Nervous System — Laboratorium

Nervous system lab

Five modes — from the whole body to fractions of a second

The nervous system has two parts. The central nervous system (CNS) is the brain and spinal cord — this is where decisions are made. The peripheral nervous system (PNS) is the nerves that connect the CNS with the rest of the body. Click a nerve or an organ on the map, or pick one from the list.

front view · diagram

Pick an element

map

Click the brain, the spinal cord or any nerve. The thick lines are nerve trunks — each holds thousands of fibers (axons) running side by side like wires in a cable.

A single neuron and its electrical impulse are shown on the Neuron page.

List of elements

keyboard works too

Somatic

Works largely consciously: it carries sensation from the skin, muscles and joints, and commands to the skeletal muscles — you walk, write, speak.

Autonomic

Works without conscious control: it runs the heart, breathing, digestion, pupils and glands. It has a sympathetic and a parasympathetic division (mode T).

Impulse race: from big toe to brain

t = distance / speed

Press “Send impulse.” Both signals leave the big toe at the same moment.

Height (≈ signal path)175 cm

Myelinated fiber100 m/s

Unmyelinated fiber (type C)1.0 m/s

Myelinated: 17.5 ms
Unmyelinated: 1.75 s
Difference: 100×
Clock: 0 ms

The fastest myelinated fibers (type Aα, to muscles) conduct at up to about 120 m/s, unmyelinated fibers only about 0.5–2 m/s. We simplify: path ≈ height, no delays at synapses.

The brain usually weighs about 1.2–1.4 kg. The cerebral cortex is folded into ridges (gyri) and grooves (sulci), which lets it fit inside the skull. Different areas specialize in different tasks, but almost every activity takes many of them working together.

left hemisphere · front on the left

Pick an area

brain

Click a lobe, a center or a structure deep in the brain. Dashed outlines mark structures that lie deeper, inside the temporal lobe.

Note: the drawing is schematic — in a real brain the borders between lobes are not this sharp.

“We only use 10% of our brain”

False

Brain imaging (e.g. fMRI, PET) shows that practically all its areas are active over the course of a day, though not all at once. Damage to almost any part has some effect. The brain uses about 20% of the body's energy — such a costly organ couldn't afford to leave 90% of itself idle.

“You're either left-brained or right-brained”

Big oversimplification

Some functions really do lean to one side — in most people, speech depends mainly on the left hemisphere. But the two hemispheres work together through the corpus callosum, and a study of brain scans from over a thousand people (2013) found no sign that people divide into “left-brained” and “right-brained.”

Homunculus — the body map in the cortex

Just in front of the central sulcus runs a strip of motor cortex; just behind it, a strip of sensory cortex. Every part of the body has “its own” patch there, and the size of the patch depends on how precisely we control that part or how finely we feel it — not on how big it is.

Share of the strip's length — approximate values based on W. Penfield's maps. The area for the feet and legs lies mostly on the inner (medial) surface of the hemisphere.

12 pairs of cranial nerves

shown on the section

No.	Nerve	What it does

Nerves I and II arise from the forebrain, III–XII from the brainstem. Type: S — sensory, M — motor, B — both (mixed). Exit points in the drawing are schematic.

Neuroplasticity

True

The brain changes throughout life. When you practice a new skill, frequently used connections between neurons (synapses) get stronger, and rarely used ones may weaken. In one study, people who spent a few months learning to juggle showed a small, temporary increase in gray matter in areas that process visual motion.

Crossed control

True

Most motor pathways cross over in the medulla oblongata (the pyramidal decussation), so the left hemisphere mainly controls the right side of the body, and the right hemisphere the left. Most sensory pathways cross over too.

A single neuron

Series results

visual stimulus

Mean: —
Median: —
Best trial: —
False starts: 0
Record (trial): —
Record (median): —

Reaction time to a visual stimulus is usually about 200–250 ms; to sound it's usually a few dozen ms shorter. A browser measurement also includes screen, mouse or touchscreen delay (often 10–50 ms). Records are saved in this browser.

Hick's law: the more options to choose from, the longer the reaction — the time grows roughly with the logarithm of the number of options.

Histogram of your trials

0 trials

Bars in 25 ms bins (all trials from this session for the selected stimulus type). Background band: the typical 200–250 ms range for a visual stimulus.

Where do the ≈ 200 ms go?

diagram · approximate values

The stages partly overlap, and their durations vary between people. A white line will show the median of your series.

Ruler test — reaction time without a computer

d = ½ g t²

A friend holds a ruler vertically so that its zero is between your thumb and index finger, then drops it without warning. You catch it as fast as you can and read how many centimeters it fell. The ruler is in free fall, so the distance tells you the time.

The ruler fell20.0 cm

Reaction time: 202 ms
Ruler speed: 1.98 m/s

When you finish a series in the test, I'll work out how far the ruler would fall at your median.

A 30-centimeter ruler falls its full length in about 0.25 s — with a slower reaction it slips through your fingers.

The autonomic system works without conscious control. Its parasympathetic division (mainly the vagus nerve) favors rest and digestion, while the sympathetic division prepares the body for effort and for responding to danger. Both work all the time — what changes is which one has the upper hand.

parasympathetic dominance

Heart rate: 70 /min
Pupil: 3.6 mm
Breaths: 14 /min
Adrenaline: low

Example values for a healthy adult. Pupil size also depends strongly on lighting.

Vagus nerve (X)

The longest cranial nerve. It leaves the medulla oblongata and “wanders” (Latin vagus) through the neck and chest all the way to the abdomen. It carries most of the body's parasympathetic fibers, yet about 80% of its fibers are sensory — they report to the brain on the state of the organs. It brakes the heart constantly: that's why the resting heart rate (about 60–80 /min) is lower than the sinus node's own rhythm (about 100 /min). See Heart.

Adrenaline from the adrenal glands

The adrenal medulla is like a sympathetic ganglion that has blood instead of a nerve as its output: when stimulated by nerves, it releases adrenaline (about 80%) and noradrenaline (about 20%) into the blood. The hormones reach the whole body and act longer than a nerve signal — that's why your heart keeps pounding for a while after a fright. The adrenal glands sit on top of the kidneys.

Who's in charge?

The autonomic system is run by centers in the brainstem and by the hypothalamus (mode W, section view). The wall of the digestive tract also has its own network — the enteric nervous system, with several hundred million neurons. Breathing and the airways are covered on the Lungs page.

Challenges

They complete themselves when you do the task in the lab.

Find the longest nerve
On the body map (mode Q), click the nerve that is the longest and thickest in the human body.
To do
Find Broca's speech area
In the side view of the brain (mode W), point to the area without which it's hard to say words.
To do
Outrun the pain
In Reflex mode (E), run the “Hot plate” reflex and watch it to the end: show that the hand moves before the brain becomes aware of the pain.
To do
Stay under 250 ms
In the reaction test (R), finish a series of 5 trials with a median of at most 250 ms — with any stimulus type.
To do
Switch on “fight or flight”
In Autonomic mode (T), push the body at least 90% toward the sympathetic system.
To do

How does it work?

The nervous system in plain words

A network in two parts

The central nervous system (brain and spinal cord) processes information and makes decisions. The peripheral nervous system is the 12 pairs of cranial nerves, the 31 pairs of spinal nerves and their branches, plus the ganglia. A nerve is a bundle of thousands of fibers — the axons of many different neurons. Sensory fibers carry signals to the CNS, and motor fibers carry them from the CNS to muscles and glands.

A signal travels along a fiber as an electrical impulse (an action potential), and between cells it is passed on by chemicals at synapses. The details are on the Neuron page.

How fast is a nerve?

Conduction speed depends on the fiber's thickness and on its myelin sheath. The signal's travel time is simply distance divided by speed:

t = sv e.g. 1.8 m100 m/s = 0.018 s = 18 ms

The same fiber without myelin (about 1 m/s) would need a full 1.8 s for that distance.

The reflex arc

A reflex has five links: receptor → sensory neuron → center (spinal cord or brainstem) → motor neuron → effector (a muscle or a gland). In the knee-jerk reflex there is only one synapse between the sensory and motor neurons (a monosynaptic reflex), which is why the signal usually reaches the muscle after only about 20 ms. The withdrawal reflex has interneurons along the way (polysynaptic), so it takes a little longer — but it is still faster than a conscious decision.

Reaction time

A conscious reaction has to travel the whole route: eye → visual cortex → decision → motor cortex → spinal cord → muscle. A single trial varies a lot, so we compute the mean and median of a series:

x = x₁ + x₂ + … + x_nn median = the middle value after sorting

The median is less sensitive than the mean to a single “daydreaming” trial.

When you have to choose between several responses, the reaction takes longer (Hick's law):

T = a + b · log₂(n + 1)

The ruler test

A dropped ruler falls freely with Earth's gravitational acceleration g ≈ 9.81 m/s². The distance grows with the square of the time, so the distance tells you the reaction time:

d = ½ g t² ⇔ t = √2dg

Example: t = 0.2 s gives d = ½ · 9.81 · 0.04 ≈ 0.196 m, or about 19.6 cm.

Two autonomic modes

The sympathetic division leaves the spinal cord in the thoracic and lumbar segments and reaches the organs through a chain of ganglia (the sympathetic trunk). It speeds up the heart, widens the pupils and airways, releases glucose from the liver and slows digestion. The parasympathetic division leaves the brainstem (mainly the vagus nerve) and the sacral spinal cord — it slows the heart, stimulates digestion and saliva, and during urination contracts the bladder muscle.

Did you know…?

The brain has about 86 billion neurons, but about 69 billion of them sit in the cerebellum, which makes up only about 10% of the brain's mass. The cerebral cortex has about 16 billion (Azevedo et al., 2009).
The brain is about 2% of body mass, yet it uses about 20% of the body's energy at rest — roughly the power of a dim light bulb (about 20 W).
Hitting your “funny bone” isn't hitting a bone at all — it's the ulnar nerve, which runs just under the skin there. Hence the tingling in your little and ring fingers.

What we simplified

The drawings are schematic: they show position and order, not exact anatomy. In reality, the borders of the lobes and cortical areas are less sharp.
Most peripheral nerves are mixed: alongside sensory and motor fibers, they carry autonomic fibers. The colors on the map are a simplification.
The reflex times and the 200 ms breakdown are model values on the order of those measured in adults — for a particular person they may differ.
The autonomic slider shows a single “level,” but in reality each organ can be controlled somewhat differently.

Neuron impulse and synapse Eye vision and the pupil Heart rhythm and the vagus nerve Lungs breathing Kidneys and the bladder

The nervous systema network that feels and decides