2.13 Basal Ganglia Flashcards
What is the basal ganglia
The basal ganglia is a large group of subcortical structures comprising of several interconnected nuclei in the forebrain, midbrain and hindbrain. It participates in control of movement as well as having roles in cognition and emotion
What are the key nuclei groups in the basal ganglia
-the striatum which is made of cuadate and putamen in the dorsal striatum and the nucleus accumbens in the ventral striatum
-the globus pallidus which has an internal segment (GPi) and external segment (GPe)
-the substantial nigra which has the substantia nigra pars compacta (SNpc) and substantia nigra pars reticulata (SNpr)
-the subthalamic nucleus (STN)
Which neurotransmitter pathways are excitatory and which neurotransmitter pathways are inhibitory
Glutamatergic pathways are excitatory, GABAergic pathways are inhibitory, and dopaminergic pathways can be excitatory or inhibitory
Describe the basic circuit feedback loop of the basal ganglia
-cerebral cortex
to (via excitatory glutamatergic neurons)
-striatum
to (via inhibitory gabaergic neurons)
-intrinsic nuclei
to (via both excitatory and inhibitory neurons)
-output nuclei
to (via inhibitory gabaergic neurons)
-thalamus
to (via excitatory glutamatergic neurons)
-cerebral cortex
Describe the projection from the cerebral cortex to the striatum
The projection from cerebral cortex to striatum is roughly topographical, so the motor cortex projects to posterior putamen and the prefrontal cortex projects to anterior caudate
Describe the parallel subcircuits within the basic circuit feedback loop of the basal ganglia
There are primarily 4 subcircuits (somatomotor, occulomotor cognitive and limbic) and they are interconnected and not completely separate. These circuits are described as parallel to each other as they all connect the basal ganglia, thalamus and cerebral cortex, however there can also be convergence and divergence between these parallel circuits so input can be received from different projections.
Describe the input nuclei
The input nuclei are the caudate and putamen of the striatum. The 2 nuclei together is one of the largest subcortical structures. There are 2 types of striatal neurons, projection neurons (aka medium sizes spiny neurons MSN) (90%) and interneurons (10%). The MSN are inhibitory and project out to the intrinsic nuclei.
What is the intrinsic nuclei
The 3 components of intrinsic nuclei are the subthalamic nucleus, globus pallidus external and substantia nigra pars compacta. The globus pallidus external and substantia nigra pars compacta receive the bulk of input from and send the bulk of output to the other basal ganglia. The projecting neurons from the intrinsic nuclei can be inhibitory or exhibitory, this determines the output function.
Describe the actions of the subthalamic nucleus in the intrinsic nuclei
It receives excitatory input from frontal cortex and inhibitory input from globus pallidus external
Describe the actions of the globus pallidus external in the intrinsic nuclei
It receives inhibitory input from the striatum and excitatory input from the subthalamic nucleus. It provides feedback inhibition to the striatum, subthalamic nucleus, globus pallidus internal and sunstantia nigra pars reticulata. The function of the globus pallidus external is to oppose/limit/focus the activity of output nuclei
Describe the actions of the substantia nigra pars compacta in the intrinsic nuclei
It is made up of larger dopaminergic neurons which can degenerate in Parkinson’s disease. The substantia nigra pars compacta receives input from the striatum and the dopaminergic neurones project back to the striatum. The function of the substantia nigra pars compacta is to modulate activity across circuits.
Describe the output nuclei
The components or the output nuclei are the globus pallidus internal and the substantia nigra pars reticulata. All outputs from the globus pallidus internus and the substantia nigra pars reticulata are inhibitory and GABAergic. Both the globus pallidus internal and the substantia nigra pars compacta have large neurons projecting out of the basal ganglia primarily towards thalamus.
The basal ganglia is involved in the control of movement (voluntary and involuntary). How does damage to different parts of the basal ganglia affect this
-damage to specific nuclei within bg leads to specific alterations in control of movement
-damage to subthalamic nucelus causes large amplitude in involuntary movement
-damage to globus pallidus causes sloweness of movement, abnormal posture and difficulty relaxing muscles
-damage to snpc causes a tremor, rigidity, slowness of movement
-damage to striatum causes slowness in voluntary movement slowness and may produce involuntary movements
Describe the direct and indirect pathways in the basal ganglia and explain the importance in keep these pathways in balance
- there are direct pathways: cortex to striatum (exc) then striatum to globus pallidus/snr (inh) then globus pallidus/snr to thalamus (inh) which ultimately causes disinhibition of thalamus which decreases activity of motor cortex
- there are indirect pathways: cortex to striatum (exc) then striatum to gpe (inh) then gpe to stn (inh) then stn to gpi/snr (exc) then gpi/snr (inh)
- the two pathways need to be kept in balance to allow for automatic execution of learned movement sequences
-The activation of direct pathway facilitates movement and the activation of indirect pathway inhibits motor output
What are the 3 current hypotheses for the exact function of the basal ganglia
-Basal ganglia are responsible for automatic execution of learned movement sequences. e.g., patients with PD have difficulty moving several body parts simultaneously or sequentially.
-Facilitating or inhibiting cortical targets by keeping the direct/indirect pathways in balance.
-The output of the basal ganglia is inhibitory to posture and movement pattern generators in the cerebral cortex. Basal ganglia acts as a brake.
