ch 18 – modulation of movement by the basal ganglia (motor selection)

basal ganglia and cerebellum

what regulates upper motor neurons?

false

true/false: basal ganglia and cerebellum regulate lower motor neurons

subcortical loop

• formed by the basal ganglia, substantia nigra, and subthalamic nucleus

• links motor cortex and upper motor circuits

• neurons in these circuits change their activity at the beginning and end of voluntary movements

basal ganglia

• striatum

  • caudate

  • putamen

• pallidum

  • globus pallidus

  • substantia nigra

    • pars reticulata

    • pars compacta

cortex → striatum → pallidum → thalamus

order of basal ganglia

projections to the basal ganglia

• caudate

• putamen

• globus pallidus

• substantia nigra

• subthalamic nuclei

medium spiny neurons

where do cortical neurons and substantia nigra project their inputs onto?

caudate and putamen

where are the medium spiny neurons located?

dopaminergic; cortical

__________ inputs from substantia nigra synapse closely to ______ synapses, modulating them

medium spiny neurons

• have spines where glutamatergic synapses form

• have inward K+ rectifying currents (even at rest) → little spontaneous activity

  • require many excitatory inputs to fire

• firing correlates with the occurrence of movement

  • fire seconds before movements and at termination of movement

  • firing helps select and initiate a movement

• GABAergic (inhibitory)

relationship between cortical neurons and medium spiny neurons

• each cortical neuron synapses on one spine, but each medium spiny neuron receives many cortical inputs

• allows for widespread information distribution

basal ganglia

one cortical neuron also connects with multiple _____ _____ neurons (telegraph poles)

firing patterns in caudate

• medium spiny neurons fire in anticipation of eye movements

firing patterns in putamen

• medium spiny neurons fire in anticipation of limb and trunk movements

globus pallidus (GP) and substantia nigra pars reticulata (SN)

where do medium spiny neurons in the caudate and putamen project to?

GABA-ergic

are the neurons of the GP and SN GABA-ergic or glutamatergic?

degree of convergence

• 100 medium spiny neurons innervate each cell in the GP (funneling of information)

GP to cortex pathway

• internal segment of GP → thalamus (ventral anterior and lateral muscle) → motor cortex (frontal lobe)

SN to superior colliculus pathway

• SN → brainstem → superior colliculus

• direct connection to upper motor neurons controlling eye and head movements

GP and SN inhibitory loop

• inhibitory neurons of the striatum synapse onto inhibitory neurons of the GP and the SN

  • inhibition of inhibition → activation (disinhibition)

• loop originates and ends in the cortex

• GP and SN have high spontaneous firing to prevent unwanted movements

• when striatum is activated, it inhibits the inhibitory neurons of the GP and SN → induces activation of thalamus and superior colliculus → initiation of movement

gate

basal ganglia acts as a ____ for the initatiation of movement

saccades

• rapid eye movements

oculomotor loop

• humans are foveating primates (spend a lot of time on moving eyes to focus on something)

• when the eyes are fixated on a target, the upper motor neurons controlling the movement are inhibited by the SN (SN inhibits superior colliculus) → no saccades occur

• before a saccade occurs: caudate inhibits SN activity via GABAergic input → upper motor neurons of superior colliculus are disinhibited → saccade occurs

direct pathway in basal ganglia

• striatum to GP

• facilitates voluntary movement initiation

indirect pathway in basal ganglia

• some medium spiny neurons project to external segment of GP → external segment neurons project to internal segment and subthalamic nucleus

• inhibits initiation of movement

  • increases inhibition provided by basal ganglia

• cortex also projects to subthalamic nucleus

balance

output of the globus pallidus (GP) results from the _______ of the activity of both the direct and indirect pathway

focused selection

• created by GP output balance between direct and indirect pathways of basal ganglia

• desired movement is allowed while unwanted movements are suppressed

• enhances behavioral contrast (movement vs no movement)

hypokinesia

• decrease in voluntary movement

• ex. Parkinson’s disease

pars compacta; striatum

dopaminergic neurons of the SN ___ _______ send projection to the _______

D1 receptors

• used by dopaminergic neurons in SN pars compacta to signal to the striatum

  • dopaminergic

  • on medium spiny neurons

  • located close to synapses between medium spiny neurons and cortical projections

• increase in cAMP → enhance excitatory input

D2 receptors

• used by dopaminergic neurons in SN pars compacta to signal to the striatum

  • dopaminergic

  • on medium spiny neurons

  • located close to synapses between medium spiny neurons and cortical projections

• decrease in cAMP → inhibit excitatory input

Parkinson’s disease

• degeneration of dopaminergic neurons in SN pars compacta

• loss of dopamine → inhibitory activity of basal ganglia is enhanced → reduces activation of upper motor neurons by thalamus → difficulty initatiating movement

• loss of neuromelanin in SN

hyperkinesia

• ex. Huntington’s disease

  • insufficient output from the pallidus → unwanted, jerky movements

  • atrophy of striatum

  • excitatory subthalamic nucleus can’t effectively oppose the direct pathway → cortex is more excited by the thalamus → excessive movement