Basal ganglia- telencephalon

• Basal ganglia

  • Group of interconnected subcortical nuclei located deep within brain
• Basal ganglia components

  1. Anatomical: Caudate nucleus; lentiform nucleus; amygdaloid body; claustrium

  
• • caudate + putamen + globus pallidus= corpus striatum
  • Caudate + putamen= neostriatum
  • Globus pallidus= paleostriatum

  
• Anatomical division of basal ganglia

  1. caudate nucleus
  2. Lentiform nucleus
  3. Amygdaloid body
  4. Clastrium
• Functional/clinical division of basal ganglia

  1. Caudate nuclei
  2. Lentiform nucleus
  3. Sub-thalamic nuclei
  4. Substancia nigra

  
• Functions of basal ganglia

  • Select + initiate appropriate motor function while inhibit unwanted movements
  • Planning + programming movement
  • Learning + refining motor skills
  • Regulate muscle tone by modulating activity of descending pathways
  • Cognitive functions: learning; goal-directed behaviour
• Caudate nuclei

  • C-shaped nuclei that lies adjancet to lateral ventricle. Located superior to thalamus and extends from frontal to accipital lobes
  • Head: anterior part, located near frontal lobe. Bulges into anterior horn of lateral ventricle and forms great part of its horn
  • Body: in floor of central part of lateral ventricle
  • Tail: curves posteriorly and lies in inf horn of lat ventricle. Ends by joining with amygdaloid body

  
• Lentiform nucleus

  • Biconcave nuclei on section
  • Forms lateral boundary of internal capsule and medial boundary of insula

  It has 2 parts

  1. Putamen: larger, separated from globus pallidus by external medullary lamina
  2. Globus pallidus: futher subdivided into internal and external segments by internal medullary lamina.

  
• Coronal section


  Basal ganglia

  
• • Slightly lateral to each thalamus is caudate nucleus, with amygdala located at the tail and lentiform nuclei located lateral and posterior to its head.
  • Lentiform nucleus: pitamen, globus pallidus internis and externus.
  • Lentiform nucleus is surronded bybwhite matter capsule medially and laterally.
  • Medial part of white matter capsule: inner capsule; separates it from thalamus
  • Lateral part of white matter: outer capsule: separates it from claustrium

  
• Three primary functions of the basal ganglia:

  o Start movement

  o Stop movement

  o Modulate movement
• Motor function of basal ganglia in context

  • Broadly speaking basal ganglia contains circuits where motor programs are stored
  • It receives info about final intented moviment of body from cortex→ with this its circuits selects appropriate UMN that need to be activated/inhibited to produce meaningful mov→relay this info back to cortex
  • Cerebellum also aids in selection of UMN by sending inputs about current proprioception of body back to cortex→ inputs of basal ganglia+cerebellum are integrated in cortex→ Cortex send info to UMN→ execution of appropriated movement
• • Basal ganglia modulates motor functions through series of interconnected pathways with other parts of brain.
  • Main pathways: direct, indirect and hyper direct pathways.
• Caudate + putamen (striatum) are considered main input region in basal ganglia.

  1. Cortical input: motor, sensory, associative, and limbic info necessary for motor control.
  2. Thalamic input: specifically from intralaminar nuclei, which provides additional sensory and motor information.
  3. Dopaminergic Input:
  4. Dopaminergic neurons from the substantia nigra pars compacta. Dopamine modulates the activity of the striatal neurons, influencing both the direct and indirect pathways. Dopamine has an excitatory effect on D1 receptors (direct pathway) and an inhibitory effect on D2 receptors (indirect pathway).

  
• Caudate + putamen (striatum) are considered main input region in basal ganglia.

  1. Cortical input: motor, sensory, associative, and limbic info necessary for motor control.
  2. Thalamic input: specifically from intralaminar nuclei, which provides additional sensory and motor information.
  3. Dopaminergic input: Dopaminergic neurons from the substantia nigra pars compacta. Dopamine modulates the activity of the striatal neurons. Dopamine has an excitatory effect on D1 receptors (direct pathway) and an inhibitory effect on D2 receptors (indirect pathway).

  
• • Globus pallidus and substancia nigra pars reticulata are considered main output nuclei of basal ganglia.

  GPi send inhibitory projections to ventral anterior and ventral lateral nuclei of thalamus which then projects to motor areas of cerebral cortex.

  Substancia nigra pars reticulata send inhibitory signals to thalamus, and superior colliculus, involved in control of eye and movements.

  
• Basal ganglia perform its function by means of 3 pathways:

  1. Direct
  2. Indirect
  3. Nigrostriatal
• Direct pathways of basal ganglia

  • Disigned to initiate motor movement→enhancing desire motor action

  Pathway:

  1. when we think of an action→ motor + SMA are activated→ corticostriated neurons sends excitatory signals (glutamate) to putamen→ which sends signals (GABA) to GPi → this decrease tonic inhibition of GPi to thalamus→ thalamus become more active→ sends excitatory signals to motor areas→ ↑ motor activity os skeletal muscle facilitationg initiation + execution of voluntary movements.

  
• Indirect pathway of basal ganglia

  Designed to inhibit undesired/unwanted motor movement

  1. Motor cortex sends excitatory signals (glutamate) to putamen→ which sends inhibitory signals (GABA) to GPe→ whcih decreases activity of GPe→ decreasing inhibition of GPe to subthalamic nucleus→ subthalamic neurons become more active→ sends excitatory signals (glutamate) to GPi + substancia nigra pars reticulata→ increase in activty→ increase inhibition to thalamus→ decrease activity of thalamus→ decrease excitatory input to cortex→ suppression of voluntary movement.

  
• Indirect pathways serves as a stop pathway inhibiting unwanted/involuntary or excessive motor movements by suppressing activity of cerebral cortex, thereby ensuring smooth and controlled motor activit. Nigrostriatal pathway acts on it modulating its activity preventing excessive inhibition of motor cortex balancing motor movements.

  • When we are doing something for exemple writing, some of arm muscle are flexed through direct pathways some extensors are relaxed through indirect pathway so that movement is precise and coordinate.
• Nigro-striatal pathway

  • Major dopaminergic pathway in brain connecting substancia nigra pars compacta to striatum.
  • Important in modulation of motor control. SNc produces dopraning and releases them into striatum. Dopamine modulates activity of neurons in striatum influencing both direct and indirect pathways of basal ganglia.
• Role of dopamine in direct pathway

  SNc releases dopamine into striatum which binds into D1 receptors→ D1 receptors has excitatory effect, enhancing activity of direct pathway.

  • dopamine→ increase stimulation of striatum→ increase GABa release to GPi→ decrease its action on thalamus→ decrease GABA release into thalamus→ increase activity of thalamus→ increase stimulation to cerebral cortex→ increase motor ac of wanted motor movements.
• Role of dopamine in indirect pathway

  Dopamine binds to D2 receptors on striatum neurons (part of indirect pathway)→ activation of d2 receptors has inhibitory effect reducing activity of indirect pathway.

  • Dopamin→ d2 receptors→ inhibition of striatum→ decrease stimulation to GPe→ decrease inhibition to subthalamic nucleus→ decrease glutamate release into GPi→ decrease stimulation to GPi→ decrease stimulation of GPi into thalamus→ decrease inhibition to thalamus→ thalamus is stimulated→ stimulation of motor cortex→ increase activity of unwanted motor movement.
• • Dopamine promotes activity of direct pathway through D1 receptors, enhancing initiation and smooth execution of voluntary movements
  • Dopamine inhibits activity of indirect pathway through D2 receptors, reducing excessive movement inhibition and preventing suppression of desired movements
• Balance between direct and indirect pathways allows for fine-tuning of motor activity

  • Direct pathway facilitates activation of motor programs necessary for intended movement.
  • Indirect pathway suppresses potentially competing motor programs that might interfere with intended movement.
  • Nigrostriatal pathway modulates both og them facilitating movement and preventing excessive inhibition
• Parkinson’s Disease


  ● Neurodegenerative disease that primarily affects the basal ganglia.

  ● Movements are slow or fail to be initiated and patients have significant resting tremor

  Damage in the direct pathway and the nigrostriatal pathway → difficulty initiating or maintaining movement

  ● Dopaminergic neurons in the pars compacta degenerate in Parkinson’s disease and striatal dopamine is profoundly reduced
• Lesion to the indirect pathway → ↑unwanted motor

  movements.

  • Huntingon disease:Caused by a mutation to the Huntington gene is observed → expansion of CAG repeats (CAG > 35) Huntington protein with long polyglutamine repeat → affects neuron function making them vulnerable to cell death.
  • degeneration of strital neurons leading to excessive and involuntary movements.