50 Fun Facts About Nervous System

In vertebrates the nervous system has two main parts: the central nervous system and which other?

The peripheral half is not shielded by the skull, the vertebral column or the blood-brain barrier, which makes it more vulnerable to injury and toxins.

Neurons are typically sorted into three functional types: sensory neurons, motor neurons and what?

They connect neurons to other neurons within the same region. Sensory neurons carry signals in from the sense organs; motor neurons carry orders out to muscles and glands.

Which part of a neuron carries impulses away from the cell body, sometimes for a metre or more?

It leaves the cell body at a swelling called the axon hillock. Dendrites, by contrast, branch profusely but only reach a few hundred micrometres.

The body's longest axons belong to which nerve, running from the spine's base to the big toe?

It is also the widest single nerve in the body, formed from the L4 to S3 spinal roots. Most axons are only about a micrometre across.

Along with neurons, the nervous system is built from support cells known collectively as what?

They provide structural and metabolic support. Star-shaped astrocytes are the most abundant kind in the brain, and microglia act as its resident immune cells.

Which cells, named after a German physiologist, form the myelin sheath outside the brain and spinal cord?

Theodor Schwann is the namesake. Inside the brain and spinal cord the same wrapping job is done by different cells, the oligodendrocytes.

Impulses in myelinated axons seem to jump between gaps in the sheath. What are those gaps called?

The leaping style is called saltatory conduction, from the Latin for 'jump'. Louis-Antoine Ranvier described the gaps in the 1870s.

Which ion floods into a neuron to trigger the rising phase of an action potential?

Voltage-gated channels open at a threshold, the interior turns positive, then potassium flows out to restore the resting state. Local anaesthetics work by blocking those sodium channels.

The fastest myelinated nerve fibres conduct impulses at roughly what speed?

That is about 432 km/h, glacial next to electricity in a wire but far quicker than blood flow. The slowest unmyelinated fibres crawl along at about half a metre per second.

Jens Christian Skou, who discovered the neuron's three-out, two-in ion pump in 1957, was of which nationality?

He collected a Nobel Prize for it forty years later. The pump burns one ATP molecule per cycle and keeps every animal cell electrically ready to fire.

Roughly how long is the adult human spinal cord?

It is much shorter than the spinal column that protects it, ending between the first and second lumbar vertebrae before tapering into a bundle of nerve roots.

The nerve-root bundle below the spinal cord takes its Latin name from its resemblance to what?

The cauda equina consists of the lower lumbar, sacral and coccygeal nerve pairs. Compressing it is a surgical emergency.

How many pairs of spinal nerves branch off the human spinal cord?

Eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal. The cervical nerves are numbered by the vertebra below them, the rest by the one above.

Conventionally, how many pairs of cranial nerves do humans have?

They are labelled with Roman numerals I to XII. Only the first two emerge from the cerebrum; the other ten arise from the brainstem.

Which cranial nerve, the tenth, takes its name from the Latin for 'wandering'?

It is the longest nerve of the autonomic system, reaching from the brainstem to the gut, and it drives the 'rest and digest' response that slows the heart.

Which cranial nerve, the fifth, serves facial sensation and chewing and is named for its three branches?

Its ophthalmic, maxillary and mandibular divisions converge just before entering the pons. It is often called the most complex of the cranial nerves.

The facial nerve also carries which sense from the front two-thirds of the tongue?

A branch called the chorda tympani carries the taste fibres. Damage to this nerve on one side is what causes the drooping of Bell's palsy.

The hypoglossal nerve, cranial nerve XII, which drives the muscles of the tongue, is which type of nerve?

Its name means 'under the tongue'. Doctors test it by asking a patient to stick their tongue out and watching for it to deviate to one side.

Which is the shortest of the twelve cranial nerves, and unusually can regenerate after damage?

It carries the sense of smell and, like the optic nerve, does not come off the brainstem. Regeneration is rare among nerve tracts of the central nervous system.

The knee-jerk reflex is the classic example of which kind of reflex arc?

A tap below the kneecap stretches the quadriceps spindle, the signal loops through L3-L4 in the spinal cord and back, and the leg kicks before the brain hears about it.

Why can a reflex like the knee-jerk fire faster than a deliberate movement?

The brain still receives the input, but it analyses the signal after the reflex has already happened.

What is the third division of the autonomic nervous system, which runs the gut, called?

Often called the 'second brain', it holds five times as many neurons as the spinal cord and can operate independently of the brain.

Which branch of the autonomic nervous system drives the 'fight or flight' response?

Its nerves emerge from the thoracic and lumbar spinal cord, and the adrenal medulla behaves like one of its ganglia, pumping out adrenaline.

With nerves emerging from the brainstem and sacral spinal cord, the parasympathetic system has what kind of 'outflow'?

The fight-or-flight system, by contrast, has thoracolumbar outflow. Parasympathetic fibres travel in the oculomotor, facial, glossopharyngeal and vagus nerves.

Roughly how many neurons does the human gut's own 'second brain' contain?

That is about 0.5% of the brain's count but five times the spinal cord's. It signals with acetylcholine, dopamine and serotonin, just like the brain.

How many layers of meninges surround the brain and spinal cord in mammals?

From outside in: the tough dura mater, the web-like arachnoid mater and the delicate pia mater. Cerebrospinal fluid fills the space under the arachnoid.

Cerebrospinal fluid is mostly produced by which structures inside the brain's ventricles?

Harvey Cushing demonstrated this in 1914. About 125 mL is present at any moment, but roughly 500 mL is made every day and absorbed back into the blood.

A lumbar puncture, or spinal tap, was first introduced in 1891 by which German physician?

The needle goes into the lower back below where the spinal cord ends, to draw cerebrospinal fluid or inject drugs and anaesthetics.

Which part of the brainstem controls breathing, heart rate, blood pressure and vomiting?

It is the lowest part of the brain and continues directly into the spinal cord through the foramen magnum.

Which almond-sized brain structure controls temperature and hunger and links to the pituitary gland?

It secretes releasing hormones that switch pituitary hormones on and off, and it also governs circadian rhythms and sleep.

Which brain structure, Latin for 'little brain', fine-tunes coordination and timing of movement?

It does not initiate movement but corrects it. Thanks to its tiny granule cells it packs more neurons than the rest of the brain combined into 10% of the volume.

Grey matter looks grey mostly because it contains lots of what?

White matter is the opposite: mostly long myelinated axons, whose fatty insulation gives it the pale colour. In the spinal cord the grey matter forms a butterfly shape in the centre.

A knot of nerve-cell bodies lying outside the brain and spinal cord is called what?

Dorsal root ganglia sit just outside the spinal cord. The chickenpox virus hides in such ganglia for decades before re-emerging as shingles along a single nerve's skin territory.

Which nerve, arising from C3 to C5 in the neck, provides the sole motor control of the diaphragm?

Medical students remember 'C3, 4, 5 keep the diaphragm alive'. A spinal injury above that level can stop a person breathing unaided.

Hitting your 'funny bone' actually strikes which nerve against the elbow?

It runs exposed behind the medial epicondyle of the humerus. The name is thought to be a pun on 'humerus' and 'humorous'.

Carpal tunnel syndrome is caused by compression of which nerve at the wrist?

It is the only nerve that passes through the carpal tunnel, squeezed in alongside nine flexor tendons under the flexor retinaculum.

Multiple sclerosis is an autoimmune disease in which the immune system attacks what?

French neurologist Jean-Martin Charcot first described it in 1868. Damaged insulation means signals in the central nervous system slow down or fail.

Parkinson's disease involves the loss of dopamine-producing neurons in which brain region?

The Latin name means 'black substance'. Levodopa, MAO-B inhibitors and dopamine agonists are the usual first treatments.

Amyotrophic lateral sclerosis is popularly named after which baseball star?

It is the most common motor neuron disease. Charcot linked the symptoms to the underlying nerve damage in 1869 and coined the name in 1874.

Why does the incubation period of rabies vary with where on the body a person is bitten?

A bite on the face reaches the brain far sooner than one on the foot. The old name hydrophobia comes from the throat spasms triggered by trying to swallow liquids.

Which bacterial infection, nicknamed lockjaw, causes muscle spasms violent enough to break bones?

Clostridium tetani produces a toxin that blocks the nerve signals which normally relax muscles. Spasms can continue for three to four weeks.

Bell's palsy, sudden one-sided facial paralysis, results from dysfunction of which cranial nerve?

It is named for the Scottish surgeon Charles Bell and accounts for about 70% of one-sided facial nerve paralysis.

Guillain-Barré syndrome is caused by the immune system attacking which part of the nervous system?

Weakness usually starts in the feet and hands and climbs. Three French doctors described it in 1916.

The name of the disease poliomyelitis literally refers to inflammation of what?

From Greek polios (grey) and myelos (marrow). The virus destroys motor neurons, and in about 1 in 200 to 1,000 cases the result is flaccid paralysis.

Which are the only multicellular animals with no nervous system at all?

Jellyfish and their relatives do have one, though it is about as simple as a nervous system gets, with nothing that could be called a brain.

Cnidarians such as jellyfish and hydras have which simplest form of nervous system?

It lets them respond to contact but with no central processing, so a hydra reacts the same way wherever it is touched. Comb jellies share the same arrangement.

Exactly how many neurons does a hermaphrodite of the roundworm C. elegans have?

Its nervous system is the first to be fully mapped as a connectome; males of the species have exactly 383 neurons.

Which giant 'command neurons' in a fish's brainstem trigger its C-shaped escape reflex?

Every fish has two, one on each side; a single action potential from one makes the fish curve into a C-shape within milliseconds.

What is the rule that a neuron releases the same neurotransmitters at all its synapses called?

It has only a few known exceptions, and a neuron can still affect targets differently because the effect depends on the receptors activated.

Which two embryologists showed that nervous tissue is 'induced' by signals from an organizer region?

Their early-20th-century experiments posed a puzzle about neural induction that was only solved by genetic approaches in the 1990s.