NEUROSCIENCE: Motor Systems
Basal Ganglia

BASAL GANGLIA

Note: the Basal Ganglia and the Cerebellum do not innervate alpha motoneurons either directly or indirectly through interneurons; instead, they sample the output of the cortex and then send input back to the cortex (via thalamic nuclei) to modify the motor cortex program; the cerebellum also provides input to certain brainstem nuclei to modify movement and posture

A. Basal Ganglia Anatomy: several nuclei (should be called the “Basal Nuclei”) at the base of the cerebrum and in the diencephalon and midbrain

1. Caudate nucleus (cerebrum)
2. Putamen (cerebrum)
3. Nucleus Accumbens (cerebrum)

Note: Caudate, Putamen, and Accumbens are collectively referred to as the Striatum, in reference to their striped appearance

Note: Caudate and Putamen function as a single nucleus even though they are separated (in part) by the internal capsule; they consist of discrete patches of cells which have different responses and output connections

Note: the Striatum is essential (along with the cerebellum) for motor learning (acquisition and retention of skilled movements based upon practice)

Note: nucleus accumbens is involved in emotional reinforcement (c.f. the limbic system) and not in voluntary motor activity. It is particularly important in addiction.

4. Globus Pallidus (diencephalon); composed of two subnuclei

a. lateral globus pallidus (GPL)
b. medial globus pallidus (GPM)

5. Subthalamic Nucleus (STN, diencephalon)

6. Substantia Nigra (midbrain) ; composed of two subnuclei

a. pars compacta (SNC)
b. pars reticulata (SNR)

B. Roles

1. control the rate of onset and cessation of voluntary activity
2. append the elaboration of voluntary activity (associated movements, e.g. swinging ones arms when walking)
3. suppress unwanted spontaneous movements

Note: the basal ganglia also influence cognition, but their exact relation to higher mental function is unclear. .  Also, the basal ganglia are related somatic activity related to emotion (e.g. facial expression).

C. Neurotransmitters involved

1. Glutamate; excitatory
2. Gamma-aminobutyric acid (GABA), inhibitory
3. Dopamine; may be excitatory or inhibitory, depending upon whether the receptor to which it binds is type D1 (excitatory) or D2 (inhibitory)
4. Other (cotransmitters: enkephalin & SubstanceP)

Note: formation of dopamine is a step in the catecholamine synthetic pathway

Tyrosine => Dopa => Dopamine => Norepinephrine => Epinephrine

D. Connections

 

E. Pathophysiology: Dyskinesia (movement disorder), of which there are two general types: Hyperkinesia and Hypokinesia

1. Hyperkinesia: unwanted spontaneous involuntary movements

a. Chorea: continuous rapid, jerky, involuntary movements that are fragments of normal movements (“St. Vitus Dance”)

b. Athetosis: slow continual writhing

c. Dystonia, two types

1) torsion dystonia: strong contraction of antagonistic muscles, leading to rigidity (locked joints)
2) spasmodic torticollis: involuntary contraction of neck muscles

d. Ballismus: involuntary, violent flinging movements (ballistic)

e. Tics: small, repeated movement fragments (e.g. sniffing) or vocalizations (e.g. Tourette’s syndrome)

2. Hypokinesia: lack of normal movements (“Poverty of Movement”)

a. Bradykinesia

1) akinesia: difficulty in initiating and terminating movement
2) slow onset and cessation of movement
3) lack of associated movements (e.g. not swinging the arms when walking)
4) lack of normal spontaneous movement -- fixed posture, fixed facial expression (“Reptilian Stare”), drooling due to lack of swallowing

EXAMPLES

3.  Huntington’s Disease (hyperkinetic)

a.  Characteristics

1)  usual age of onset 30-50
2)  hyperkinetic, initially chorea, usually progressing to other hyperkinetic conditions
3)  initially personality changes, eventually progressing to dementia
4)  course: invariably fatal, due to disease complications

b.  Etiology:  inherited (autosomal dominant)

c.  Pathology:  degeneration of the striatum, particularly patches of cells containing the D2 receptor, leading to spontaneous excitation of the motor cortex

4.  Ballismus (hyperkinetic)

a. Pathology:  discrete lesion of the Subthalamic nucleus

5.  Parkinson’s Disease (hypokinetic)

a.  Characteristics

1)  usual age of onset 50-70

2)  prevalence: 1% of population over 50; prevalence increases with age

3)  signs

a)  bradykinesia
b)  passive (resting) tremor; often “pill rolling”
c)  rigidity

4)  course:  generally fatal, due to complications of immobility; life expectancy  about 5-15 years after onset if untreated, about twice as long with treatment

 

b.  Etiology:  various: idiopathic, toxins (carbon monoxide, manganese, MPTP, etc.), head trauma, encephalitis lethargic (influenza variety)

c.  Pathology:  degeneration of dopamine producing cells in the substantia nigra, resulting in decreased activation of the motor cortex

d.  Treatment

1)  restore brain dopamine by administration of L-Dopa (levadopa)

Note:  dopamine does not cross the blood-brain barrier, but L-Dopa does cross and is converted to dopamine in the substantia nigra

Note:  effectiveness of L-Dopa is limited to about 5 years

2)  administration of a D2 agonist

3)  MAO inhibitor (note: dopamine is catabolized by MAO)

Note:  these treatments reduce the progress of the disease but eventually become ineffective as the substantia nigra cells continue to degenerate

4)  deep brain stimulation to inhibit GPL or STN

(For a video illustration of deep brain stimulation, see "Parkinson's Surgery - UK" on YouTube)