P6: Basal Ganglia

flow charts are not include- learn from lecture slides

What is the basal ganglia?

• large and functionally diverse set of nuclei that lies deep within the cerebral hemispheres

What does the basal ganglia facilitate?

• the initiation of motor programs that express movement and suppression of competing or non-synergistic motor programs that would otherwise interfere with the expression of sensory driven & goal directed behaviours

Basal ganglia and cerebellum do not directly influence the ___________ circuitory but influence _________ by regulating the activity of ____________. 

Basal ganglia and cerebellum do not directly influence the lower motor neuron circuitry but influence movement by regulating the activity of upper motor neurons. 

Describe facilitation and use referred pain as an example. 

Facilitaiton:

• excitatory neurotransmitters (glutamate) are released from the presynaptic neurons onto the postsynaptic neurons

Example: Referred Pain 

• sensory neurons from the local and referred pain peripheral sites synapse with the secondary or projection neurons in the dorsal horn of the spinal cord 

• an axon collateral from the local sensory neuron synapses with the projection neuron of the referred pain 

• as a result, the local neuron sensory signal can induce a sensation of pain to the referred pain site 

Describe inhibition and give an example from the cerebral cortex and the cerebellar cortex. 

Inhibition

• inhibitory neurotransmitters (GABA & glycine) are released from the presynaptic neuron onto the postsynaptic neuron 

Example in the Cerebral Cortex 

• basket cells are GABAergic interneurons 

• basket cells will form synapses around the soma of pyramidal cells (main excitatory output of the cortex) 

• when basket cells are activated, they release GABA onto the pyramidal neurons, reducing their activity (hyperpolarisation) 

Example in the Cerebellar Cortex 

• granule cells receive excitatory input from the mossy fibres and send their axons up to form parallel fibres

• parallel fibres excite the Golgi cells

• Golgi cells inhibit the granule cells

Describe disinhibition and give an example from the cerebellar cortex. 

Disinhibition

• inhibition of an inhibitory neuron, which results in a net excitatory effect on a target neuron 

Example in Cerebellar Cortex:

• Purkinje cells are GABAergic neurons that project to the deep cerebellar nuclei and normally suppress them

• Molecular layer (basket cells and stellate cells), which inhibits the Purkinje cells 

• As a result, there is less inhibition of the deep cerebellar nuclei 

Describe disfacilitation and give an example in the cerebellar cortex.

Disfacilitation

• reduction in the excitatory input to a neuron which causes a decrease in its activity without a direct inhibitory effect 

Example in the cerebellar cortex: 

• Golgi cells are inhibitory interneurons which suppresses the granule cells 

• granule cells which normally excite the Purkinje cells are now inhibited; thereby the Purkinje cells are disfacilitated 

What is the main input component of the basal ganglia?

Striatum 

• caudate and putamen

What is the main output components of the basal ganglia?

Pallidum 

• globus pallidus 

• substantia nigra 

What structure does the striatum contain that allows it to collect and integrate a variety of information?

From which structures do they collect and integrate information from? 

medium spiny neurons

• collects and integrates information from cortical, thalamic & brainstem structures 

How does the striatum get its striped appearance?

the caudate and the dorsal part of the putamen are connected by a slender bridge of grey matter that extend to the internal capsule and confer a striped appearance in the parasagittal sections through this area

What are the motor components of the basal ganglia mostly comprised of and where is it mainly buried?

• comprised of a set of grey matter structures

• mainly buried in the telencephalon, some in the diencephalon and midbrain

What 2 structures provide input to the striatum and pallidum respectively?

• substantia nigra pars compacta

• subthalamic nucleus 

Where does output from the basal ganglia arise and where is it sent to?

Arises in the internal segment of the globus pallidus and is sent to the VA/VL complex of the thalamus which interacts with the circuits of the UMN in the frontal lobe

What structure control head and eye movements?

• substantia nigra pars reticulata projects to UMN in the superior colliculus 

Medium spiny neurons which originate in the substantia nigra pars reticulata are __________.

Medium spiny neurons which originate in the substantia nigra pars reticulata is dopaminergic.

Medium spiny neurons express _________________________ that tend to remain open near resting membrane potentials. 

When the medium spiny neurons do become active, their firing is associated with the occurrence of ____________. Neurons in the putamen tend to discharge in anticipation of __________________. Neurons in the caudate fire prior to ______________. 

Medium spiny neurons express inward rectifier potassium conductances that tend to remain open near resting membrane potentials. 

When the medium spiny neurons do become active, their firing is associated with the occurrence of movement. Neurons in the putamen tend to discharge in anticipation of limb and trunk movements. Neurons in the caudate fire prior to eye movement. 

What type of projections do the medium spiny neurons of the caudate and putamen give rise to and where do they terminate? 

• gives rise to: inhibitory GABAergic projections 

• teminate in: globus pallidus and substantia nigra pars reticulata 

Efferent cells of the caudate and putamen are __________ and therefore the main output of the basal ganglia is ________. 

Efferent cells of the caudate and putamen are GABAergic and therefore the main output of the basal ganglia is inhibitory. 

Neurons in the caudate and putamen have _________________ that prevent unwanted movement by __________________ cells of the UMN in the cortex. 

Disinhibition allows the UMN to send commands to ________________ & LMN in turn initiate ____________. 

Neurons in the caudate and putamen have high levels of spontaneous activity that prevent unwanted movement by tonically inhibiting cells of the UMN in the cortex. 

Disinhibition allows the UMN to send commands to local circuit neurons & LMN in turn initiate movement. 

What is the source of the largest input to the basal ganglia? 

striatum 

In eye movement, the substantia nigra pars reticulata sends axons mainly to the deep layers of which structure?

superior colliculus

What are saccades?

rapid, orienting movements of the the eyes