Neural Circuits

Patterns of neuronal connections in CNS circuits

  • Diverging

  • Converging

  • Reverberating

  • Parallel-after-discharge

**NOTE: Focus on diverging/converging only

Simple Spinal Motor Reflexes

Myotatic reflex ~ knee-jerk response

  • Stimulation of peripheral sensors: muscle stretch receptor

    • Initiates receptor potentials → triggers action potentials (travel along afferent axons of sensory neurons)

  • Information stimulates spinal motor nerons thru synaptic contacts

  • Action potential from the spinal motor neurons travel peripherally in efferent axons

    • Causes muscle contraction and behaviral response

  • 1 purpose of this reflex: help maintain an upright posture in the face of unexpected changes

What is the role of interneurons?

Why is the flexor showing no/little activity?

Golgi tendon organ reflex (opp. of myotatic reflex)

  • contraction of quadricepts → relaxes muscle + contraction of antagonist

  • axon synapse on both inhibitory + excitatory interneurons in the spinal cord

    • inhibitory neurons - innervate a motor neurons to the quadricepts (relaxes the muscle)

    • excitatory interneurons - innervate a motor neurons to the antagonistic semitendinosus muscle (contracts muscle)

Spatial Organization in the Spinal Cord

Spatial Motor Pattern Generators

Model for alternating rhytm generation in flexor + extensor motor neurons

  • Stimulation of the upper excitatory interneurons has 2 effects

    • stimulated excitatory interneuron → excites motor neuron to the flexor muscle

    • stimulates excitatory interneuron → exites an inhibitory interneuron → inhibits lower pathway

  • Stimulation of the lower excitatory intereneuron has the opposite effects

    • when one motor neuron is active, the opposite one is inhibited

Cerebral Circuitry - Layers Lamination

Columnar processing circuits~

  • Basic local circuit in the neocortex consists of:

    • Inputs

    • Excitatory and inhibitory interneurons

    • Output neurons

Connections Among Areas ~

Organization of Movement

Descending systems (upper motor neurons)

  • Motor cortex (planning, initiating, and directing voluntary movements)   

    • Basal Ganglia - gating proper initiation of movement

  • Brainstem centers (basic movements and postural control)

    • Cerebellum - Sensory motor coordination

Spinal cord and brainstem circuits

Signal from brainstem centers towards:

  • Local circuit nurons - reflex coordination

AND

  • Motor neuron pools - lower motor neurons

    • sends signals to skeletal muscle

Organization of movement

Movement: Connections Among Areas

Striatum:

  • Is composed of:

    • Caudate nucleus (orange)

    • Putamen (green)

  • Major target for cortical afferents of the basal ganglia

Globus Pallidus:

  • Gives rise to efferents from the basal ganglia to the thalamus

Pars Oralis~

  • Part of ventrolateral nucleus of thalamus

    • projects to area 6

  • Usually designated by the symbol VLo

Other structures of the basal ganglia:

  • Form various internal loops → modulate the activity of the main loop

    • in which info passes thru the following brain structures in succession:

      • Cortex - straitum - globus pallidus - VLo - cortex (supplementary motor area, or SMA)

Which of the connections in this main loop are excitatory and which ones are inhibitory?

  • Excitatory influence of the substantia nigra and the subthalamic nucleus on various parts of circuit?