No medical authority gives a total count of nerves in the human body, because named nerves branch into ever smaller ones that nobody has tallied. What anatomy does count is 12 pairs of cranial nerves and 31 pairs of spinal nerves, according to Cleveland Clinic, and about 86 billion neurons in the brain, according to a 2009 count published in Frontiers in Human Neuroscience. This guide explains those numbers, how the nervous system is organised, which nerve is the largest, and how doctors test nerves.
How many nerves are in the human body?
Nobody has counted every nerve in the human body, and none of the authorities used for this guide gives a total. The honest answer is a set of smaller, verified numbers. Cleveland Clinic states that 12 pairs of cranial nerves originate in the brain and 31 pairs of spinal nerves branch out from the spinal cord. That is 43 named pairs, or 86 named nerves, leaving the brain and spinal cord. Each of these divides again and again into smaller nerves in the limbs, skin and organs, and those branches have never been tallied in a way a medical source will stand behind.
The number people usually want is the number of nerve cells, and that one has been measured. Suzana Herculano-Houzel, writing in Frontiers in Human Neuroscience in 2009, reported that the adult male human brain, at an average of 1.5 kg, has 86 billion neurons and 85 billion non-neuronal cells. She called the older figure of 100 billion neurons a "ubiquitous quote" that lies outside the range measured in real brains. Cleveland Clinic's nervous system page still rounds up to 100 billion; the measured count is 86 billion.
So the three numbers worth remembering are 12, 31 and 86 billion. Anyone quoting a figure in the trillions for "nerves" is not drawing on a medical source.
What is the difference between a nerve and a neuron?
A neuron is a single nerve cell; a nerve is a cable made of many neuron fibres bundled together. Cleveland Clinic explains that nerve cells are also called neurons, and that when a doctor says "nerve" they usually mean the part of the nervous system outside the brain and spinal cord. Inside a nerve, long fibres called axons sit at the centre. Groups of axons form fascicles. Connective tissue wraps each layer: endoneurium around axons, perineurium around fascicles and epineurium around the whole nerve.
Many axons are coated in myelin, a fatty sheath that Cleveland Clinic compares to the insulation on electrical wiring. Oligodendrocytes make myelin in the brain and spinal cord; Schwann cells make it in the peripheral nerves. Damage to myelin slows the signal, which is one of the things a nerve test looks for.
Neurons come in three types. Cleveland Clinic lists motor neurons, which carry signals from the brain and spinal cord to muscles; sensory neurons, which relay information from the senses to the brain; and interneurons, which link the other two and take part in learning, thinking and memory. Not all neurons live in the brain. Cleveland Clinic's page on the autonomic nervous system says the enteric nervous system in the gut contains about as many neurons as the spinal cord.
How is the nervous system organised?
The nervous system has two main parts: the central nervous system (brain and spinal cord) and the peripheral nervous system, the network of nerves that branch from the spinal cord to the organs and limbs.
The peripheral nervous system itself has two parts. The somatic nervous system controls voluntary movement. The autonomic nervous system controls involuntary activity, and Cleveland Clinic describes it as a web of nerves that runs breathing and heartbeat without conscious effort, staying active even during sleep. It has three divisions: sympathetic ("fight-or-flight"), parasympathetic ("rest-and-digest") and enteric, which manages digestion. Autonomic fibres travel in four cranial nerves (the third, seventh, ninth and tenth) and most spinal nerves.
| Division | What it includes | What it does |
|---|---|---|
| Central nervous system | Brain, spinal cord | Processes signals, controls movement, thought and reflexes |
| Peripheral: somatic | Nerves to skeletal muscle and skin | Voluntary movement, touch, pain, temperature |
| Peripheral: autonomic, sympathetic | Nerves to heart, vessels, glands | Fight-or-flight: faster heart rate, more blood to muscle |
| Peripheral: autonomic, parasympathetic | Mainly the vagus nerve and sacral nerves | Rest-and-digest: slows heart, drives digestion |
| Peripheral: autonomic, enteric | Nerve network in the gut wall | Controls digestion; about as many neurons as the spinal cord |
Source: Cleveland Clinic pages on the nervous system and the autonomic nervous system.
What are the 12 cranial and 31 spinal nerves?
The 12 cranial nerve pairs come from the brain and serve the head and neck, and through the vagus the chest and abdomen. In Cleveland Clinic's order they are: olfactory (smell), optic (vision), oculomotor (eye movement and pupil size), trochlear (eye movement), trigeminal (face sensation and chewing), abducens (sideways eye movement), facial (facial expression and taste), vestibulocochlear (hearing and balance), glossopharyngeal (taste and swallowing), vagus (heart rate, breathing, digestion), accessory (shoulder and neck movement) and hypoglossal (tongue movement). Olfactory and optic begin in the cerebrum; the other ten begin in the brainstem.
The 31 spinal nerve pairs leave the spinal cord between the vertebrae. Cleveland Clinic says the cord ends in the lower back in a cone called the conus medullaris; below it the remaining roots hang as a bundle, the cauda equina, which gives sensation to the lower body.
| Region | Pairs | Where they go |
|---|---|---|
| Cervical (neck) | 8 | Head, neck, arms and hands |
| Thoracic (upper back) | 12 | Chest, upper back and abdomen |
| Lumbar (lower back) | 5 | Legs and feet |
| Sacral | 5 | Pelvis, back of the legs |
| Coccygeal | 1 | Base of the spine |
Cleveland Clinic names 8, 12, 5 and 5 pairs and counts 31 in total; the single coccygeal pair makes up the difference. Some reflexes run through the cord without involving the brain at all.
Which is the longest and largest nerve in the body?
The sciatic nerve is the largest nerve in the body, and the vagus is the longest cranial nerve. Cleveland Clinic describes the sciatic nerve as "the largest nerve you have", with one on each side. It starts near the base of the spine in the lower back, passes through the pelvis and buttock, and runs down the back of each thigh into the lower leg. It begins narrow and widens as it travels down the leg; at its widest point it is about the size of a penny. It carries both motor signals, helping the leg and foot muscles move, and sensory signals from the leg.
Because it runs from the lower back to the foot, the sciatic nerve is also the nerve most people mean when they ask about the longest nerve. Irritation or compression of it causes sciatica. The NHS says a slipped disc is the most common cause, and that sciatica usually gets better in a few weeks to a few months.
Among the cranial nerves, Cleveland Clinic names the vagus (the tenth) as the longest, running from the brain to the large intestine. It is the main parasympathetic nerve and regulates heart rate, breathing, digestion and blood pressure. The smallest named nerves are the fine branches in the fingertips and skin, which have no single count.
How are nerves tested?
Nerves are tested with a nerve conduction study, often called an NCV test, and electromyography (EMG); MedlinePlus says the two are usually done together in one visit, nerve conduction first. In a nerve conduction study, electrodes on the skin deliver a mild electrical pulse to a nerve, and recording electrodes over the related muscle time how long the response takes. MedlinePlus calls that speed the conduction velocity. The pulse feels like mild static electricity. The study takes from about 15 minutes to over an hour depending on how many nerves are tested.
In EMG, a small needle electrode goes into a muscle and records its electrical activity at rest and when you tighten it. A healthy muscle is electrically silent at rest; a damaged one is not. EMG takes about 30 to 60 minutes and can leave the muscle sore for a few days. MedlinePlus lists conditions the tests help diagnose: carpal tunnel syndrome, Guillain-Barré syndrome, myasthenia gravis, ALS, herniated disc, muscular dystrophy and Charcot-Marie-Tooth disease.
MedlinePlus says do not wear lotions, sunscreen or perfume on the day, keep warm because cold alters nerve conduction and can give false results, and mention a pacemaker or blood thinners. Abnormal results point to axon damage, a conduction block or loss of myelin. The test reflects the best surviving fibres, so a normal result does not rule out some damage.
An EEG is different: it records the brain's electrical activity through scalp electrodes for seizures and epilepsy, not peripheral nerves. HealthFinder lists prices for the NCV test and the EEG by city.
What are the most common nerve problems?
Peripheral neuropathy, carpal tunnel syndrome and sciatica are the nerve problems people most often see a doctor for. The first is damage to nerves in the hands, feet and arms. The NHS estimates almost 1 in 10 people aged 55 or over in the UK are affected, and names diabetes as the most common cause; injury, shingles, some medicines and heavy drinking are others. Symptoms are numbness and tingling, burning or stabbing pain, loss of balance and weakness, usually starting in the feet because the longest nerves are hit first. The NHS says the sooner it is diagnosed, the better the chance of limiting the damage.
Carpal tunnel syndrome is pressure on the median nerve at the wrist. The NHS lists aching fingers, numbness, pins and needles and a weak thumb, often worse at night. A night splint is the first step, then a steroid injection; surgery usually cures it.
Go to an emergency department or call 112 if back or leg pain comes with numbness around the genitals or anus, weakness in both legs, or new trouble passing or controlling urine or stool. The NHS lists these as signs of a serious spinal problem that needs treatment the same day.
For numbness, weakness or nerve pain lasting more than a few weeks, a neurologist is the specialist, which the NHS defines as the specialist in health problems affecting the nervous system. This guide is general information from the sources listed below, not medical advice; a registered doctor makes decisions with you.
