2. Basal ganglia

Question 1

What are ‘ganglia’?

Answer 1

Concentration of neuronal cell bodies

Question 2

What are the ‘basal ganglia’?

Answer 2

This refers to the nuclei deep within the brain

Question 3

What is the role of the basal ganglia?

Answer 3

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

Question 4

Which nuclei compose the basal ganglia?

Answer 4

Caudate nucleus
Putamen nucleus
Globus pallidus

Question 5

What is the striatum and the corpus striatum?

Answer 5

Due to the striated structure in the brain:
Caudate nucleus + putamen nucleus = (dorsal) striatum

Striatum (caudate + putamen) + globus pallidus = corpus striatum

Question 6

Which other regions of the brain, does the basal ganglia have motor association with?

Answer 6

Substantia nigra

Subthalamic nucleus

Question 7

What is the susbtantia nigra?

Answer 7

This is a brain structure in the base of the midbrain

Plays a role in reward and movement

Question 8

What is the subthalamic nucleus?

Answer 8

Part of the subthalamus - involved in the basal ganglia pathway

Question 9

Specifically, where in the brain are the basal ganglia nuclei located? (caudate and putamen)

What other nucleus is closely associated with these two?

Answer 9

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

Question 10

What are the different parts of the globus pallidus nucleus?

Answer 10

There is an internal region and an external region

Internal - more medial and inferior
External - more lateral and superior

Question 11

What is the dorsal striatum?

Answer 11

This is the caudate and the putamen nuclei

Question 12

What is the ventral striatum?

Answer 12

This is the nucleus accumbens

Question 13

Where does input into the basal ganglia come from?

Answer 13

Input is to the caudate nad putamen nuclei

Comes from the entire region of the cortex and also from the substantia nigra

Question 14

What type of innervation (input) goes to the basal ganglia from each region?

Answer 14

From the cortex - glutaminergic innervation

From the substantia nigra pars compacta - dopaminergic innervation

Question 15

What are the different components of the substantia nigra?

Answer 15

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

Question 16

Where is output from the basal ganglia from and where does this lead to?

Answer 16

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)

Question 17

SUMMARY:

What is the dorstal striatum?

What is the ventral striatum?

What is included in the ‘basal ganglia’?

Answer 17

(Dorsal) striatum - caudate + putamen

Ventral striatum - accumbens

Basal ganglia - caudate, putamen, both parts of the globus pallidus, substantia nigra and the subthalamic nucleus

Question 18

What are the functions of the basal ganglia?

Answer 18

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

Question 19

What is the main neurone in the basal ganglia nuclei and what are the two main types?

Answer 19

The medium spiny neurone:
D1 - excitatory (direct pathway)
D2 - inhibitory (indirect pathway)

Question 20

What is the input to the medium spiny neurones (MSN)?

Answer 20

Input is dopaminergic

Question 21

What is the output from the MSNs?

Answer 21

Output is GABAnergic (remember this is inhibitory)

Question 22

Where does the MSN connect to and from?

Answer 22

Connects from the putamen and the caudate to the globus pallidus internal or external and the substantia nigra reticulata (the outer, lateral part)

Question 23

What is ‘convergence’?

Answer 23

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

Question 24

What are the two pathways via which the basal ganglia can coordinate movement?

Answer 24

Via the direct pathway or via the indirect pathway

Question 25

Describe the direct pathway of the basal ganglia

Answer 25

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

Question 26

What is disinhibition?

Answer 26

Where you inhibit the inhibition so this in turn results in a form of excitation

Question 27

How does disinhibiton occur in the direct pathway?

Answer 27

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

Question 28

What is the action of the direct pathway on motor programming?

Answer 28

The direct pathway is excitatory - allows a switch in the motor program

Question 29

What is the action of the indirect pathway on motor programming?

Answer 29

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

Question 30

How does the indirect pathway differ from the direct pathway?

Answer 30

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

Question 31

What are the current theories on the function of the basal ganglia?

Answer 31

Idea that the basal ganglia controls the decision making of movement change/motor switch rather than just simply enabling the change

Question 32

Summarise the actions of the direct/indirect pathways when you are at rest or are moving

Answer 32

When you are at rest - the indirect pathway is switched on

To have a change in movement, the direct pathway then becomes active

Question 33

What are the neurotransmitter receptors involved in the direct and indirect pathways

Answer 33

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