The Nervous System and the Brain
Unit 2: Anatomy, Physiology, and Medical Terminology. Lesson 7 of 53 in BMET Academy.
The nervous system runs on electricity, in tiny amounts, which makes it the system a biomed should respect most. It is the reason a shock hurts, the reason one frequency of alternating current is more dangerous than others, and the reason several of your devices either listen to electrical signals or supply them.
Two halves
- The central nervous system is the brain and the spinal cord: the processor and the main cable.
- The peripheral nervous system is every nerve that branches off, out to muscle, skin, and organs.
The wire in both halves is the neuron, one nerve cell that passes a signal along as a travelling change in charge and hands it to the next cell at a junction. Sensory receptors in the skin, the muscles, and the organs start those signals, and muscles and glands are where they end. A nerve is a bundle of nerve fibers (axons), which is why a device that stimulates a nerve reaches many cells at once.
Parts of the brain worth knowing
- The cerebrum is the big wrinkled top: thinking, speech, decisions, and voluntary movement.
- The cerebellum sits underneath at the back and handles balance and smooth coordination.
- The brainstem connects the brain to the spinal cord and runs the things nobody chooses: breathing and heart rate. Damage here is why a patient ends up on a ventilator.
- The spinal cord carries signals both ways inside the backbone, and nerves fan out from it.
- A reflex is the shortcut: the signal reaches the spinal cord and comes straight back out as movement without waiting for the brain. That is why a hand pulls off a hot pan before you notice it is hot.
Why this is the safety lesson in disguise
Nerves signal by moving charge, and muscles contract when a nerve tells them to. An outside current does the same thing, whether the body asked for it or not. That is the whole of electrical shock: outside current forcing nerves and muscles to fire.
Two consequences follow:
- Muscles clamp. At roughly 10 to 20 mA through the body a person cannot let go of what they are holding, because the muscles that grip are stronger than the ones that release.
- The heart is a muscle driven by its own electrical timing, and 60 Hz wall power happens to reverse direction at close to the worst possible rate for it. A current large enough to interfere can leave the heart quivering instead of beating.
The numbers behind those sentences come later in the safety unit; what you need here is why the body is vulnerable at all. The answer is that your equipment and the patient both run on electricity.
Devices on this system
At the level of recognizing what they are for:
- EEG: records the brain's electrical activity from electrodes on the scalp, in microvolts, which is far smaller than an ECG signal and therefore far fussier about electrode contact and interference.
- Nerve stimulators: deliver a small controlled pulse to a nerve, used in surgery and in testing.
- TENS: a small unit that sends mild pulses through skin electrodes to reduce pain.
- Evoked-potential monitors: supply a stimulus (a click, a flash, a pulse) and record the much smaller response from the nervous system, averaged over many repetitions to dig it out of the noise.
The Nervous System and the Brain, explained slowly
Two systems in one
The central nervous system is the brain and spinal cord; the peripheral nervous system is every nerve leaving them. Signals travel as brief electrical pulses, created by charged particles moving across a nerve cell's membrane. Those pulses are tiny: a nerve or brain signal at the skin is measured in microvolts, tens of microvolts against about a millivolt from the heart, so roughly 10 to 100 times smaller than the signal an ECG picks up from the heart.
That single fact explains most of the equipment in this lesson. The smaller the signal, the more the amplifier has to magnify, and the more everything else in the room gets magnified with it: the 60 Hz hum from the wiring, the movement of a cable, a poorly prepped electrode, a nearby motor. An EEG technologist fussing for twenty minutes over electrode impedance is not being precious; below a certain contact quality the signal is simply buried.
The map
Structure What it does Equipment connection cerebrum thinking, speech, decisions, voluntary motion EEG electrodes sit over it cerebellum balance and smooth coordination balance and gait testing brainstem breathing and heart rate, automatically ventilator patients, brain death testing spinal cord two-way cable inside the backbone evoked-potential monitoring in surgery peripheral nerves signals out to muscle and in from skin nerve stimulators, TENS, nerve studies reflex arc sensation in, straight back out as movement reflex testing, no device needed
Why electricity hurts
A nerve fires when the voltage across its membrane shifts far enough. A current from outside the body does the same thing, indiscriminately: it makes nerves fire and muscles contract whether or not the brain asked. Everything about electrical safety follows from that.
- At about 1 mA through the body you feel a tingle: the threshold of perception.
- 5 mA is the accepted maximum harmless current.
- At 10 to 20 mA the flexor muscles that close a hand overpower the extensors that open it, so a person cannot let go. That is the let-go threshold, and it is the reason a brief contact becomes a long one.
- Around 50 mA there is pain, possible fainting, and respiratory arrest, because the muscles that drive breathing are affected too.
- From 100 to 300 mA ventricular fibrillation is likely: the heart's own electrical timing is overwhelmed and the muscle quivers instead of pumping.
- At 6 A and above the whole heart muscle clamps in sustained contraction and burns follow.
And 60 Hz matters, not just the size of the current. Wall power reverses 60 times a second, which lands close to the worst case for interfering with the heart's own rhythm. Those numbers belong to the safety unit, but the reason behind them belongs here: the body runs on electricity, so electricity can run the body.
Devices to recognize
EEG records brain activity from scalp electrodes and lives or dies by electrode contact. Nerve stimulators deliver a controlled pulse to find or test a nerve, often during surgery. TENS units send mild pulses through skin electrodes for pain. Evoked-potential monitors supply a repeated stimulus and average hundreds of responses together, because any single response is smaller than the noise around it. Averaging is the trick that makes an invisible signal visible, and it is worth knowing by name.