Download Basal Ganglia

Basal Ganglia


Masses of gray matter found deep within the
cortical white matter
Composed of three parts
Caudate nucleus
 The putamen and the globus pallidus
(The lentiform nuclei)
 Fibers of internal capsule running between and
through caudate and lentiform nuclei

Basal Ganglia
Putamen
Globus Pallidus
The basal nuclei (ganglia) have an inhibitory role in motor control
Functions of Basal Ganglia

Act by modifying ongoing activity in motor pathways

Inhibit muscle tone (proper tone – balance the excitatory
and inhibitory inputs to motor neurons that innervate
skeletal muscle)

Select and maintain purposeful motor activity while
suppressing unwanted patterns of movement
(Action Selection: behavioral switching or decision making)

Monitor and coordinate slow and sustained contractions,
especially those related to posture and support

Regulate attention and cognition

Control timing and switching

Motor planning and learning
The basal ganglia are a collection of interconnected areas deep below the
cerebral cortex. They receive information from the frontal cortex about
behavior that is being planned for a particular situation. In turn, the basal ganglia
affect activity in the frontal cortex through a series of neural projections that
ultimately go back up to the same cortical areas from which they received the
initial input. This circuit enables the basal ganglia to transform and amplify the
pattern of neural firing in the frontal cortex that is associated with adaptive, or
appropriate, behaviors, while suppressing those that are less adaptive. The
neurotransmitter dopamine plays a critical role in the basal ganglia in determining,
as a result of experience, which plans are adaptive and which are not.
Evidence from several lines of research supports this understanding of the role
of basal ganglia and dopamine as major players in learning and selecting adaptive
behaviors.
In rats, the more a behavior is ingrained, the more its neural representations in
the basal ganglia are strengthened and honed. Rats depleted of basal ganglia
dopamine show profound deficits in acquiring new behaviors that lead to a reward.
Experiments pioneered by Wolfram Schultz, M.D., Ph.D., at the University of
Cambridge have shown that dopamine neurons fire in bursts when a monkey
receives an unexpected juice reward. Conversely, when an expected reward is not
delivered, these dopamine cells actually cease firing altogether, that is, their
firing rates “dip” below what is normal. These dopamine bursts and dips are
thought to drive changes in the strength of synaptic connections—the neural
mechanism for learning—in the basal ganglia so that actions are reinforced (in
the case of dopamine bursts) or punished (in the case of dopamine dips)
Humphries MD, Gurney KN (2002) The role of intra-thalamic and
thalamocortical circuits in action selection, Network 13:131-56
We embed our basal ganglia model into a wider circuit containing the
motor thalamocortical loop and thalamic reticular nucleus (TRN).
Simulation of this extended model showed that the additions gave five
main results which are desirable in a selection/switching mechanism.
First, low salience actions (i.e. those with low urgency) could be selected.
Second, the range of salience values over which actions could be
switched between was increased. Third, the contrast between the
selected and non-selected actions was enhanced via improved
differentiation of outputs from the BG. Fourth, transient increases in the
salience of a non-selected action were prevented from interrupting the
ongoing action, unless the transient was of sufficient magnitude. Finally,
the selection of the ongoing action persisted when a new closely matched
salience action became active.
The first result was facilitated by the thalamocortical loop; the rest were
dependent on the presence of the TRN. Thus, we conclude that the results
are consistent with these structures having clearly defined functions in
action selection.
Connectivity between different sections
BG anatomy
?
Basal ‫اتصاالت داخلی‬
‫ مسیرهای‬:ganglia
،‫ با رنگ قرمز‬glutamate
‫ با رنگ‬dopamine‫مسیرهای‬
‫ با‬GABA‫صورتی و مسیرهای‬
. ‫رنگ آبی نشان داده شدهاند‬
External
pathways
1. Direct pathway:
♦ Inhibition of internal GP
so no longer keeps
thalamus from exciting
cortex
♦ Facilitate the ongoing
action
DIRECT
2. Indirect PW:
♦ Causes subthalamus
to activate internal
GP, suppressing
thalamic activation
♦ Suppressing
unwanted movements
INDIRECT
Damage to Basal Ganglia
Parkinson’s disease
 Degeneration of DA neurons within the
substantia nigra (SN) which project to the striatum.
 Under normal circumstances these terminals convert
tyrosine to L-dopa which is synthesized to DA.
Loss of the pre-synaptic neurons results in DA release
 Possible to lose ~80% of DAergic cells in the SN without
manifesting symptoms
 Examination of a PD brain reveals loss of SN cells.
Parkinson’s Disease
• Loss of excitation from
substantia nigra to caudate &
putamen
• Results in increased activity in
indirect pathway
• Causes overactivity in the
internal GP
• Inhibits the thalamus, resulting
in decreased cortical motor
activity
INDIRECT
Parkinsons’ Disease: symptoms
1.
Akinesia/Bradykinesia: poverty or
slowness of automatic and voluntary
movements, incl. speech
2.
Rigidity: abnormal muscle tone
consisting of stiffness (poor range or
motion), cogwheeling, spontaneous
facial movement
3.
Resting tremor: (4-7/sec freq.),
referred to as “pill rolling”; may
lessen with intentional movement
4.
Postural instability: patients often
unsteady, may carry centre of gravity
out front (falls); difficulty righting
5.
Gait disturbance: fixed, stooped
posture and shuffling gait
6.
Non-motor features may also occur
Parkinson’s Disease: Cognition
Symptoms
 Bradyphrenia: slowing of thought processes
 Memory, specifically retrieving info in nonstructured
situations/spatial working memory
 Emotional functioning: depression is common
 Decrease in executive functioning