Nervous System — revision sheet

The nervous system detects changes inside and outside the body, integrates that information against memory and goals, and commands muscles and glands within milliseconds. It comprises the central nervous system (brain and spinal cord) and the peripheral nervous system (12 pairs of cranial nerves, 31 pairs of spinal nerves, their ganglia and plexuses), with an autonomic division that runs the viscera largely below awareness.

Adult brain mass: ≈1.2–1.4 kg (≈2% of body mass)Neurons in the brain: ≈86 billion (≈69 bn cerebellum, ≈16 bn cerebral cortex)Glia : neuron ratio (brain): ≈1 : 1Cerebral blood flow: ≈750 mL/min (≈50 mL/100 g/min; ≈15% of cardiac output)Brain share of resting O₂ use: ≈20%CSF volume / production: ≈150 mL / ≈500 mL per dayNormal intracranial pressure (adult, supine): ≈5–15 mmHgResting membrane potential (neuron): ≈ −70 mVAP threshold / peak: ≈ −55 mV / ≈ +30 mVFastest conduction (Aα fibres): ≈70–120 m/s (C fibres 0.5–2 m/s)Spinal cord: ≈42–45 cm; ends at L1–L2 (conus medullaris)Nerve pairs: 12 cranial; 31 spinal (8 C, 12 T, 5 L, 5 S, 1 Co)

Organisation

Central nervous system Region: The brain and spinal cord: the integrating centre of the nervous system, enclosed in the skull and vertebral canal and cushioned by meninges and cerebrospinal fluid.
Peripheral nervous system Region: All nervous tissue outside the brain and spinal cord: 12 pairs of cranial nerves, 31 pairs of spinal nerves, their ganglia, plexuses and sensory endings.
Somatic nervous system Nerve group: The part of the PNS serving the body wall and limbs: somatic sensation from skin, muscles and joints, and voluntary control of skeletal muscle through single lower motor neurons.
Autonomic nervous system Nerve group: The visceral motor system controlling smooth muscle, cardiac muscle and glands through two-neuron chains, organised into sympathetic, parasympathetic and enteric divisions and directed by the hypothalamus and brainstem.
Sympathetic nervous system Nerve group: The thoracolumbar (T1–L2) division of the ANS that mobilises the body for exertion, stress and thermoregulation through noradrenergic postganglionic fibres and adrenaline from the adrenal medulla.
Parasympathetic nervous system Nerve group: The craniosacral division of the ANS (CN III, VII, IX, X and S2–S4) that promotes digestion, conservation of energy and visceral emptying, using long preganglionic fibres, terminal ganglia and acetylcholine on muscarinic receptors.
Enteric nervous system Plexus: An intrinsic network of hundreds of millions of neurons in the wall of the gut, from oesophagus to anus, that can independently control motility, secretion and local blood flow and is modulated by sympathetic and parasympathetic input.

Brain

Brain Organ: The ≈1.3 kg organ within the skull containing ≈86 billion neurons, made up of the cerebrum, diencephalon, brainstem and cerebellum; it generates perception, movement, cognition and consciousness and regulates homeostasis.
Cerebrum Organ part: The largest part of the brain: two cerebral hemispheres, each with a folded cortex, underlying white matter, basal nuclei and limbic structures, responsible for perception, voluntary movement and all higher mental functions.
Cerebral cortex Brain region: The 2–4 mm-thick folded sheet of grey matter covering the cerebral hemispheres; mostly six-layered neocortex holding ≈16 billion neurons, organised into primary, unimodal and multimodal association areas.

Cerebral lobes

Frontal lobe Brain region: The largest lobe, anterior to the central sulcus and above the lateral sulcus; it contains the primary motor, premotor and supplementary motor cortex, the frontal eye field, Broca's area and the prefrontal cortex for executive function and personality.
Parietal lobe Brain region: The lobe between the central and parieto-occipital sulci, containing the primary somatosensory cortex (postcentral gyrus) and posterior parietal association cortex for spatial attention, body schema, praxis, reading and calculation.
Temporal lobe Brain region: The lobe below the lateral sulcus, housing the primary auditory cortex, Wernicke's area for language comprehension, the ventral visual "what" stream, and medial structures (hippocampus, amygdala) for memory and emotion.
Occipital lobe Brain region: The posterior pole of each hemisphere, devoted to vision: the primary visual (striate) cortex lines the calcarine sulcus, and the surrounding visual association areas analyse form, colour and motion.
Insula Brain region: A cortical lobe hidden deep within the lateral sulcus, covered by the frontal, parietal and temporal opercula; it integrates interoceptive, visceral, gustatory, pain and emotional information.

Deep structures & limbic system

Limbic lobe Brain region: The C-shaped ring of cortex on the medial surface of each hemisphere that borders the corpus callosum and diencephalon, made up mainly of the cingulate and parahippocampal gyri, and the cortical core of the limbic system.
Limbic system Neural pathway: A functional network of cortical and subcortical structures (limbic lobe, hippocampus, amygdala, septal nuclei, mammillary bodies, anterior thalamus, nucleus accumbens and hypothalamus) underlying emotion, memory, motivation and their autonomic and endocrine expression.
Hippocampus Brain region: A curved ridge of three-layered archicortex in the floor of the temporal horn of the lateral ventricle, essential for forming new declarative (episodic) memories and for spatial navigation.
Amygdala Brain region: An almond-shaped complex of nuclei in the anteromedial temporal lobe that detects threat and emotional salience, drives fear learning, and triggers the autonomic, endocrine and behavioural responses to emotion.
Corpus callosum Brain region: The largest commissure of the brain: a broad arched band of ≈200 million axons in the floor of the longitudinal fissure connecting corresponding areas of the two cerebral hemispheres.
Internal capsule Brain region: A compact V-shaped sheet of projection fibres between the thalamus and caudate nucleus medially and the lentiform nucleus laterally, carrying almost all traffic between the cortex and the brainstem and spinal cord; a small lesion here causes dense contralateral hemiplegia.
Basal nuclei Brain region: Deep grey-matter nuclei of the cerebrum (caudate, putamen, globus pallidus), functionally linked with the subthalamic nucleus and substantia nigra, that select and scale movements, form habits and modulate cognition and motivation through cortico-basal-thalamo-cortical loops.

Diencephalon

Diencephalon Brain region: The central core of the forebrain surrounding the third ventricle, made up of the thalamus, hypothalamus, epithalamus and subthalamus; it relays almost all information to the cortex and links the nervous and endocrine systems.
Thalamus Brain region: A paired egg-shaped mass of nuclei forming most of the diencephalon; the obligatory relay through which almost all sensory, motor and limbic information reaches the cortex, and a regulator of arousal and attention.

Brainstem & cerebellum

Brainstem Brain region: The stalk of the brain (midbrain, pons and medulla oblongata) connecting the forebrain to the spinal cord; it carries every ascending and descending tract, contains the nuclei of cranial nerves III–XII and the vital cardiorespiratory centres, and sustains consciousness.
Midbrain Brain region: The shortest and most rostral part of the brainstem (≈2 cm), passing through the tentorial notch; it contains the nuclei of CN III and IV, the red nucleus, substantia nigra and periaqueductal grey, and the colliculi for visual and auditory reflexes.
Substantia nigra Brain region: A pigmented nucleus in the midbrain between the cerebral peduncle and tegmentum; its pars compacta supplies dopamine to the striatum and its loss causes Parkinson disease.
Pons Brain region: The middle part of the brainstem, a bulging "bridge" of transverse fibres to the cerebellum; its tegmentum holds the nuclei of CN V, VI, VII and VIII, the horizontal gaze centre and respiratory centres.
Medulla oblongata Brain region: The lowest part of the brainstem (≈3 cm), continuous with the spinal cord at the foramen magnum; it contains the pyramidal and sensory decussations, the nuclei of CN IX–XII and the vital centres for breathing, heart rate and blood pressure.
Reticular formation Brain region: A diffuse network of interconnected neurons running through the core of the brainstem that controls arousal and consciousness, muscle tone and posture, autonomic and respiratory rhythms, and pain transmission.
Cerebellum Brain region: The "little brain" in the posterior cranial fossa (≈10% of brain mass but ≈80% of its neurons); it compares intended with actual movement to coordinate posture, balance, gait, eye movements and skilled motor learning, acting on the same side of the body.

Meninges, ventricles & CSF

Meninges Membrane: The three connective-tissue membranes enclosing the brain and spinal cord: tough outer dura mater, delicate avascular arachnoid mater and the thin vascular pia mater that clings to the neural surface. CSF fills the subarachnoid space between arachnoid and pia.
Dura mater Membrane: The thick, tough outermost meninx; in the skull it has periosteal and meningeal layers that separate to enclose the dural venous sinuses and fold inwards as the falx cerebri and tentorium cerebelli; in the vertebral canal it forms a single tube ending at S2.
Arachnoid mater Membrane: The thin, avascular middle meninx lying against the inner surface of the dura; its barrier cell layer seals the CSF in, its trabeculae span the subarachnoid space, and its granulations return CSF to the venous sinuses.
Pia mater Membrane: The delicate, vascular innermost meninx closely applied to the brain and spinal cord, following every gyrus and sulcus; it forms the denticulate ligaments and filum terminale and, with ependyma, the tela choroidea.
Subarachnoid space Space: The CSF-filled space between the arachnoid and pia surrounding the brain and spinal cord; it carries the major cerebral vessels and widens into cisterns, including the lumbar cistern used for lumbar puncture.
Ventricles of the brain Space: Four interconnected CSF-filled cavities within the brain (two lateral ventricles, the third and fourth ventricles) lined by ependyma, containing the choroid plexus, and continuous with the central canal of the spinal cord and, through three apertures, with the subarachnoid space.
Lateral ventricles Space: The largest ventricles: a C-shaped CSF cavity in each cerebral hemisphere, with a frontal horn, body, atrium and occipital and temporal horns, communicating with the third ventricle through the interventricular foramen.
Third ventricle Space: The narrow midline slit-like cavity of the diencephalon between the two thalami and hypothalami; it receives CSF from both lateral ventricles and drains into the cerebral aqueduct.
Cerebral aqueduct Space: The narrow (≈1–2 mm) channel through the midbrain that carries CSF from the third to the fourth ventricle; the narrowest point of the ventricular system and the commonest site of congenital obstruction.
Fourth ventricle Space: A tent-shaped cavity of the hindbrain between the pons and medulla in front and the cerebellum behind; its diamond-shaped floor (rhomboid fossa) carries the cranial nerve nuclei, and its three apertures release CSF into the subarachnoid space.
Choroid plexus Tissue: Fronds of fenestrated capillaries covered by a specialised cuboidal epithelium projecting into the ventricles; it actively secretes most of the CSF and its tight-junctioned epithelium forms the blood–CSF barrier.
Cerebrospinal fluid Fluid: The clear, colourless fluid (≈150 mL, renewed 3–4 times a day) that fills the ventricles and subarachnoid space; it buoys and cushions the CNS, stabilises its chemical environment and clears waste, and its analysis is a key diagnostic test.
Arachnoid granulations Organ part: Protrusions of the arachnoid through the dura into the dural venous sinuses, especially the superior sagittal sinus, that act as pressure-dependent one-way valves returning CSF to the venous blood.
Blood–brain barrier Tissue: The selective barrier formed by tight-junctioned, non-fenestrated brain capillary endothelium (supported by pericytes and astrocyte end-feet) that tightly controls what passes from blood into brain tissue.

Spinal cord

Spinal cord Spinal structure: The caudal part of the CNS, a cylindrical cord ≈42–45 cm long running from the foramen magnum to about the L1–L2 vertebral level. It carries ascending sensory and descending motor tracts and contains the segmental circuits for spinal reflexes.
Conus medullaris Spinal structure: The tapered, cone-shaped lower end of the spinal cord, lying at about the L1–L2 vertebral level in adults. It contains the sacral and coccygeal segments that control the bladder, bowel and sexual function.
Filum terminale Spinal structure: A slender thread of pia mater, ≈20 cm long, that extends from the apex of the conus medullaris to the dorsum of the coccyx. It anchors the lower spinal cord.
Cauda equina Nerve group: The bundle of lumbar, sacral and coccygeal nerve roots (L2–Co1) that descends through the lumbar cistern below the conus medullaris. It is named for its resemblance to a horse's tail.
Grey matter of the spinal cord Spinal structure: The H- or butterfly-shaped central core of the spinal cord. It contains neuronal cell bodies organised into posterior (sensory), lateral (autonomic) and anterior (motor) horns, and into Rexed laminae I–X.
White matter of the spinal cord Spinal structure: The peripheral mantle of myelinated axons surrounding the spinal grey matter. It is organised into posterior, lateral and anterior funiculi, which contain the ascending and descending tracts.
Dorsal root ganglion Ganglion: An oval swelling on each dorsal root, lying in or near the intervertebral foramen. It contains the cell bodies of the pseudounipolar primary sensory neurons. No synapses occur within it.

Spinal cord — tracts

Dorsal column–medial lemniscus pathway Neural pathway: The three-neuron pathway for fine (discriminative) touch, vibration, pressure and conscious proprioception. It ascends ipsilaterally in the dorsal columns and crosses in the medulla.
Spinothalamic tract (anterolateral system) Neural pathway: The ascending pathway for pain, temperature, crude touch, pressure and itch. Its second-order fibres cross in the anterior white commissure within 1–2 segments of entry and ascend contralaterally to the thalamus.
Corticospinal tract Neural pathway: The principal voluntary motor pathway. Upper motor neurons in the cerebral cortex project through the internal capsule, cerebral peduncle, pons and medullary pyramid. About 85–90% of fibres cross at the pyramidal decussation to form the lateral corticospinal tract.
Spinocerebellar tracts Neural pathway: Ascending pathways that carry unconscious proprioception from muscle spindles, Golgi tendon organs and spinal interneurons to the cerebellum. Each one ultimately reaches the cerebellar hemisphere on the same side as its origin.

Spinal nerves & reflexes

Spinal nerves Nerve group: Thirty-one pairs of mixed nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal), each formed by the union of a dorsal (sensory) and a ventral (motor) root in the intervertebral foramen. Each then divides into dorsal and ventral rami.
Dermatomes Region: A dermatome is the area of skin supplied by the sensory fibres of a single spinal nerve (dorsal root). The dermatome map is used to localise root and cord lesions.
Myotomes Muscle group: A myotome is the group of muscles innervated by a single spinal segment (ventral root). Testing a key movement for each root, together with the deep tendon reflexes, localises radiculopathies and cord lesions.
Reflex arc Physiological process: The neural pathway of a reflex: a rapid, involuntary, stereotyped response to a stimulus. It runs receptor → afferent neuron → integrating centre → efferent neuron → effector.

Cranial nerves

Cranial nerves (overview) Nerve group: Twelve pairs of nerves that attach to the brain rather than the spinal cord, numbered I–XII in rostrocaudal order of their attachment. They carry sensory, motor and parasympathetic fibres for the head and neck, and through the vagus nerve they also reach the thoracic and abdominal viscera.
Olfactory nerve (CN I) Nerve: The nerve of smell (purely sensory, SVA). About 20 bundles of unmyelinated axons from olfactory receptor neurons pass through the cribriform plate of the ethmoid to synapse in the olfactory bulb.
Optic nerve (CN II) Nerve: The nerve of vision (purely sensory, SSA). It is formed by about 1.2 million retinal ganglion cell axons running from the optic disc through the optic canal to the optic chiasm. Developmentally it is a CNS tract, not a peripheral nerve.
Oculomotor nerve (CN III) Nerve: A motor nerve (GSE + GVE) supplying four of the six extraocular muscles and levator palpebrae superioris. It also carries the parasympathetic fibres that constrict the pupil and allow accommodation.
Trochlear nerve (CN IV) Nerve: A purely motor nerve (GSE) to the superior oblique muscle. It is the thinnest cranial nerve, has the longest intracranial course, is the only one to leave the dorsal brainstem, and is the only one whose fibres fully decussate.
Trigeminal nerve (CN V) Nerve: The largest cranial nerve and a mixed nerve. It is the main sensory nerve of the face, scalp, eye, nasal and oral cavities, teeth and most of the dura. Through its mandibular division it is also the motor nerve to the muscles of mastication (1st pharyngeal arch).
Abducens nerve (CN VI) Nerve: A purely motor nerve (GSE) to the lateral rectus, which abducts the eye. Its long course over the petrous apex and through the middle of the cavernous sinus makes it the cranial nerve most often affected by raised intracranial pressure.
Facial nerve (CN VII) Nerve: A mixed nerve of the 2nd pharyngeal arch. It supplies the muscles of facial expression and stapedius, carries taste from the anterior two-thirds of the tongue, and is secretomotor to the lacrimal, submandibular and sublingual glands.
Vestibulocochlear nerve (CN VIII) Nerve: A purely sensory nerve (SSA) with two parts. The cochlear nerve carries hearing and the vestibular nerve carries balance and head movement, both from the inner ear through the internal acoustic meatus to the brainstem at the cerebellopontine angle.
Glossopharyngeal nerve (CN IX) Nerve: A mixed nerve of the 3rd pharyngeal arch. It carries taste and general sensation from the posterior third of the tongue and the oropharynx, information from the carotid body and sinus, parasympathetic fibres to the parotid gland, and motor fibres to the single muscle stylopharyngeus.
Vagus nerve (CN X) Nerve: The longest and most widely distributed cranial nerve ("vagus" means wandering). It supplies the pharynx, larynx and soft palate, and carries most of the body's parasympathetic outflow to the heart, lungs and gut as far as the left colic flexure. About 80% of its fibres are afferent, carrying visceral sensation from these organs.
Accessory nerve (CN XI) Nerve: A motor nerve to sternocleidomastoid and trapezius. It arises from the upper cervical spinal cord, ascends into the skull through the foramen magnum and leaves again through the jugular foramen. Its superficial course across the posterior triangle of the neck makes it vulnerable during surgery.
Hypoglossal nerve (CN XII) Nerve: A purely motor nerve (GSE) to all the intrinsic muscles of the tongue and to all its extrinsic muscles except palatoglossus. Through it, the tongue moves for speech, chewing and swallowing.

Cervical & brachial plexuses

Cervical plexus Plexus: A network formed by the ventral rami of C1–C4, lying deep to sternocleidomastoid. It supplies the skin of the neck, ear and shoulder, the infrahyoid strap muscles (via the ansa cervicalis), and, most importantly, the diaphragm (via the phrenic nerve, C3–C5).
Phrenic nerve Nerve: The sole motor nerve to the diaphragm, arising from C3–C5 (mainly C4). It descends on the anterior scalene, through the superior thoracic aperture, and between the pericardium and pleura. It also carries sensation from the central diaphragm, the pericardium and the mediastinal pleura.
Brachial plexus Plexus: The network formed by the ventral rami of C5–T1 that supplies the whole upper limb. It is organised from medial to lateral into roots, trunks, divisions, cords and terminal branches, running from the neck through the cervicoaxillary canal into the axilla.

Upper-limb nerves

Musculocutaneous nerve Nerve: The nerve of the anterior (flexor) compartment of the arm, from the lateral cord (C5–C7). It supplies coracobrachialis, biceps brachii and brachialis, then continues as the lateral cutaneous nerve of the forearm.
Axillary nerve Nerve: A branch of the posterior cord (C5–C6). It winds around the surgical neck of the humerus through the quadrangular space to supply deltoid and teres minor, and the skin over the lower deltoid (the "regimental badge" area).
Radial nerve Nerve: The largest branch of the brachial plexus (posterior cord, C5–T1) and the nerve of all the extensor muscles of the upper limb. It spirals around the humerus in the radial groove, which makes it vulnerable to fractures, and damage causes the classic "wrist drop".
Median nerve Nerve: The main nerve of the anterior forearm and of the thumb (thenar) muscles (C5/6–T1). It forms from the lateral and medial cords, passes through the carpal tunnel, and supplies the skin of the palmar lateral three and a half digits. Compression causes carpal tunnel syndrome, the commonest entrapment neuropathy.
Ulnar nerve Nerve: A branch of the medial cord (C8–T1, often with C7). It passes behind the medial epicondyle (the "funny bone") and supplies flexor carpi ulnaris, the medial half of FDP and most of the intrinsic hand muscles. Its lesions produce a claw hand and loss of fine finger movement.
Long thoracic nerve Nerve: A branch from the roots of the brachial plexus (C5–C7) that runs down the lateral chest wall on the surface of serratus anterior, which it supplies. Injury causes medial "winging" of the scapula.

Thoracic & autonomic nerves

Intercostal nerves Nerve group: The ventral rami of T1–T11, which run in the intercostal spaces and supply the intercostal muscles, the skin of the chest wall and, through the lower nerves (T7–T11), the anterior abdominal wall. T12, below the 12th rib, is the subcostal nerve. Unlike other ventral rami, they do not form plexuses.
Sympathetic trunk Ganglion: A paired chain of interconnected paravertebral sympathetic ganglia running alongside the vertebral column from the base of the skull to the coccyx. It distributes sympathetic fibres to every spinal nerve, to the head, and to the thoracic viscera.
Splanchnic nerves Nerve group: Visceral nerves that carry autonomic fibres to the organs of the body cavities. The thoracic, lumbar and sacral splanchnic nerves carry sympathetic fibres to prevertebral ganglia. The pelvic splanchnic nerves (S2–S4) are the sacral parasympathetic outflow.

Lumbosacral plexus & lower-limb nerves

Lumbar plexus Plexus: A plexus formed within psoas major by the ventral rami of L1–L4, with a small contribution from T12. It supplies the lower abdominal wall, the groin and genital skin, and the anterior and medial thigh through the femoral and obturator nerves.
Femoral nerve Nerve: The largest branch of the lumbar plexus (L2–L4, posterior divisions). It passes beneath the inguinal ligament lateral to the femoral artery to supply the anterior thigh muscles, including quadriceps, and the skin of the anterior thigh. Its saphenous branch supplies the medial leg and foot.
Obturator nerve Nerve: The nerve of the medial (adductor) compartment of the thigh (L2–L4, anterior divisions). It runs along the lateral pelvic wall and through the obturator canal to supply the hip adductors and a patch of skin on the medial thigh.
Sacral plexus Plexus: A plexus on the posterior pelvic wall formed by the lumbosacral trunk (L4–L5) and the ventral rami of S1–S4. It supplies the gluteal region, the posterior thigh, the whole leg and foot below the knee (except the saphenous territory), and the perineum.
Superior gluteal nerve Nerve: A branch of the sacral plexus (L4–S1) that leaves the pelvis above piriformis to supply gluteus medius, gluteus minimus and tensor fasciae latae. These are the hip abductors that keep the pelvis level when walking, and their paralysis produces a positive Trendelenburg sign.
Sciatic nerve Nerve: The largest and thickest nerve in the body (L4–S3, about 2 cm wide). It leaves the pelvis below piriformis and descends through the buttock and posterior thigh, supplying the hamstrings. It then divides into the tibial and common fibular nerves, which supply every muscle below the knee.
Tibial nerve Nerve: The larger terminal branch of the sciatic nerve (L4–S3). It descends through the popliteal fossa and the posterior compartment of the leg, passes behind the medial malleolus in the tarsal tunnel, and supplies the plantarflexors, the toe flexors and, through the plantar nerves, the sole of the foot.
Common fibular nerve Nerve: The smaller terminal branch of the sciatic nerve (L4–S2). It winds superficially around the neck of the fibula and divides into the superficial and deep fibular nerves. It is the most commonly injured nerve of the lower limb, and the classic result is foot drop.
Deep fibular nerve Nerve: The motor nerve of the anterior compartment of the leg, the ankle dorsiflexors and toe extensors, and of the dorsal foot muscles. Its only cutaneous territory is the first web space between the great and second toes.
Superficial fibular nerve Nerve: The nerve of the lateral compartment of the leg. It supplies fibularis longus and brevis, the evertors of the foot, and then becomes cutaneous to supply most of the dorsum of the foot.
Pudendal nerve Nerve: The main somatic nerve of the perineum (S2–S4). It supplies the external anal and urethral sphincters and the perineal muscles, and carries sensation from the perineum and external genitalia. It is therefore central to continence and sexual function.

Neurons & glia

Neuron Cell: The excitable cell that is the structural and functional unit of the nervous system. It receives input on its dendrites and soma, integrates it at the axon hillock, and transmits action potentials along its axon to synapses on other cells.
Classification of neurons Cell: Neurons are classified structurally by the number of processes (multipolar, bipolar, pseudounipolar, anaxonic), functionally by the direction of conduction (sensory, motor, interneuron), and by fibre diameter and conduction velocity (A, B and C fibres).
Neuroglia (glial cells) Cell: The non-neuronal supporting cells of the nervous system. There are four types in the CNS (astrocytes, oligodendrocytes, microglia, ependymal cells) and two in the PNS (Schwann cells, satellite cells). They myelinate, nourish, protect and regulate neurons, and they keep the ability to divide.
Astrocyte Cell: The most abundant glial cell of the CNS. It is star-shaped, with processes whose end-feet ensheathe capillaries and synapses. Astrocytes maintain the blood–brain barrier and the ionic and neurotransmitter environment, and they form glial scars after injury.
Oligodendrocyte Cell: The myelin-forming glial cell of the CNS. Each oligodendrocyte extends several processes and myelinates one segment (internode) on each of many axons, up to ≈50.
Schwann cell Cell: The principal glial cell of the peripheral nervous system. A myelinating Schwann cell wraps a single internode of one axon, whereas non-myelinating (Remak) Schwann cells ensheathe bundles of small unmyelinated axons. Schwann cells are essential for peripheral nerve regeneration.
Microglia Cell: The resident macrophages of the CNS, making up ≈5–12% of CNS cells. They derive from yolk-sac progenitors, not neuroectoderm. They survey the brain continuously, phagocytose debris and pathogens, and prune synapses during development.
Ependymal cell Cell: Cuboidal to columnar ciliated glial cells that line the ventricles of the brain and the central canal of the spinal cord. Modified ependymal cells form the CSF-secreting epithelium of the choroid plexus.
Satellite glial cell Cell: Flattened glial cells of the PNS that form a capsule around each neuronal cell body in sensory and autonomic ganglia. They regulate the neurons' microenvironment, in a role analogous to that of astrocytes.
Myelin sheath Membrane: A segmented, multilayered, lipid-rich wrapping of glial membrane around axons. Oligodendrocytes form it in the CNS and Schwann cells in the PNS. It insulates the internodes so that action potentials jump between the nodes of Ranvier (saltatory conduction).

Neurophysiology

Synapse Cell process: The specialised junction where a neuron transmits a signal to another neuron or to an effector cell. Most synapses are chemical, using neurotransmitter released into a 20–40 nm cleft. A minority are electrical, using gap junctions.
Resting membrane potential Physiological process: The steady voltage across the membrane of an unstimulated neuron, inside negative relative to outside, typically ≈ −70 mV. It arises mainly from K⁺ leaking out down its concentration gradient through K⁺ leak channels, and the Na⁺/K⁺-ATPase maintains the gradients.
Action potential Physiological process: A brief (≈1–2 ms), all-or-none reversal of membrane potential, from ≈ −70 mV to ≈ +30 mV and back. Voltage-gated Na⁺ and K⁺ channels generate it, and it propagates without decrement along the axon.
Neurotransmitters Molecule: Chemical messengers released from presynaptic terminals that act on specific receptors of target cells. They include amino acids (glutamate, GABA, glycine), acetylcholine, monoamines (dopamine, noradrenaline, adrenaline, serotonin, histamine), neuropeptides, purines, gases and lipid messengers.

Mnemonics

Cranial nerve names (I–XII): "Oh Oh Oh To Touch And Feel Very Good Velvet, Such Heaven" — Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, (Spinal) accessory, Hypoglossal.
Cranial nerve types (sensory / motor / both): "Some Say Marry Money But My Brother Says Big Brains Matter More" — I S, II S, III M, IV M, V B, VI M, VII B, VIII S, IX B, X B, XI M, XII M. Parasympathetic fibres travel in III, VII, IX and X ("1973").
Exit of the trigeminal divisions: "Standing Room Only" — V1 through the Superior orbital fissure, V2 through foramen Rotundum, V3 through foramen Ovale.
Brachial plexus: "Randy Travis Drinks Cold Beer" — Roots (C5–T1), Trunks (3), Divisions (6), Cords (3), Branches. The terminal branches are "MARMU": Musculocutaneous, Axillary, Radial, Median, Ulnar.
Tendon reflex roots: "S1, 2 — buckle my shoe (ankle jerk); L3, 4 — kick the door (knee jerk); C5, 6 — pick up sticks (biceps, brachioradialis); C7, 8 — lay them straight (triceps)."
The diaphragm: "C3, 4, 5 keeps the diaphragm alive" — the phrenic nerve. A complete cord injury at or above C3 abolishes spontaneous breathing.
Cerebellar signs: "DANISH" — Dysdiadochokinesia, Ataxia (wide-based gait), Nystagmus, Intention tremor, Slurred (scanning) speech, Hypotonia/past-pointing (dysmetria). The signs are ipsilateral to the lesion.
Autonomic actions: Parasympathetic "SLUDD" plus three decreases: Salivation, Lacrimation, Urination, Digestion, Defecation, with decreased heart rate, pupil size and airway diameter. Male sexual function: "Point and Shoot" — Parasympathetic erection, Sympathetic emission.

From the Corpus anatomy atlas — for education only.