Basal Ganglia

BASAL GANGLIA

BASAL GANGLIA• Derived from Telencephalon

– From thickening of the lateral telencephalon vesicle—STRIATAL RIDGE

• Control:– Background tone– Posture for movement

• Initiated in the cerebral cortex

• Participates in autonomic movements– Ex. Walking

Learning new motor

behavior

• w/ direct and indirect pathways

• w/ excitatory and inhibitory fxns

• Include:– Caudate nucleus– Globus pallidus– Putamen– Amygdaloid– Claustrum– Partly

• Substancia nigra• Subthalamic nucleus

Parts• Corpus Striatum

– Concerned with somatic motor fxn

– Neostriatum• Caudate nucleus• Putamen

– Paleostriatum• Globus Pallidus

– Forming the smaller and most medial part of corpus striatum

– Lies medial to the putamen, lateral to internal capsule

• Amygdaloid nuclear complex– Component of the limbic

system– Located beneath uncus

(temporal lobe)– Has primarily olfactory

input– Has receiprocal

connections w/:• Hypothalamus

• Prepyriform cortex

– Archistriatum• oldest part of basal ganglia

• Amygdaloid nuclear complex (cont’d)– Divisions: (main nuclear

masses)– Corticomedial nuclear

group– Basolateral nuclear group

– Largest and most differentiated part of the amygdaloid complex

– Dorsal Striatum• Caudate nucleus• Putamen

– Receives afferent input to the basal ganglia

– w/ high conc. of Dopamine

• Contained in small granular vesicles in terminal buttons (terminals of Nigrostriatal fibers)

• Glutamate• Conveyed by corticostriate

fibers

• Serotonin• Transmitted by raph nuclei of

the midbrain

• Inhibits Globus Pallidus through axonal projections containing GABA• Striatal efferent neurons

• P. GABA• Transported to

– Globus pallidus

– Substantia nigra

Striatum (Corpus Striatum)

• Lentiform/ lenticular nucleus– Putamen– Globus pallidus

• Basal Ganglia Circuits– Fibers emanating and

going to basal ganglia– Efferent and Afferent

types– StriatumGlobus

palldusThalamusCortexStriatum

– StriatumSubstancia nigraStriatum

– Globus Pallidus SubthalamusGlobus pallidus

1. Caudate nucleus– C-shaped cellular mass– related throughout its

extent to the lateral ventricle

– Suprathalamic• HEAD

– Rostral to thalamus

• BODY– arches along the

dorsolateral border of the thalamus

– Lateral to fibers of stria terminalis

– TAIL• Evident caudal to the

thalamus• Lies in the roof of the

inferior horn of lat. vent.• Terminal part

– comes into a relathionship with central nucleus of ANC

2. Putamen– Largest and most lateral

portion of striatum– Lies bet ext. capsule

and lat. medullary lamina of the globus pallidus

– Medial to the insular cortex

– Separated from insula by:

• External capsule• Claustrum• Extreme capsule

– Cytology:• Considered identical w/

caudate nucleus• Cells

– Densely packed

– No laminations on special arrangements

– 2 types:

» Small achromatic neurons

» Large multipolar neurons w/ rounded contours and irregularly clumped Nissl subs.

3. Globus Pallidus – Forms smaller and inner

part of lentiform nucleus– Medial to putamen– Dorsomedial margin

• borders the posterior limb of internal capsule

– Thin lateral medullary lamina, lies on

• External surface of pallidum

• Pallidum’s junction with putamen

– Medial medullary lamina• Divides GP into medial

and lateral segments

– Pallidal neurons• Large• Ovoid or polygonal cells

with long, thick and smooth dendrites

– Thin plate of gray mater– Lies in medullary subs.

of hemisphere between:• Lentiform nucleus (LN)• Insular cortex (IC)

– 2 white laminae• External capsule

– Separated from LN and IC medially

• Extreme capsule– separated from LN and

IC laterally.

– More closely related to the cerebral cortex than striatum

– 2 parts:• Insular part

– Large cells underlying the insular cortex

• Temporal part– Loosely arranged cells

located bet. Putamen and temporal lobe

– Contains discrete visual and somatosensory subdivisions

• Have interconnections w/ corresponding primary sensory areas of the neocortex

Claustrum

– Forms a broad compact, fiber bands

– Borders:• M – thalamus

- caudate• L – lentiform nucleus

– Composed of all fibers (afferent and efferent) which go to and from the cerebral cortex

– Mostly formed of thalamic radiations

– the rest, by corticofugal fibers or efferent cortical fibers

• Corticospinal• Corticobulbar• Corticoreticular• Corticopontine tracts

Internal Capsule

– Genu of internal capsule• Contains corticobulbar

and corticoreticular tracts• FUSION OF:• Shorter anterior limb of

internal capsule– Ant. thalamic radiations

– Prefrontal corticopontine tracts

• Longer posterior limb of internal capsule

– Corticospinal tracts

– Frontopontine tracts

– Superior thalamic radiations

– Small numbers of coritcotectal, corticobulbar and corticoreticular fibers

• Neurotransmitters involved– Glutamate

• Corticostriate fibers• Subthalamic nucleus to

globus pallidus

– Acetylcholine• Utilized by neurons with

short axons within the nuclei

– Dopamine• Manufactured in the

substantia nigra• Utilized by striatal neurons

– GABA• StriatumGlobus Pallidus• Globus PallidusSubstantia

Nigra

• Afferent Fibers– Corticostriate– Thalamostriate– Nigrostriate– Striatal Brainstem

Afferents

• Efferent Fibers– Striopallidal– Strionigral

• Pallidal connections– Pallidal afferent fibers– Pallidofugal fiber system

• Ansa lenticularis• Lenticular fasciculus• Pallidotegmental fibers• Pallidosubthalamis fibers

Afferent Fibers

• Corticostriate– From cortex to

striatum– glutamate• Thalamostriate• Comes from subthalamic

nucleus going to striatum

• Nigrostriate• From substantia nigra to

striatum• Dopamine

• Striatal Brainstem Afferents

– From the brainstem– GABA

Efferent Fibers

• Striopallidal– From striatum to

globus pallidus

• Stionigral– Striatum to

substantia nigra– Dopamine– Lesions:

PREMOTOR AND SUPPLEMENTARY MOTOR CORTEX

PRIMARY MOTOR CORTEX

STRIATUMVENTRAL ANTERIOR

NUCLEUS OF THALAMUS

SUBTHALAMIC NUCLEUS

GLOBUS PALLIDUS

SUBSTANTIA NIGRA

BRAINSTEM

SPINAL CORD

DIRECT ACTIVATION PATHWAYS

FINAL COMMON PATHWAY

INDIRECT ACTIVATION PATHWAYS

KEY:

DOPAMINE

GABA

GLUTAMATE

SCHEMA OF DIRECT AND INDIRECT ACTIVATION PATHWAYS

VA/VL complex of Thalamus

Motor Cortex

Spinal Cord

InternalGlobus Pallidus

ExternalGlobus Pallidus

Striatum

Subthalamic Nucleus

pars reticularis pars compacta

Substancia nigra

Motivation and

association cortices

The Corticothalamic Loop As the cortex determines that a voluntary movement is needed, the basal ganglia become engaged in selecting and presenting the motor cortex with the right motor programs needed to perform the movement. The basal ganglia integrates all the necessary data streams for the various cortex areas, processes them, and the result is served back to the frontal motor cortex as a buffet of carefully chosen motor programs, ready to be performed in a synchronized symphony of muscle contractions.

Thalamus

Basal Ganglia

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


From Stimulus to Action Here are the basal ganglia nuclei laid out for clarity. Let’s suppose that the body is idle, so that no voluntary movement occurs. Now assume a ball has been spotted, and the motivation to grab the ball is born within the motivation areas of the cortex. The motor has currently no idea of how to actually get the ball, and cannot execute any movement yet because the motor thalamus, that acts as a motion “gatekeeper,” is inhibited. Without this inhibition, wild and random movement would occur. So, before a motion is started, the thalamus is prohibited to allow any movements because one of the efferent parts of the basal ganglia, the internal segment of the globous pallidus, is inhibiting it.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


A Decision is Born The cortical and subcortical motivation and association cortices decide that a certain action is to be taken, e.g. to get the ball, but cannot execute the “reach” and “grasp” motor programs on its own. Of course, there are different reaching and grasping programs for different types of objects at different positions, and the programs need not only be chosen and started—they must also be halted at the right time. Thus, the motor cortex needs to have the correct motor programs chosen and unlocked by the basal ganglia.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


The Duality of the Striatum The striatum consists mainly of medium spiny neurons, that are usually silent because they require strong input signals to fire an action potential. The inputs are not only from the cortex, but also from the dopaminergic neurons of the substantia nigra pars compacta. The striatum’s dopamine receptors are both of excitatory D1 and inhibitory D2 types, which selects for the balance between the motion starting the direct and indirect pathways. Keep in mind that the caudate and putamen are parts of the striatum, and that both are reached by inhibitory and excitatory nigral neurons. But for now, let’s just focus on the motion starting the direct pathway.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


The Direct Pathway When the striatum receives the input from the cortex, together with dopamine from the 5Nc to the D1 receptors. Its GABAergic neurons will inhibit the GPi. The GPi has in itself a tonically inhibitory effect on the motor thalamus, and this is the “gate” for preventing unwanted movements. With the GPi inbihited, the motor thalamus is now disinhibited, and it can now present the frontal motor cortex with the appropriate motor programs for the desired movement, their temporal sequence and the strength of the muscle contactions.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


The Indirect Pathway Let’s say the brain changes its mind about the ball, and decides it’s best not to grasp it afterall. But the movement to reach out and grasp the ball has already begun. This is where the indirect pathway kicks in. It serves as a way to nullify the disinhibitory actions of the direct pathway. In short, it acts as a brake, restoring the inhibition of the motor thalamus. The key structure in accomplishing this brake, is the subthalamic nucleus.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


Inhibiting the Inhibitor The subthalamic nucleus (STN) is normally under tonic inhibition of the external segment of the globus pallidus (GPe). When this inhibition is lifted by the striatum, the STN, excited the inhibitory GPi, which means that the GPi will “brake” the motor thalamus to its original state.

Basal Ganglia

Thalamus

Globus Pallidus


Motor Cortex

Spinal Cord



Striatum

Subthalamic Nucleus


Substancia nigra

Motivation and


A Black Brake The STN also excites the substantia nigra pars reticulata, causing it to also inhibit the motor thalamus. This way, by influencing both the substantia nigra the GPi, the STN performs as an effective 2-way brake that stops the thalamus from permissing the cortex to execute motor programs.

Basal Ganglia

Thalamus

Globus Pallidus

CLINICAL CORRELATION

• Damping or modulating system

• Excess discharge– slowing

• Lack of discharge– hyperactivity

• Disorders may be present with– Abnormal

movements• Hypokinesia or• hyperkinesia

– Changes in tone

• HYPOKINESIA– Increase in tone rigidity

• ATHETOSIS– Slow and writhing

• DYSTONIA– Abnormal posturing of trunk

and extremeties

• HEMIBALLISMUS– Rapid flinging movements in

subthalamic lesions

• CHOREA– Brief rapid jerks in disease

of striatum

• TREMOR– 3-4 Hz at rest

• PARKINSON’S DISEASE– 1817 by James Parkinson– Involuntary tremulous motion,

w/ lessened muscular power, in parts not in action and even when supported; w/ propensity to a running pace, the senses and intellect being uninjured.

– Masked facies, poverty and slowness of voluntary movements “resting tremor”, stooped posture, rigidity and festinating gait, infrequenct of blinking (5 to 10 blinks/min, normal=20/min)

– Loss of pigmented cells in substantia nigra

– Encephalitis, CO poisoning, Mn poisoning, toxicity from psychoactive drugs (MPTP)

• HUNTINGTON’S CHOREA– By George Huntington– TRIAD of dominant

inheritance, choreoathetosis, and dementia

– Bilateral atrophy of the head of the caudate nucleus and putamen

– Widespread loss of cholinergic and GABAergic neurons

• SYDENHAM’S CHOREA– Immunologic disorder

• Associated w/ Rheumatic fever

– Protein structure of streptococcal antigen similar to that of proteins in the membrane of striatal neurons

– Transient, full recovery