Some circuit operations in the mammalian brain.

There is an account of the basis neuronal connectivities of the spinal cord with the Sherringtonian principles of divergence and convergence. Neurones act synaptically either as excitatory or as inhibitory, depending on the specific transmitter substances liberated. Inhibitory neurones usually act either in a feedback or a feedforward manner. Voluntary movement is considered in relation to the instructions delivered to the motor cortex in order to produce the discharges down the pyramidal tract that evoke the required movement. There is an account of the three lines of evidence which indicate that in voluntary movements the primary neural event arises in discharges of neurones of the supplementary motor area (SMA). There are three main circuits from the SMA that activate subroutines concerned in the preprogramming of movements: (1) SMA to the basal ganglia, thence to the thalamus with a collateral line through the substantia nigra, thence to the association cortex; (2) SMA to cerebellar hemisphere via the pontine nuclei, thence to the nucleus dentatus, to the thalamus, to the association cortex, and (3) SMA to association cortex both frontal and parietal. According to the SMA hypothesis the liaison brain for intention is located in the SMA, there being reciprocity of informational flow from the mental events of intention to the neuronal events in the SMA.