2. Basal ganglia Flashcards
What are ‘ganglia’?
Concentration of neuronal cell bodies
What are the ‘basal ganglia’?
This refers to the nuclei deep within the brain
What is the role of the basal ganglia?
Involved in the switching of movement from the resting state to a switch in the motor program
e.g. when walking - constant walking is the resting state - switch to sitting down - this involves the basal ganglia to initiate the switch in movement
Which nuclei compose the basal ganglia?
Caudate nucleus
Putamen nucleus
Globus pallidus
What is the striatum and the corpus striatum?
Due to the striated structure in the brain:
Caudate nucleus + putamen nucleus = (dorsal) striatum
Striatum (caudate + putamen) + globus pallidus = corpus striatum
Which other regions of the brain, does the basal ganglia have motor association with?
Substantia nigra
Subthalamic nucleus
What is the susbtantia nigra?
This is a brain structure in the base of the midbrain
Plays a role in reward and movement
What is the subthalamic nucleus?
Part of the subthalamus - involved in the basal ganglia pathway
Specifically, where in the brain are the basal ganglia nuclei located? (caudate and putamen)
What other nucleus is closely associated with these two?
Caudate nucleus:
This is long
Has a head and a tail
Follows the path of the lateral horn of the lateral ventricle
The top of the ventricle holds the head of the caudate and the tail of the caudate lies by the third and fourth ventricles
Putamen nucleus:
This connects to the caudate nucleus at the third ventricle
NB. The nucleus accumbens can be seen between these two nuclei on the ventral basal aspect
What are the different parts of the globus pallidus nucleus?
There is an internal region and an external region
Internal - more medial and inferior
External - more lateral and superior
What is the dorsal striatum?
This is the caudate and the putamen nuclei
What is the ventral striatum?
This is the nucleus accumbens
Where does input into the basal ganglia come from?
Input is to the caudate nad putamen nuclei
Comes from the entire region of the cortex and also from the substantia nigra
What type of innervation (input) goes to the basal ganglia from each region?
From the cortex - glutaminergic innervation
From the substantia nigra pars compacta - dopaminergic innervation
What are the different components of the substantia nigra?
NB. Substantia nigra is composed of dopaminergic neurones
Two parts - parts compacta and pars reticulata
Pars compacta - input to the basal ganglia - dopaminergic
Pars reticulata - output from the basal ganglia - GABAergic
Where is output from the basal ganglia from and where does this lead to?
Output from the basal ganglia is from the caudate and putamen nuclei
Output goes only to the frontal cortex and the globus pallidus (either internal or external regions)
SUMMARY:
What is the dorstal striatum?
What is the ventral striatum?
What is included in the ‘basal ganglia’?
(Dorsal) striatum - caudate + putamen
Ventral striatum - accumbens
Basal ganglia - caudate, putamen, both parts of the globus pallidus, substantia nigra and the subthalamic nucleus
What are the functions of the basal ganglia?
Regulate the intensity of movements
Inhibit antagonistic or unnecessary movements i.e. when at rest the basal ganglia is active to inhibit the unnecessary movement
Regulate attention and cognition
Motor program switch
What is the main neurone in the basal ganglia nuclei and what are the two main types?
The medium spiny neurone:
D1 - excitatory (direct pathway)
D2 - inhibitory (indirect pathway)
What is the input to the medium spiny neurones (MSN)?
Input is dopaminergic
What is the output from the MSNs?
Output is GABAnergic (remember this is inhibitory)
Where does the MSN connect to and from?
Connects from the putamen and the caudate to the globus pallidus internal or external and the substantia nigra reticulata (the outer, lateral part)
What is ‘convergence’?
The reduction of action potentials being transmitted from 150,000,000 neurones in the cortex to 30,000 neurones in the striatum to 100 neurones in the GPe to one singular neurone in the GPi/SNr
What are the two pathways via which the basal ganglia can coordinate movement?
Via the direct pathway or via the indirect pathway
Describe the direct pathway of the basal ganglia
Excitatory (glutamate) axons from the sensory and association cortices project to the caudate and putamen nuclei
These two nuclei project into the internal global pallidus
Then projects to the motor thalamus
Then to the supplementary motor cortex (SMA)
There is also discreet dopaminergic innervation from the substantia nigra to the caudate and putamen nuclei
SEE IMAGE IN ANATOMY BOOKLET
SO the direct pathway is excitatory to the via disinhibiton to the thalamus
The removal of tonic inhibition of the thalamus allows for excitation of the motor thalamus and movement of the body occurs
What is disinhibition?
Where you inhibit the inhibition so this in turn results in a form of excitation
How does disinhibiton occur in the direct pathway?
The caudate and putamen nuclei both have GABAnergic (inhibitory) neurones so when they are both stimualted from the cortex, two lots of inhibitory neurones are sent to the globus pallidus and hence this results in excitation of the neurone
What is the action of the direct pathway on motor programming?
The direct pathway is excitatory - allows a switch in the motor program
What is the action of the indirect pathway on motor programming?
The indirect pathway is inhibitory - inhibits a switch in the motor program
SO when at rest, the indirect pathway is always switched on - allows you to stay still
How does the indirect pathway differ from the direct pathway?
Rather than going straight to the GPi, the signal from the caudate and putamen nuclei goes to the GPe and then to the subthalamic nucleus before then going to the GPi
SEE IMAGE IN ANATOMY BOOKLET
What are the current theories on the function of the basal ganglia?
Idea that the basal ganglia controls the decision making of movement change/motor switch rather than just simply enabling the change
Summarise the actions of the direct/indirect pathways when you are at rest or are moving
When you are at rest - the indirect pathway is switched on
To have a change in movement, the direct pathway then becomes active
What are the neurotransmitter receptors involved in the direct and indirect pathways
Dopamine neurotransmitters are present in the brain and exerts function via D1 and D2 receptors
D1 - direct pathway - motor program change
D2 - indirect pathway - blocks motor program change
