#12 Peripheral Nervous System

Peripheral Nervous System:
puts your brain in touch with the physical environment, and allows it to respond.

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<Sensoory Nerve Receptors>

The peripheral nervous system's sensory nerve receptors spy on the world for the centeral nervous system, and each type responds to different kinds of stimuli.

Thermoreceptors respond to changes in temperature.

Photoreceptors react to light.

Chemoreceptors pay attention to chemicals.

Mechanoreceptors respond to pressure, touch, and vibration.

Nociceptors:
fire only to indicate pain

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<What is Pain?>

It helps protect us from ourselves, and from the outside world.

Such congenital conditions are very rare.

The rest of us have a whole nervous system dedicated to making sure our bodies react with a predictable chain of events at the first sign of damage.

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<How Pain is Processed>

The first step was a change in your environment - that is, a stimulus that activated some of your sensory receptors.

In this case, it was a change from the probably completely ignored feeling of bare skin on a smooth flooer to a distinct feeling of discomfort - the sharp metal tack piercing your skin.

Your peripheral nervous system's mechano- and nociceptors provided that base sensation, or awareness that something had changed.

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<Pain Threshold vs Pain Tolerance>

We all have the same pain threshold.

That is, the point where a stimulus is intense enough to trigger action potentials in those nociceptors is the same for everybody.

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<How the Brain Processes Pain>

Some neurons have mechanically-gated receptors that respond to a stretch in their membranes - in this case, that happens when the tack punches through them.

Meanwhile, other neurons have ligand-gated receptors that open when the damaged skin tissue releases chemicals like histamine ore potassum ions.

Signal travel between neurons either by electrical or chemical synapses.

The electrical ones send an electrical impulse, while the chemical ones - the ones I'm talking about now - first convert that signal from electrical to chemical, by activating neurotransmitters to bridge the synaptic gap, before the receiving neuron converts that chemical signal back into an electrical one.

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<Afferent and Efferent Divisions>

The responsibility changes hands. The torch is passed.

Because the pain signal has just triggered an action potential in a neuron in the spinal cord, which is part of the central nervous system, and there it reaches an integration center.

Once the integration center interprets the signal, it transmits the message to motor neurons, which send an action potential back down your leg, where it reaches an effector.

An effector is just any structure that receives and reacts to a motor neuron's signal, like a muscle contracting or a gland secreting a hormone.

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<Five Steps of the Reflex Arc>

1. receptor sense a stimulus

2. sensory neuron transmits signal up the PNS to the CNS

3. integration center decodes the signal

4. motor neuron sends directions back to the site of the stimulus

5. effector cells respond by contracting or secreting 

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<What the Brain Says About Pain>

News of the tack arrived first at your thalamus, the information switchboard that then split the message and sent it to the somatosensory cortex - which identifies and localizes the pain, like: "sharp, and foot";