal movts on one side of the body (esp the arm while walking). Aka flinging movt. Occurs due to degeneration of the sub-thalamic nucleus of Luys.

• Kernicterus: Staining of the BG with bile during episodes of severe jaundice in neonates. High pitched crying, poor motor development, rigidity etc.\