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Response to stimuli and the spinal cord

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The nervous system processes information throughout the body by carrying signals between the central nervous system (CNS) and peripheral nervous system (PNS). The CNS is composed of the brain and spinal cord, while the PNS consists of nerves which are connected to the CNS and reach the rest of the body.

The spinal cord consists of a central canal filled with cerebrospinal fluid, and grey (H shape) and white matter. Nerves connect to the cord, while the meninges act to protect the cord (alongside the brain).

The so-called spinal nerves are attached in the same pattern across the spinal cord, with a dorsal root and a ventral root.

The nerves and their roots can be observed under a microscope of a spinal cord cross-section.

Reflex - think of sensory neuron --> interneuron --> motor neuron

The knee-jerk reflex is too famous so it shall be ignored on this occasion (also, what is the point of it anyway? I never recall it saving me from anything, do you?!). Instead we turn to the iris reflex responsible for controlling the amount of light entering your eyes.

(Fun experiment that's also an opportunity for justified procrastination: go in the bathroom and keep covering an eye and uncovering quickly while staring in the mirror to see how quickly the pupil appears to change size!)

...back now.

How does this reflex work? Most reflexes have multiple steps, but the outline goes something like this:

receptor --> sensory neuron --> integration centre --> motor neuron --> effector

In other words: the retina (receptor) at the back of the eyes senses the amount of light present. This information is relayed (sensory neuron) to a centre in the brain (integration centre) which then acts upon it by stimulating the motor neuron to carry out a response command to the effector - the iris muscles which contract.

The above example uses the spinal cord as the integration centre instead of the brain.

Nerve net in Cnidaria

Cnidaria includes hydra, sea stars and jellyfish. Pretty. They exhibit the simplest nervous system among multicellular organisms. 

They don't have a centralised system with a brain or a spinal cord. Their neurones are simply spread across their body and interconnected, producing signals that are not traceable depending on the location of the stimulus.

This means that if there is food or danger on their left side, they wouldn't know, and instead respond overall. They move accordingly towards food or away from a negative stimulus, as a whole and without regard for the stimulus location.

The sea star, however, does have nerve nets in each arm, connected centrally by a ring, so it can have more specific responses compared to a full-body net such as that of hydra.

Compared to the more complex nervous systems such as the one aforementioned, these nerve nets are decentralised, and therefore these organisms cannot pinpoint stimulus location. Their motor response will be the same regardless of where the stimulus is coming from.

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