Day-4: Its all about connectivity!
How information flows from one neural element to another
The flow of information and its processing was pioneered by Cajal and Sherington, with the help of cybernetics studies done by Norbert Wiener and John von Nuemann.
Interestingly Norbert Wiener had already postulated in the 1940s about the clear similarity between the brain and the nervous system and how the communication between the elements of the nervous system is similar to that of machines.
A model was created which suggested that the behavior of an individual is determined by three inputs- the sensory system, the cognitive system and the intrinsic behavioral state, which in turn provides us with an output generated by the CNS motor system. The motor output or the behavior varies qualitatively in different species and quantitatively in different individuals.
The wiring of the nervous system can be understood in the following diagram:-
B- Behavior
M- Motor System
S- Sensory Input
I- Intrinsic behavioral state
C- Cognitive Input
v- voluntary response
c- control signal
r- reflex response
According to the above model:-
B is determined by M.
M is a result of three input signals- S, I and C.
S creates an “r”
C mediates “v” and
I acts as control signals “c”
Let us discuss a bit more about the three cardinal neural systems in a rather detailed manner
The Sensory System
- One-mode or unimodal sensory pathways branch out into motor pathway and the cerebral cortex.
- In the cerebral cortex, processing with respect to perception and sensation occurs.
- Perception is important for cognition and consciousness whereas sensation evokes reflex reaction.
- Various receptors in our body, present in various regions- eg,tactile receptors, olfactory receptors, aortic receptor, hypothalamic insulin receptors etc. receive signal, after this each of the three motor systems (MPG, MPI and MN) receive these input signals, this information enters the CNS through cranial and spinal nerves.
- This methodology is starkly similar in all vertebrates. Lastly, depending upon the number of synapses present between the sensory receptor and cerebral cortex, the reaction takes place accordingly. The synapse number varies from one sensory receptor system to another sensory receptor system.
Cognitive System
- The cerebral cortex is responsible for the prioritization, evaluation and initiation of the voluntary behaviour.
- For understanding the voluntary behaviour of the cerebral cortex, it is compared with reflexive behaviour.
Intrinsic System
- Motoneurons and their related MPGs generate intrinsic behaviour.
- Intrinsic system can generate its own set of behaviours, unlike the reflexive or voluntary response.
- Certain regions in the CNS generates rhythmic activity pattern, the most common being the sleep-wake cycle behavioural pattern which is part of the endogenous circadian cycle and generates oscillations to maintain the day-night cycle.
- During the sleep-wake cycle in the night, the body is completely under the activity of the intrinsic and reflexive systems. For ex- respiration and sustained sphincter contractions whereas in the day, the voluntary mechanisms dominate the functionality.
- Another good example of the intrinsic behavioural pattern is the arousal system in animals. Here the hypothalamus plays a crucial role in determining the attainment of specific goals. Eg- foraging, procreation etc.
- The behavioural consequences of pleasure and pain is also an example of intrinsic behaviour, which is still a mysterious realm. But, till date it has been found out that the subcortical regions of the brain control this intrinsic behavioural aspect.
- According to this view, thinking or cognition arises in cerebral cortex whereas feeling or affect arises subcortically. It is also possible that all aspects of consciousness (thinking and feeling) arise only from cortical neural activity.
A brief case of the neurotransmitters
The neurotransmitters became a part of the CNS study models during the 1950s. The two of the major sets of CNS neurotransmitters are- noradhrenergic and cholinergic. They release acetylecholine and noradrenaline neurotransmitters (NT).
But, the NT systems are not clearly defined functional elements of the CNS, but exceptions are present.
They help operationally in defining circuits of the CNS when the administration of a drug takes place. For example, administration of acetylecholine agonists affect the synapses of the functional elements of the systems. These systems are pharmacological systems with highly specific characteristics, but definitely not independent from other pharmacological systems . So, if a drug is being administered for certain therapeutic effects, the response can also be noticed in other systems which manifests in the form of “side effects”, which can be good or bad.
Similarly, the gene products that are released by the nervous systems elements affect the behavioural patterns as well.
Role of genetic networks in NS functionality
The neural gene expression system, which determines the correct expression of various genes important for neural functionality determines the macro-circuitry as well as the micro-circuitry of the individual.
A good example of a gene is the μ-opiod receptor gene which expresses calbindin in neurons, that mediates calcium absorption in neurons.The macrocircuitry essentially develops during embryogenesis whereas the microcircuitry refers to the quantitative aspect of the number of synapses and the individual strength of each neuron.
The CNS working with respect to its pharmacological systems as well as genetic systems still remains a not so ventured arena and research is still ongoing regarding how crucial genes play a roles in certain diseases as well as how those genes relate to the other functionally known elements of the nervous system.
