The Science of Orphan Black by Casey Griffin & Nina Nesseth

Author:Casey Griffin & Nina Nesseth
Language: eng
Format: epub
Publisher: ECW Press
Published: 2017-08-22T00:00:00+00:00

Neurons fire once that signal reaches a certain threshold, like squeezing hard enough on a trigger to fire a gun. Regardless of whether the stimulus that triggers the neuron-firing is weak or strong, the neuron fires in the same way, with the same intensity. To continue the gun analogy, you can’t half-fire a gun: it either fires or it doesn’t. This is called an all-or-nothing response. If the stimulus is more intense, the neuron may fire faster, or more neurons may fire overall.

But neurons don’t fire bullets. The electrical signal that travels down the axons can’t jump gaps between neurons, called synapses, like a static shock jumping from your finger to a metal doorknob. That’s where neurotransmitters come in. Neurons take the electrical impulse and translate it into a chemical neurotransmitter, depending on what sort of signal it needs to transmit. There are myriad neurotransmitters that use as many different mechanisms to influence neurons, but their main action is the same: activate receptors on neighboring neurons’ dendrites and cell bodies. Once neurotransmitters are released into the synapse, they can lock into the appropriate receptors on the next neuron’s dendrites and then trigger ion channels to open. Ion channels are like gateways that allow ions like sodium, potassium, or calcium to move into and out of the cell. These ions carry a charge, and the movement of ions changes the neuron’s overall charge from a negative state (its usual state) to a positive one. This positive charge rushes down the neuron body to the end of the axon, where, if the threshold is met again, neurotransmitters will be released into the gap between neurons. And then the cycle begins again, to trigger the dendrites of the next neuron.

Nerve tracts are arranged in well-tread pathways for the right signals to activate or inhibit cells in different areas of the brain. Another reason why nerve pathways in the brain are often referred to in terms of wiring and circuits is to describe consistent routes that signals take in response to a stimulus. The Papez circuit, associated with emotional expression and spatial and episodic memory, begins and ends in the hippocampus, which is a jelly roll–shaped deep temporal lobe structure, underneath the cerebral cortex, associated with converting short-term memories to long-term (a fun way to remember this function is to think that you’d “remember a hippo on campus”). It follows a set path through nodes in the cingulate cortex and hypothalamus, each with their own role in emotional response and regulating hormones. The entire circuit is only about 350 millimeters of nerve pathway. Anything that damages any of these circuits in the brain, anything that disrupts neurotransmitters and alters their balanced levels, can alter cell signaling: imbalance or disruption could trigger neurons to fire when they shouldn’t, or it could prevent neurons from firing when they should.

You’ve probably heard of the brain being separated into lobes by function, but even the lobes are mapped out by different cell types — based on their cytoarchitecture, or how the cells look when stained and observed under a microscope — that further define function.

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