5.5 Internal loops connecting subcortical structures to the cerebral cortex

The cerebral cortex (bark in latin) is the external part of the cerebral hemispheres and can be easily identified in sections of fresh or fixed slices of brain. The cortex appears grey (the “grey matter”) because it contains the cell bodies and dendrites of the cortical neurons, contrasting with the “white matter” underneath which consists of the axons arriving and leaving the cortex. These axons are surrounded by myelin, a fat (lipid) that produces the white appearance. 

Several subcortical structures are connected reciprocally with the cerebral cortex forming extensive internal loops. 

Cortico-thalamic loops 

Most external sensory inputs to the cortex, namely mechanical stimuli from the body, visual stimuli and auditory stimuli, arrive via the thalamus. ‘First order’ thalamo-cortical neurons project to the corresponding primary sensory cortices transmitting messages largely from the opposite side of the body.

There is good topological organisation of sensory pathways to the thalamus with mechanical inputs arriving to ventral posterior nucleus, visual inputs to the lateral geniculate body, and auditory inputs to the medial geniculate body. These pathways project to the primary sensory cortices via thalamo-cortical neurons referred to as ‘drivers’. The mediodorsal thalamus and pulvinar are reticular ‘higher-order’ thalamic nuclei, with ‘modulator’ neurons that project widely to the cortex. Modulator neurons far outnumber drivers in terms of the numbers of synaptic connections¹.

Every cortical region receives projections from the thalamus and, in turn, every cortical region sends back signals to one or multiple thalamic nuclei². Thus, extensive cortico-thalamic loops are involved in modulating cortical functions.

Cortico-cerebellar loops

Massive cortico-cerebellar loops, involving the cerebro-cerebellum, are the most recent evolutionary development, forming the bulk of the cerebellar lobes. These loops are involved in shaping and correcting ongoing motor behaviour.

The main motor output from the neocortex originates from the pyramidal cells of the primary motor cortex and projects down to the motor circuits of the spinal cord. For motor tasks to be completed effectively, the motor cortex requires information about what the muscles are currently doing.

The pathways from the human motor cortex to spinal motor circuits to execute intentional movements involves about 1 million cortico-spinal neurons (the corticospinal tract). The remaining 19 million of the 20 million neurons in the primary motor cortex project to the brain stem and from there to the cerebellum. Here, this barrage of signals associated with the generation of intentional movements is compared with the actual activity of the muscles, primarily via sensors of muscle length (muscle spindles). From the cerebro-cerebellum, there are massive neural pathways back to the entire cerebral cortex, thereby closing a giant internal loop, the cortico-cerebellar loop. This ensures correct output from the motor cortex ahead of the planned movements.

As Jordan (2020) stated: “Many researchers refer to such cortico-cerebellar loops as forward models, and/or cerebellar control models. Others refer to them as anticipatory motor error virtual feedback, and dynamic state estimation”³. Lesions of the cerebellum affecting this massive internal loop result in the cortex becoming unable to accurately command intentional movements which become unsteady, oscillating like an inexpert driver zigzagging on a straight road, continuously correcting the direction. Such neurological condition is called ‘cerebellar tremor’ or ‘intention tremor’.

Loops between basal ganglia and cortex 

The basal ganglia are deep brain structures involved in coordinating many of the bodily movements, particularly those involved in multi-limb motor activities such as walking, climbing, dancing, playing sports, etc. The connections from the cortex to basal ganglia are complex but fundamentally form internal loops that control the amount of movement (force) required to achieving the desired behaviour.

Neurological lesions of certain parts of the internal loops result in disorders characterised by with too much or too little movement, eg Huntington’s Disease and Parkinson’s Disease, respectively. The loops between cortex and basal ganglia are also extensively involved in what LeDoux (2023) describes as a habit learning and goal directed system⁴. 

Taken together, these examples of well-established internal neural circuits indicate that a hierarchy of superimposed neural loops is the common architecture of the vertebrate nervous system⁵.

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1. S Murray Sherman & RW Guillery (2013): Functional Connections of Cortical Areas: A New View from the Thalamus. MIT Press. ↩︎
2. EG Jones (2001): The thalamic matrix and thalamocortical synchrony. Trends in Neuroscience 24, 595-601. ↩︎
3. JS Jordan (2020): Wild Stories: Science, consciousness, and the anticipatory narratives in which we live. Journal of Consciousness Studies 27, 128-151. ↩︎
4. Joseph LeDoux (2023): The Four Realms of Existence: A New Theory of Being Human. Belknap. ↩︎
5. RJ Douglas & KAC Martin (2004): Neuronal Circuits of the Neocortex. Annual Review of Neuroscience 27, 419-451.
   S Murray Sherman (2017): Functioning of Circuits Connecting Thalamus and Cortex. Comprehensive Physiology 7, 713-739. ↩︎

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