Nerve
A nerve is an enclosed, cable-like bundle of nerve fibers or axons, which includes the glia that ensheath the axons in myelin. Neurons are sometimes called nerve cells, though this term is technically imprecise since many neurons do not form nerves.

Anatomy
Nerves are part of the peripheral nervous system. Afferent nerves convey sensory signals to the central nervous system, for example from skin or organs, while efferent nerves conduct stimulatory signals from the central nervous system to the muscles and glands. Afferent and efferent fibers are often arranged together, forming mixed nerves.

Most nerves connect to the central nervous system through the spinal cord. The twelve cranial nerves, however, connect directly to parts of the brain. Spinal nerves are given letter-number combinations according to the vertebra through which they connect to the spinal column. Cranial nerves are assigned numbers, usually expressed as Roman numerals from I to XII. In addition, most nerves and major branches of nerves have descriptive names. Inside the central nervous system, bundles of axons are termed tracts rather than nerves.

The signals nerves carry, sometimes called nerve impulses, are also known as action potentials: rapidly traveling electrical waves, which begin typically in the cell body of a neuron and propagate rapidly down the axon to its tip or "terminus." The signals cross over from the terminus to the adjacent neurotransmitter receptor through a gap called the synapse.

Clinical Importance

Damage to nerves can be caused by physical injury, swelling (e.g. carpal tunnel syndrome), autoimmune diseases (e.g. Guillain-Barré syndrome), infection (neuritis), diabetes, or failure of the blood vessels surrounding the nerve. Pinched nerves occur when pressure is placed on a nerve, usually from swelling due to an injury or pregnancy. Nerve damage or pinched nerves are usually accompanied by pain, numbness, weakness, or paralysis. Patients may feel these symptoms in areas far from the actual site of damage, a phenomenon called referred pain. Referred pain occurs because when a nerve is damaged, signaling is defective from all parts of the area which the nerve receives input, not just the site of the damage.

Neurologists usually first diagnose disorders of the nerves by a physical examination, including the testing of reflexes, walking, muscle weakness, directed movement, proprioception, and the sense of touch. During the exam, the doctor asks questions about the patients' symptoms and medical history. The initial exam can be followed with tests such as nerve conduction study and electromyography (EMG).

Cranial Nerves

Cranial nerves are nerves that emerge from the brainstem instead of the spinal cord.

Names of nerves

There are 12 pairs of cranial nerves, traditionally abbreviated by the corresponding Roman numerals. They are numbered according to where their nuclei lie in the brain stem, i.e. Cranial Nerve I (the Olfactory nerve) leaves the brainstem at a higher position then Cranial nerve XII, whose origin is located more caudally (lower) than the other cranial nerves.

All cranial nerves except for the olfactory and optic nerves belong to the peripheral nervous system, having axons that are myelinated by Schwann cells (myelinating cells of the peripheral nervous system) rather than oligodendrocytes (myelinating cells of the central nervous system). The olfactory and optic nerves are named as such, but are technically not nerves, and are continuations of the central nervous system.

Note: technically the accessory nerve splits into the spinal accessory nerve and the cranial accessory nerve, but the latter quickly combines with the vagus nerve.

# Name       Nuclei
I Olfactory nerve     Anterior olfactory nucleus
II Optic nerve      Lateral geniculate nucleus
III Oculomotor nerve     Oculomotor, Edinger-Westphal nucleus
IV Trochlear nerve     Trochlear nucleus
V Trigeminal nerve     Principal sensory trigeminal nucleus, Spinal trigeminal nucleus, Mesencephalic trigeminal nucleus, Trigeminal motor nucleus
VI Abducens nerve (or abducent nerve) Abducens nucleus
VII Facial nerve (VII)     Facial nucleus, Solitary nucleus, Superior salivary nucleus
VIII Vestibulocochlear nerve (or auditory nerve) Vestibular nuclei, Cochlear nuclei
IX Glossopharyngeal nerve    Nucleus ambiguus, Inferior salivary nucleus, Solitary nucleus
X Vagus nerve      Nucleus ambiguus, Dorsal motor vagal nucleus, Solitary nucleus
XI Accessory nerve     Nucleus ambiguus, Spinal accessory nucleus
XII Hypoglossal nerve     Hypoglossal nucleus

Olfactory Nerve

The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerve consists of a collection of sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate, a sieve-like structure.

Olfactory receptor neurons continue to be born throughout life and extend new axons to the olfactory bulb. Olfactory ensheathing glia wrap bundles of these axons and are thought to facilitate their passage into the central nervous system.

The sense of smell (olfaction) arises from the stimulation of the olfactory receptors by activation from gas molecules that pass by the nose during respiration. The resulting electrical activity is transduced into the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

The olfactory nerve is the shortest of all the twelve cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.

To test the function of the olfactory nerve, doctors block one of the patient's nostrils and place a pungent odour (such as camp coffee essence) under the open nostril. The test to see if the patient can detect the smell is then repeated on the other nostril.

Optic Nerve

The optic nerve, also called cranial nerve II, is the nerve that transmits visual information from the retina to the brain.

The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during embryonic development.

Consequently, the fibers are covered with myelin produced by oligodendrocytes rather than the Schwann cells of the peripheral nervous system. Similarly, the optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in peripheral nerves.

This is an important issue, as fiber tracks of the mammalian central nervous system (as opposed to the peripheral nervous system) are incapable of regeneration and hence optic nerve damage produces irreversible blindness. The fibers from the retina run along the optic nerve to nine primary visual nuclei in the brain, from whence a major relay inputs into the primary visual cortex.

Oculomotor Nerve

The oculomotor nerve is the third of twelve paired cranial nerves. It controls most of the eye movements (cranial nerves IV and VI also do some), constriction of the pupil, and holding the eyelid open.

The oculomotor nerve arises from the anterior aspect of mesencephalon (midbrain). The muscles it controls are the ciliary muscle (affecting accommodation), and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle. In addition, it supplies parasympathetic fibers-which originate in the Edinger-Westphal nucleus - to the eye via the ciliary ganglion, and thus controls pupil constriction.

Testing the oculomotor nerve

Cranial nerves III, IV and VI are usually tested together. The examiner typically instructs the patient to hold his head still and follow only with the eyes a finger or penlight that circumscribes a large "H" in front of the patient. By observing the eye movements and eyelids, the examiner is able to obtain more information about the extraocular muscles, the levator palpebrae superioris muscle, and cranial nerves III, IV, and VI.

Trochlear Nerve

The fourth of twelve cranial nerves, the trochlear nerve controls the function of the superior oblique muscle, which rotates the eye away from the nose and also moves the eye downward. It is the only cranial nerve to originate from the dorsal part of the brainstem. It is also the smallest of the cranial nerves.

A brief survey of neurology and physiology texts and websites reveals much confusion about the role of the superior oblique muscle, with many sources claiming that its role is to move the eye towards the nose. In fact, because of its positioning, it is able to rotate the eye away from the nose so that when the eye is already adducted (looking directly "inwards") its rotational action turns the pupil downwards to look towards the mouth, which many texts misinterpret as its primary action.

This nerve is sometimes called the 'pathetic' nerve as loss of its function paralyses the superior oblique muscle giving the eyes a pathetic look.

Trigeminal nerve

The trigeminal nerve (the fifth cranial nerve, also called the fifth nerve or simply V) is responsible for sensation in the face. It is similar to the spinal nerves C2-S5 that are responsible for sensation in the rest of the body. Sensory information from the face and body is processed by parallel pathways in the central nervous system.

The fifth nerve is primarily a sensory nerve, but it also has certain motor functions (biting, chewing and swallowing). These are discussed separately.

The trigeminal nerve is the largest of the cranial nerves. Its name derives from the fact that it has three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2) and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.
In classical anatomy, the trigeminal nerve is said to have general somatic afferent (sensory) components, as well as special visceral efferent (motor) components. The motor branches of the trigeminal nerve control the movement of eight muscles:

Masseter
Temporalis
Medial pterygoid
Lateral pterygoid
Tensor veli palatini
Mylohyoid
Anterior belly digastric
Tensor tympani

With the exception of tensor tympani, all of these muscles are involved in biting, chewing and swallowing. All have bilateral cortical representation. A central lesion (e.g., a stroke), no matter how large, is unlikely to produce any observable deficit. However, injury to the peripheral nerve can cause paralysis of muscles on one side of the jaw. The jaw deviates to the paralyzed side when it opens.

Wallenberg Syndrome

Wallenberg syndrome (also called the lateral medullary syndrome) is a classic clinical demonstration of the anatomy of the fifth nerve. It provides a useful summary of essential points about the processing of sensory information by the trigeminal nerve.

A stroke usually affects only one side of the body. If a stroke causes loss of sensation, the deficit will be lateralized to the right side or the left side of the body. The only exceptions to this rule are certain spinal cord lesions and the medullary syndromes, of which Wallenberg syndrome is the most famous example. In Wallenberg syndrome, a stroke causes loss of pain/temperature sensation from one side of the face and the other side of the body.

The explanation involves the anatomy of the brainstem. In the medulla, the ascending spinothalamic tract (which carries pain/temperature information from the opposite side of the body) is adjacent to the descending spinal tract of the fifth nerve (which carries pain/temperature information from the same side of the face). A stroke that cuts off the blood supply to this area (e.g., a clot in the vertebral artery) destroys both tracts simultaneously. The result is loss of pain/temperature sensation (but not touch/position sensation) in a unique “checkerboard” pattern (ipsilateral face, contralateral body) that is entirely diagnostic.

Abducens Nerve

The sixth of twelve cranial nerves, the abducens nerve is a motor nerve that innervates the lateral rectus muscle and therefore controls each eye's ability to abduct (move away from the midline). The abducens nerve emerges from the ipsilateral abducens nucleus between the caudal pons beneath the floor of the fourth ventricle and the medulla (the pontomedullary junction). The abducens nerve exits the skull through the superior orbital fissure, one of the holes in the skull behind the eye.

Facial Nerve

The facial nerve is seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and taste to the anterior two-thirds of the tongue.

Its main function is motor control of most of the facial muscles and the stapedius muscle of the middle ear. It also supplies parasympathetic fibers to the submandibular gland and sublingual glands via chorda tympani nerve and the submandibular ganglion, and to the lacrimal gland via the pterygopalatine ganglion. In addition, it receives taste sensations from the anterior two-thirds of the tongue and sends them to the nucleus of solitary tract. There is also a small amount of cutaneous sensation carried by the nervus intermedius from the skin in and around the auricle (earlobe).

Five branches

The five main branches of the facial nerve are, from top to bottom:

Temporal branch of the facial nerve
Zygomatic branch of the facial nerve
Buccal branch of the facial nerve
Marginal mandibular branch of the facial nerve
Cervical branch of the facial nerve

A helpful mnemonic device for remembering the major branches is the phrase, "To Zanzibar By Motor Car".

Testing the facial nerve

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly, pursing the lips and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.

In an upper motor neuron lesion, only the lower part of the face will be affected, due to the bilateral control to the upper facial muscles.

Taste can be tested on the anterior of the tongue, this can be tested with a swab dipped in a flavored solution, or with electronic stimulation (similar to putting your tongue on a battery).

Facial Nerve Pathology

People may suffer from acute facial nerve paralysis, which is usually manifested by facial paralysis. Bell's palsy is one type of idiopathic acute facial nerve paralysis, which is more accurately described as a multiple cranial nerve ganglionitis that involves the facial nerve, and most likely results from viral infection.

Vestibulocochlear Nerve

The vestibulocochlear nerve is the eighth of twelve cranial nerves and also known as the auditory nerve or acoustic nerve. It is the nerve along which the sensory cells (the hair cells) of the inner ear transmit information to the brain. It consists of the cochlear nerve, carrying information about hearing, and the vestibular nerve, carrying information about balance. It emerges from the medulla oblongata and enters the internal acoustic meatus in the temporal bone, along with the facial nerve.

Glossopharyngeal Nerve

The glossopharyngeal nerve is the ninth of twelve cranial nerves. It exits the brainstem out from the sides of the upper medulla, just rostral (closer to the nose) to the vagus nerve.

Functions

There are a number of functions of the glossopharyngeal nerve:

It receives sensory fibers from the posterior one-third of the tongue, the tonsils, the pharynx, the middle ear and the carotid body.
It supplies parasympathetic fibers to the parotid gland via the otic ganglion.
It also supplies motor fibers to stylopharyngeus muscle and the upper pharyngeal muscles.

Vagus Nerve

The vagus nerve (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. The vagus nerve is arguably the single most important nerve in the body.

The medieval Latin word vagus means literally "wandering" (the words vagrant, vagabond, and vague come from the same root). It is also called the pneumogastric nerve since it innervates both the lungs and the stomach.

The vagus nerve supplies sensory parasympathetic fibers to all the organs except the suprarenal glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, namely:

Levator veli palatini muscle
Salpingopharyngeus muscle
Palatoglossus muscle
Palatopharyngeus muscle
Superior, middle and inferior pharyngeal constrictors
Muscles of the larynx (speech).

This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve) and keeping the larynx open for breathing. It also receives some sensation from the outer ear, via the Auricular branch (also known as Alderman's nerve) and part of the meninges.

Parasympathetic innervation of the heart is mediated by the vagus nerve. The right vagus innervates the Sinoatrial node. Parasympathetic hyperstimulation predisposes those affected to bradyarrhythmias. The left vagus when hyperstimulated predisposes the heart to Atrioventricular (AV) blocks.

Accessory Nerve

The accessory nerve (or "Spinal accessory nerve") is the eleventh of twelve cranial nerves. It leaves the cranium through the jugular foramen along with the glossopharyngeal nerve (IX) and vagus nerve (X). It innervates the sternocleidomastoid muscle (sternomastoid) and trapezius muscle on the ipsilateral side.

There are two parts to the accessory nerve:

A spinal part, that innervates the muscles around the neck.
A cranial part, that splits off, and quickly combines with the vagus nerve. The cranial part of nerve XI can be thought of doing the exact same things as the vagus.

Testing the accessory nerve

Getting a person to shrug their shoulders while you push down tests trapezius. When a person turns their head, especially against force, sternocleidomastoid should be prominent.

Hypoglossal Nerve

The hypoglossal nerve is the twelfth cranial nerve. The nerve arises from the hypoglossal nucleus and emerges from the medulla oblongata in the preolivary sulcus separating the olive and the pyramid. It then passes through the hypoglossal canal. On emerging from the hypoglossal canal, the nerve picks up a branch from the anterior ramus of C1. It spirals behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. After passing deep to the posterior belly of the digastric muscle, it passes to the tongue.

It supplies motor fibers to all of the muscles of the tongue, except the palatoglossus muscle, which is innervated by the vagus nerve (X) and the accessory nerve (XI).

Aside from the tongue, the hypoglossal nerve also controls, via the ansa cervicalis, thyrohyoid muscle, omohyoid muscle, sternothyroid muscle and sternohyoid muscle. The nerve fibers supplying these muscles all come from the C1 contribution.

Testing the hypoglossal nerve
To test the function of the nerve, a person is asked to poke out their tongue. If there is a loss of function on one side (unilateral paralysis) the tongue will point towards the affected side.

Peripheral Nervous System

The peripheral nervous system or PNS, is part of the nervous system, and consists of the nerves and neurons that reside or extend outside the central nervous system--to serve the limbs and organs, for example. Unlike the central nervous system however, the PNS is not protected by bone or the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

Naming of specific nerves

The 10 out of the 12 cranial nerves originate from the brainstem and mainly control the functions of the anatomic structures of the head with some exceptions. CN X receives visceral sensory information from the thorax and abdomen, and CN XI is responsible for innervating the sternocleidomastoid and trapezius muscles, neither of which are exclusively in the head.

Spinal nerves take their origins from the spinal cord. They control the functions of the rest of the body. In humans, there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The naming convention for spinal nerves is to name it after the vertebra immediately above it. Thus the fourth thoracic nerve originates just below the fourth thoracic vertebra. This convention breaks down in the cervical spine. The first spinal nerve originates above the first cervical vertebra and is called C1. This continues down to the last cervical spinal nerve, C8. There are only 7 cervical vertebrae and 8 cervical spinal nerves.

Cervical spinal nerves (C1-C4)

The first four cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve the neck and back of head. Spinal nerve C1 is called the suboccipital nerve which provides motor innervation to muscles at the base of the skull. C2 and C3 form many of the nerves of the neck, providing both sensory and motor control. These include the greater occipital nerve which provides sensation to the back of the head, the lesser occipital nerve which provides sensation to the area behind the ears, the greater auricular nerve and the lesser auricular nerve. See occipital neuralgia. The phrenic nerve arises from nerve roots C3, C4 and C5. It innervates the diaphragm, enabling breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.

Brachial plexus (C5-T1)

The last 4 cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the brachial plexus, or plexus brachialis, a tangled array of nerves, splitting, combining and recombining, to form the nerves that subserve the arm and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable, with little variation between people

Other thoracic spinal nerves (T3-T12)

The remainder of the thoracic spinal nerves, T3 through T12, do little recombining. They form the intercostal nerves, so named because the run between the ribs. For points of reference, the 7th intercostal nerve terminates at the lower end of the sternum, also known as the xiphoid process. The 10th intercostal nerve terminates at the umbilicus, aka the belly button.

Pelvis and Perineal Nerves

Lumbar spinal nerves
Sacral spinal nerves
Coccygeal spinal nerves

Autonomic Nervous System

The autonomic nervous system (ANS) is the part of the nervous system of the higher life forms that is not consciously controlled. It is commonly divided into two usually antagonistic subsystems: the sympathetic and parasympathetic nervous system, and involves the homeostasis of organs and physiological functions.

A third and less commonly considered part of the autonomic nervous system is the enteric nervous system, which controls the digestive organs, and is, for the most part, independent of central nervous system (CNS) input.

In general, the parasympathetic nervous system is involved with digestion and energy conservation, while the sympathetic nervous system is involved with energy expenditure and the 'fight or flight' response.

Function

The autonomic nervous system regulates bodily functions and the activity of specific organs. As examples, the ANS plays a role in the diameter of blood vessels, heart rate, force of contraction of the heart, diameter of the pupils, salivation, perspiration, bronchiole diameter, peristaltic movements in the intestine, sphincter diameter, erection, ejaculation, and parturition.

Although the bodily functions that the ANS regulates are typically portrayed as being involuntary, they are not completely outside our awareness, and some schools of thought believe that one's state of mind impacts the functioning of the ANS. It remains open to debate whether the term 'involuntary nervous system' is a precise description of the ANS. Many autonomic functions are beyond conscious control, but others are impacted voluntarily including the sphincters in urination (micturition).

The autonomic nervous system is divided into subsystems: the sympathetic (SNS) and the parasympathetic (PNS) nervous systems. The SNS and PNS often have opposing effects in the same organs or physiological systems, and the ANS is a major factor in homeostasis.

Sympathetic Nervous System

The sympathetic nervous system (SNS) is part of the autonomic nervous system (ANS), which also includes the parasympathetic nervous system (PNS).

Function

The sympathetic nervous system activates what is often termed the fight or flight response. This response is also known as sympathetico-adrenal response of the body, as the pre-ganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.

Parasympathetic Nervous System

The parasympathetic nervous system is one of three divisions of the autonomic nervous system. Sometimes called the rest and digest system, the parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.

Relationship to sympathetic
While an oversimplification, it is said that the parasympathetic system acts in a reciprocal manner to the effects of the sympathetic nervous system; in fact, in some tissues innervated by both systems, the effects are synergistic.

Receptors
The parasympathetic nervous system uses only acetylcholine (ACh) as its neurotransmitter. The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. Most transmissions occur in two stages: When stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on nicotinic receptors of the postganglionic nerve. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ.

Somatic Nervous System

The somatic nervous system is that part of the peripheral nervous system associated with the voluntary control of body movements through the action of skeletal muscles, and also reception of external stimuli. The somatic nervous system consists of afferent fibers that receive information from external sources, and efferent fibers that are responsible for muscle contraction.

The basic route of the efferent somatic nervous system includes a two neuron sequence. The first is the upper motor neuron, whose cell body is located in the precentral gyrus (Brodman Area 4) of the brain. It receives stimuli from this area to control skeletal (voluntary) muscle. The upper motor neuron carries this stimulus down the corticospinal tract and synapses in the ventral horn of the spinal cord with the alpha motor neuron, a lower motor neuron. The upper motor neuron releases acetylcholine from its axon terminal knobs and these are received by nicotinic receptors on the alpha motor neuron. The alpha motor neurons cell body sends the stimulus down its axon via the ventral root of the spinal cord and proceeds to its neuromuscular junction of its skeletal muscle. There, it releases acetylcholine from its axon terminal knobs to the muscles nicotinic receptors, resulting in stimulus to contract the muscle.

Central Nervous System

The CNS is covered by the meninges, the brain is protected by the skull and the spinal cord by the vertebrae. The rhombencephalon gives rise to the pons, the cerebellum and the medulla oblongata, its cavity becomes the fourth ventricle. The mesencephalon gives rise to the tectum, pretectum, cerebral peduncle and its cavity develops into the mesencephalic duct or cerebral aqueduct. The diencephalon gives rise to the subthalamus, hypothalamus, thalamus and epithalamus, its cavity to the third ventricle. Finally, the telencephalon gives rise to the striatum (caudate nucleus and putamen), the hippocampus and the neocortex, its cavity becomes the lateral (first and second) ventricles.

Parts of the CNS

Central Nervous System Infection

Central nervous system infections are those infections of the central nervous system (CNS). There are four main causes of infections of the nervous system: bacterial, viral, fungal and protozoal.

Bacterial infections can be pyogenic infections (e.g., meningitis; brain abscess; subdural and epidural abscesses), tuberculosis, neurosyphilis, or leprosy.

Viral infections may be meningitis, encephalitis, poliomyelitis, slow virus infections, acquired immune deficiency syndrome (AIDS), and post-infectious syndromes.

Fungal infections may be meningitis or meningoencephalitis, brain abscess, or spinal epidural infection.

Protozoal infections include toxoplasmosis, malaria or amoebic infection.

Brain

In animals, the brain, or encephalon (Greek for "in the head"), is the control center of the central nervous system. In most animals, the brain is located in the head close to the primary sensory apparatus and the mouth. While all vertebrate nervous systems have a brain, invertebrate nervous systems have either a centralized brain or collections of individual ganglia. The brain is extremely complex; the human brain contains 100 billion or more neurons, each linked to as many as 10,000 others[1]. This enormous number of interconnections, however, does not indicate intelligence.

Overview

The brain is not only important as the site of reason and intelligence, it is also the source of cognition, emotion, memory, and motor, and other forms of learning, and it controls and coordinates most sensory systems, movement, behavior, but it also controls homeostatic body functions such as heart rate, blood pressure, fluid balance, and body temperature. Some behaviors such as simple reflexes and basic locomotion, can be executed under spinal cord control alone.

Occupying the skull cavity (cranium), the adult human brain normally weighs from 2 1/4 to 3 1/4 lb (1–1.5 kg). Differences in weight and size do not correlate with differences in mental ability; an elephant's brain weighs more than four times that of a human. In invertebrates a group of ganglia or even a single ganglion may serve as a rudimentary brain.

Most brains exhibit a visible distinction between grey matter and white matter. Grey matter consists of the cell bodies of the neurons, while white matter consists of the fibers (axons) that connect neurons. The axons are surrounded by a fatty insulating sheath called myelin, giving the white matter its distinctive color. The outer, visible layers of the brain are the cortex, and consist mainly of grey matter.

By means of electrochemical impulses the brain directly controls conscious or voluntary behavior, such as walking and thinking. It also monitors, through feedback circuitry, most involuntary behavior: connections with the autonomic nervous system enable the brain to adjust heartbeat, blood pressure, fluid balance, posture, and other functions : and influences automatic activities of the internal organs. There are no pain receptors in brain tissue. A headache is felt because of sensory impulses coming chiefly from the meninges or scalp.

The study of the brain is known as neuroscience, a field of biology aimed at understanding the functions of the brain at every level, from the molecular up to the psychological.

The human brain is the anteriormost part of the central nervous system in humans as well as the primary control center for the peripheral nervous system.

Anatomically the brain has three major parts, the hindbrain (including the cerebellum and the brain stem), the midbrain, and the forebrain (including the diencephalon and the cerebrum). Every brain area has an associated function, although many functions may involve a number of different areas. The cerebellum coordinates muscular movements and, along with the midbrain, monitors posture. The brain stem, which incorporates the medulla and the pons, monitors involuntary activities such as breathing and vomiting.

The thalamus, which forms the major part of the diencephalon, receives incoming sensory impulses and routes them to the appropriate higher centers. The hypothalamus, occupying the rest of the diencephalon, regulates heartbeat, body temperature, and fluid balance. Above the thalamus extends the corpus callosum, a neuron-rich membrane connecting the two hemispheres of the cerebrum.

The cerebrum, occupying the topmost portion of the skull, is by far the largest sector of the brain. Split vertically into left and right hemispheres, it appears deeply fissured and grooved. Its upper surface, the cerebral cortex, contains most of the master controls of the body. In the cortex ultimate analysis of sensory data occurs, and motor impulses originate that initiate, reinforce, or inhibit the entire spectrum of muscle and gland activity. The parts of the cerebrum intercommunicate through association tracts consisting of connector neurons. Association neurons account for approximately half of the total number of nerve cells in the brain. The tracts are believed to be involved with reasoning, learning, and memory. The left half of the cerebrum controls the right side of the body; the right half controls the left side.

Other important parts of the brain include the pituitary gland, the basal ganglia, and the reticular activating system (RAS). The pituitary participates in growth regulation. The basal ganglia, located just above the diencephalon in each cerebral hemisphere, handle coordination and habitual but acquired skills like chewing and playing the piano. The RAS forms a special system of nerve cells linking the medulla, pons, midbrain, and cerebral cortex. The RAS functions as a sentry. In a noisy crowd, for example, the RAS alerts a person when a friend speaks and enables that person to ignore other sounds.

Nerve fibers in the brain are sheathed in a near-white substance called myelin and form the white matter of the brain. Nerve cell bodies, which are not covered by myelin sheaths, form the gray matter. The billions of nerve cells in the brain are structurally supported by the hairlike filaments of glial cells. Smaller than nerve cells and ten times as numerous, the glia account for an estimated half of the brain's weight. Cranial blood vessels in the brain have certain selective permeability characteristics that largely constitute the "blood-brain barrier." The entire brain is enveloped in three protective sheets known as the meninges, continuations of the membranes that wrap the spinal cord. The two inner sheets enclose a shock-absorbing cushion of cerebrospinal fluid.

The brain controls "lower" or involuntary activities such as heartbeat, respiration, and digestion - these are known as autonomic functions. The brain also controls "higher" order, conscious activities, such as thought, reasoning, and abstraction. The human brain is generally regarded as more capable of these higher order activities than any other species.

Function

The human brain is the source of the conscious, cognitive mind. The mind is the set of cognitive processes related to perception, interpretation, imagination, and memories, of which a person may or may not be aware. Beyond cognitive functions, the brain regulates autonomic processes related to essential body functions such as respiration and heartbeat.

Glossary of Anatomical Terminology, Definitions And Abbreviations

This is a glossary of anatomical terminology, definitions and abbreviations pertinent to the description of the Central Nervous System.
AC Anterior Commissure.
Afferent An afferent fibre is a fibre originating at the present point. For example a striatal afferent is an afferent originating at the striatum. See Efferent.
Anatomy The determination of the regions in an organism that are to be considered its "parts". From the Latin anatomia, dissection, from the Greek, anatome, where ana means up, and temnein means to cut.
Annectent Gyrus A small gyrus hidden in the depth of a sulcus. Also known as transition gyrus.
Axial A plane orthogonal to the superior-inferior axis. In the brain, it shows left and right, face and back of the head. Equivalent to Horizontal.

Calcarine Sulcus.
Caudal Towards the tail.
CC Corpus Callosum.
Cingulate Sulcus.
Commissure A groupe of fibers that cross the sagittal plane.
Contralateral At the opposite side.
Coronal A plane orthogonal to the anterior-posterior axis. In the brain, it shows left and right, the top of the head and the neck.
CS Central Sulcus.
Dorsal Towards the back (dorsum).
DTI Diffusion tensor imaging.
DWI Diffusion weighted imaging.
Efferent An efferent fibre is a fibre that arrives at the present point. For example, a cortical efferent is a fibre coming from somewhere else, and arriving to the cortex. See also Afferent.
EPI Echo planar imaging.
Fissure A deep groove produced by opercularisation like the Sylvian fissure, or by the differentiation of the telencephalic vesicles like the Medial (Interhemispheric) Fissure.
Fundus pl Fundi. The bottom of an inward fold.
Gyral Crown The top of a Gyrus.
pl. Gyri. An outward fold (a hill), as for example the gyri of the cerebellar or cerebral cortex. Important gyri are the precentral and postcentral gyri, for example.
Horizontal
A plane orthogonal to the superior-inferior axis. In the brain, it shows left and right, face and back of the head. Equivalent to Axial.
IFG Inferior Frontal Gyrus.
IFS Inferior Frontal Sulcus.
IPS Intraparietal Sulcus.
Ipsilateral At the same side. See Contralateral.
ITS Inferior Temporal Sulcus.
Mesencephalic duct aka Cerebral aqueduct or Aqueduct of Sylvius, is part of the liquidean system of the CNS.
Medial At the middle. For example, the medial cortex is the cortex near the plane that divides left and right hemispheres;
Mesial Directed towards the middle.
MRI Magnetic Resonance Imaging.
Operculum pl. Opercula.
Parasagittal A plane parallel to the sagittal plane (which divides left and right sides). See Sagittal, Anatomical localization.
Posterior Commissure.
Postcentral Sulcus.
Precentral Sulcus.
Putamen.
Rostral Towards the face.
Sagittal A plane that divides exactly the left and right sides. See Parasagittal, Anatomical localization.
SFS Superior Frontal Sulcus.
STS Superior Temporal Sulcus.
Sulcal Fundus pl Sulcal Fundi. The bottom of an inward fold. See Sulcus.
Sulcal Wall One of the two sides of a sulcus (an inward fold). See Sulcus.
Sulcus pl. Sulci. An inward fold (a valley), as for example the central and calcarine sulci of the cerebral cortex. Examples of important sulci are the central sulcus (CS) and the alcarine sulcus (CalcS).
Thalamus.
Transition Gyrus A small gyrus hidden in the depth of a sulcus. Also known as annectent gyrus.
Ventral Towards the belly.