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What are basal ganglia? 

Basal ganglia is a huge subcortical mass of the grey matter in the cerebral hemisphere of the brain, derived from the telencephalon.

The basal ganglia include the caudate nucleus, lentiform nucleus (putamen and globus pallidus), amygdaloid nuclear complex, and claustrum.

basal ganglia
Source of figure: External source

According to Physiology, Basal ganglia is a group of nuclei in the forebrain and upper part of the brainstem.

Basal ganglia include neostriatum (caudate nucleus and putamen), lenticular nucleus (putamen and globus pallidus), subthalamic nucleus, and substantia nigra.

Neuronal circuit

  • Direct pathway: excitatory pathway
  • Begins in the premotor & supplementary areas, primary motor cortex and somatosensory cortex- pass to the striatum- GPIS –thalamus- motor cortex
  • Cortex to striatum-glutaminergic pathway
  • Striatum to GPIS and GPIS to thalamus-GABAnergic pathway
  • Thalamus to motor cortex-Glutaminergic pathway

Indirect pathway: inhibitory

  • Motor cortex-striatum(Glutaminergic)
  • Striatum-GPES (GABAnergic)
  • GPES-subthalamus (GABAnergic)
  • Subthalamus-GPIS (Glutaminergic)
  • GPIS-Thalamus (GABAnergic)
  • Thalamus-cortex (Glutaminergic)

Dopaminergic pathway

  • Influence both direct and indirect pathway by nigrostriatal pathway
  • Dopamine stimulates direct pathway by D1 receptor
  • Dopamine inhibits the indirect pathway by the D2 receptor, this inhibition leads to excitation of the indirect pathway too.

Corpus striatum

It lies anterior and lateral to the thalamus and incompletely divided into two parts by the anterior limb of the internal capsule. It is a caudate nucleus and lentiform nucleus.

Caudate nucleus

  • It is formed on the floor of the anterior horn of the lateral ventricle of the brain
  • It is formed on the floor of the anterior horn of the lateral ventricle of the brain
  • It is formed on the floor of the body  of the lateral  ventricle of the brain
  • It is formed on the roof of the inferior  horn  of the lateral  ventricle of the brain
  • It is covered  by the ependymal of the ventricle
  • Forms  an  arched  mass  of  grey  mater
  • It is divided into three parts, they are head, body, and tail.

HEAD

  • Large rounded projection lying anteriorly
  • The head of the caudate nucleus is formed on the floor of the anterior horn of the lateral ventricle of the brain

BODY

  • Narrow portion succeeding the head
  • The floor of the body of the lateral ventricle
  • Ends in the tail part.

TAIL

  • Runs forwards
  • The tail of the caudate nucleus is formed on the roof of the inferior horn of the lateral ventricle.
  • Stria terminalis  on  its  medial  side.
  • Anteriorly continuous with the amygdaloid nucleus.

Lentiform nucleus

It is lies between the insula,  caudate nucleus, and thalamus. It is laterally covered by an external capsule (white mater). The claustrum lateral to the external capsule. It is a biconvex lens shape. On section divided into two parts by the external medullary lamina: putamen and globus pallidus.

Claustrum

It is a fine mass of grey matter which is saucer-shaped and located between the putamen and insula of the brain. Its significance is unknown.

Amygdaloid body (amygdala)

It is an almond-shaped mass of grey matter in the temporal lobe of the brain and situated anterosuperior to the tip of the inferior horn of the lateral ventricle. It is situated deep to uncus, which serves as a surface landmark for its location. The fibers arising from the amygdaloid body form stria terminalis, which follow the inner curve of the caudate nucleus and terminate into a septal area, anterior perforated substance, and anterior hypothalamic nuclei. The stria terminalis is the main efferent tract of the amygdaloid body.

Connections of the corpus striatum of the basal ganglia

  • Regarded as an important integrating center playing an important role in the motor activities.
  • Striatum (caudate nucleus and putamen)—mainly afferent fibers
  • Globus pallidus-Efferent fibers.
  • Afferents from
    • Cerebral cortex
      • Corticostriatal fibers from almost all parts of the cerebral cortex;  mainly from frontal and parietal lobes.
    • Thalamus
      • Thalamostriate fibers from intralaminar and medial thalamic nuclei.
    • Substantia nigra
      • Nigrostriatal fibers
  • Efferent to:
    • Globus pallidus
      • Striopallidal fibers,  main  efferent fibers
    • Thalamus
      • Striothalamic
    • Substantia nigra
      • Strionigral fibers

Connection of the Globus pallidus of the basal ganglia

  • Afferents from:
    • Striatum
      • Striopallidal fibers
    • Substantia nigra
      • Nigropallidal fibers
    • Thalamus
      • Thalamopallidal fibers
    • Subthalamus
      • Through subthalamic fasciculus
  • Efferents to:
    • Thalamus
      • Through lenticular fasciculus
      • Ventralis anterior and ventralis lateral nuclei.
    • Subthalamus
      • Subthalamic fasciculus
    • Substantia nigra
      • Pallido-nigral   fibers  through  the ansa lenticularis
    • Red nucleus
      • Pallido-rubral  fibers  through  the ansa  lenticularis
    • Midbrain reticular formation
      • Pallido-tegmental (pallido-reticular) fibers  through  ansa  lenticularis
    • Inferior olivary nucleus
      • Pallido-olivary  fibers  through  ansa lenticularis

Functions of the basal ganglia

  • Planning and Initiation of movements.
  • Controlling complex patterns of motor activities- a function of putamen circuit. Executes skilled motor activities like cutting paper with a scissor, hammering on a nail, shooting a basketball.
  • Cognitive control of a sequence of motor patterns functions of the caudate circuit.
  • Timing and scaling of the movement.
  • Regulation of muscle tone
  • Initiation of movements generated by internal cues such as scratching, moving the hands over the face, facial expression, etc.
  • It suppresses unwanted actions which might interfere with the desired movement whilst amplifying and facilitating those processes which contribute towards the desired movement.

Applied anatomy of basal ganglia

  • Parkinson’s disease
  • Huntington chorea
  • Hemiballism( hemiballismus)
  • Athetosis

Parkinson’s disease

  • Resting tremor
    • Rigidity affects all somatic musculature predilection to the flexure-stooped posture
    • Lead pipe rigidity or cogwheel type of muscular rigidity
    • Bradykinesia
    • Pin rolling movements of fingers.
    • Mask-like face or loss of facial expression
  • Onset > 60 years
  • Causes due to degeneration of substantia nigra
  • The concentration of dopamine in the nigrostriatal system is reduced.
  • Drugs that block D2 dopamine receptors
  • MPTP (methyl-phenyl-tetrahydropyridine) induced Parkinsonism.
  • Treatment
    • Drug Therapy
      • L-DOPA
    • Surgical:
      • Pallidotomy
      • Thalamotomy.
    • Tissue transplants: adrenal medulla, carotid body, fetal striatal tissues.

Huntington’s Disease

It is a hereditary disorder and most commonly occurs in the age between 30 to 40 years. It occurs due to damage to GABAergic neurons in the caudate nucleus and putamen. Damage to this inhibitory pathway results in hyperkinetic features which include: Hyperkinetic choreiform movements, slurred speech, and progressive loss of memory.

Athetosis

It is primarily due to a lesion of globus pallidus. It shows spontaneous and continuous writhing movements of a hand, arm, neck, or face.

Hemiballism

It is because of damage to the subthalamic core of the nucleus. It is characterized by sudden flailing movements affecting the whole of the opposite side of the body.

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