Lecture 12: Basal Ganglia - Modulation of movement

What is the basal ganglia

A collection of functionally diverse nuclei that lies deep within the cerebral hemispheres. A collection of deep-lying nuclei that modulates descending motor systems. Its primary roles are the initiation of intended movement and suppression of unwanted movement.

What is the striatum?

The main input area of the basal ganglia that receives signals from the cortex. It is made up of the Caudate and Putamen

The caudate nucleus

Part of the striatum involved mainly in eye movement control

The putamen

Part of the striatum involved in controlling trunk and limb movements

The globus pallidus

Part of the basal ganglia that sends inhibitory output to the thalamus. One of the two output structures of the basal ganglia.

The internal segment of the globus pallidus

Major output center that inhibits motor thalamus activity

The external segment of the globus pallidus

Part of the indirect pathway that modulates basal ganglia output

The substantia nigra

A basal ganglia nucleus that contains dopaminergic neurons

The substantia nigra pars compacta

Produces and releases dopamine that modulates striatal activity. The source of dopaminergic input to the striatum (Caudate and Putamen). Degeneration of these cells is a feature of Parkinson's disease.

The substantia nigra pars reticulata

Sends inhibitory output to control eye movements through the superior colliculus

The subthalamic nucleus

A small nucleus that helps regulate basal ganglia output in the indirect pathway

The motor cortex

The area of the brain that sends movement commands to the basal ganglia

The motor thalamus (VA/VL complex)

A relay nucleus that is tonically inhibited by the basal ganglia output structures; its disinhibition is necessary to activate the motor cortex.

Medium spiny neurons

GABAergic inhibitory projection neurons, making up 95% of striatal population (principal cells)

What is the direct pathway of the basal Ganglia?

A basal ganglia circuit that promotes movement by disinhibiting the motor cortex

What is the indirect pathway of the Basal Ganglia?

A basal ganglia circuit that inhibits unwanted movements

What is the hyperdirect pathway of the Basal Ganglia?

A fast pathway that can quickly stop or adjust movement

Dopamine

A neurotransmitter from substantia nigra that influences movement control

Disinhibition

The process of reducing inhibition to allow a target neuron or movement to activate

Tonic inhibition

Continuous inhibitory output from the basal ganglia that prevents unwanted movement

The superior colliculus

A structure that controls eye movements and receives basal ganglia output

Parkinson’s disease

A disorder caused by loss of dopamine, leading to reduced movement

Huntington’s disease

A disorder caused by degeneration of striatal neurons, leading to excess movement

The gating function of basal ganglia

Controls when motor commands are allowed to reach the cortex

The main role of the basal ganglia at rest

To keep movement circuits suppressed until activation is needed

The basal ganglia is located deep within which part of the brain?

Cerebral hemispheres

The striatum consists of which two structures?

Caudate and Putamen

The lentiform nucleus is formed by the putamen and which other structure?

Globus pallidus

The basal ganglia forms a functional network with the motor cortex and which thalamic complex?

VA/VL complex

The substantia nigra and subthalamic nucleus are components of which system?

Basal ganglia motor circuit

Which structure is considered part of the pallidum?

Globus pallidus

The motor parts of the basal ganglia primarily influence which function?

Modulation of movement

The subthalamic nucleus is functionally related to which structure?

Globus pallidus

The basal ganglia nuclei are described as functionally diverse because they

Participate in multiple motor and cognitive processes

The basal ganglia interacts with the motor cortex mainly through

Thalamic relay nuclei

The major source of input to the basal ganglia is the

Cerebral cortex

The input zone of the basal ganglia is formed by

Caudate and Putamen

Cortical inputs to the caudate mainly originate from

Motor areas controlling eye movements

Cortical inputs to the putamen mainly originate from

Premotor and primary motor cortices

The caudate receives projections primarily from

Multimodal association cortices

The putamen receives projections involved in controlling

Trunk and limb movements

Dopaminergic input to the striatum arises from

Substantia nigra pars compacta

Cortical inputs to the striatum are primarily

Glutamatergic and excitatory

The striatum receives dopaminergic modulation that

Selectively alters cortical input effectiveness

The basal ganglia input neurons use which neurotransmitter for excitation?

Glutamate

The principal cells of the striatum are

Medium spiny neurons

Medium spiny neurons make up approximately what percentage of the striatal population?

95%

Medium spiny neurons are primarily

GABAergic inhibitory neurons

Cortical inputs synapse on which part of medium spiny neurons?

Distal dendrites

Dopaminergic synapses are located

Near cortical synapses on distal dendrites

The perisomatic region of medium spiny neurons receives

Local circuit and thalamic synapses

The proximity of dopamine and cortical synapses allows

Modulation of cortical excitatory input

The main output of medium spiny neurons is

Inhibitory projections to pallidal and nigral structures

The neurotransmitter released by medium spiny neurons is

GABA

The somatodendritic arrangement of inputs to medium spiny neurons allows

Differential modulation of excitatory signals

The main output nuclei of the basal ganglia are

Internal segment of globus pallidus and substantia nigra pars reticulata

Basal ganglia output neurons are primarily

Inhibitory

The basal ganglia influence the motor cortex via

Motor thalamus (VA/VL complex)

The output of basal ganglia exerts

Tonic inhibition on upper motor circuits

The substantia nigra pars reticulata projects mainly to

Superior colliculus

The internal segment of the globus pallidus projects to

VA/VL nuclei of the thalamus

Disinhibition in basal ganglia function refers to

Inhibition of inhibitory output neurons

The effect of disinhibition on motor cortex is

Facilitation of movement initiation

The tonic inhibitory output of the basal ganglia is essential for

Preventing unwanted movements

During movement, output neurons in the basal ganglia are

Transiently inhibited

At rest, medium spiny neurons in the striatum have

Low spontaneous firing rate

Output neurons in the globus pallidus and substantia nigra pars reticulata are

Tonically active

The tonic inhibition from output nuclei

Suppresses thalamocortical activation

Excitation of striatal neurons causes

Transient inhibition of output nuclei

Disinhibition of thalamic neurons leads to

Activation of motor cortex

The basal ganglia act as a

Gatekeeper for motor commands

The superior colliculus is involved in

Eye movement control

In saccadic movement studies, firing of medium spiny neurons corresponds to

Suppression of substantia nigra pars reticulata neurons

The gating model explains

How basal ganglia permit voluntary movement

The VA/VL thalamic complex serves as

Relay for motor information to cortex

Disinhibition mechanism of the basal ganglia is

Inhibition of inhibition leading to excitation

A key feature of basal ganglia function is

Timing and selection of motor actions

Motor suppression at rest is achieved by

Tonic inhibitory output

Disruption of tonic inhibition may result in

Involuntary or excessive movements

The direct pathway of the basal ganglia facilitates

Movement initiation

The indirect pathway functions to

Inhibit unwanted movements

The external segment of the globus pallidus is part of the

Indirect pathway

The subthalamic nucleus belongs to the

Indirect pathway circuit

The direct pathway output reduces

Inhibitory outflow from the basal ganglia

The indirect pathway increases

Inhibitory outflow from the basal ganglia

The hyperdirect pathway involves

Direct cortical input to subthalamic nucleus

The antagonistic relationship between direct and indirect pathways ensures

Smooth movement control

Activation of the direct pathway

Disinhibits thalamocortical neurons

Activation of the indirect pathway

Enhances inhibition of thalamocortical neurons

The main neurotransmitter of both pathways is

GABA

The subthalamic nucleus provides

Excitatory input to internal globus pallidus

The indirect pathway modulates

Output neuron activity

Dysfunction of the indirect pathway can result in

Hyperkinetic movements

Balanced activity of both pathways is necessary for

Normal motor control

Dopaminergic projections to the striatum arise from

Substantia nigra pars compacta

Dopamine acts on

D1 and D2 receptors in the striatum

D1 receptors are primarily associated with

Direct pathway facilitation

D2 receptors are primarily associated with

Indirect pathway inhibition

Dopaminergic input to the striatum

Modulates cortical excitation

Degeneration of dopaminergic neurons

Leads to impaired movement initiation