A&P 1: Brain Development, Cranial Nerves and Accessory Structures

Gyri

Brain folds; allows more surface area for neurons

Sulci

Shallow depressions between gyri

Fissures

Deeper grooves between gyri

Rostral

Anterior; toward the nose

Caudal

Posterior; toward the tail

TBI

Acute brain damage due to accident or trauma; includes concussion, contusion, SIS

May cause long-term cognitive deficits, motor impairment

Progesterone may or may not hasten recovery

Concussion

Most common type of TBI; characterized by temporary, abrupt loss of consciousness after head impact from blow or sudden stop.

Causes headache, drowsiness, lack of concentration, confusion, amnesia

Cumulative effect with multiple episodes with personality changes, depression, intellectual decline as abilities are lost

Contusion

Bruising of brain due to trauma causing blook leak from small vessels into subarachnoid space

Immediate loss of consciousness <5 min, sometimes respiration abnormalities, decreased BP

May appear on CT

Second Impact Syndrome

Second brain injury occurs prior to resolution of a first injury causing more severe brain swelling, possible death

TBI should be fully healed before returning to risky activities like sports

Neurulation

Formation of nervous tissue beginning in embryo during third week of development

1) Neural plate develops neural groove central longitudinal indentation; cells along lateral margins of neural plate proliferate & become neural folds forming neural crest

2) Neural folds elevate & approach each other as neural groove deepens;

3) Neural crest cells begin to pinch off from neural folds to form other structures

4) EO 3rd week neural folds have met/fused to form neural tube with internal neural canal; zips together superiorly and inferiorly from midline

Notocord

Tightly packed group of mesoderm cells positioned on developing embryo midline

Neural plate

Thickened neural plate induced by notocord to form neural tube

Neuropores

Opening of neural tube which close at EO week 4; cranial (future head) and caudal neuropore (future spinal cord)

Folate deficiency increases risk of neural tube dfect, adequate diet/supplementation recommended for women of childbearing age due to early development of notocord

Ancephaly

Substantial or complete absense of brain and cranial bones due ot failure of cranial neuropore closure

Rare neural tube defect detected by ultrasound

Spina bifida

Caudal neuropore fails to close, often in lumbar or sacral region; 2 forms

Cystica: almost no vertebral arch forms, leaving posterior aspect of spinal cord unprotected typically with large cystic structure filled with CSF

Oculta: much more common, less serious defect of vertebral arch involving lamina and spinous process with tuft of hair; may be asymptomatic

Primary brain vesicles

Late week four: cranial neural tube forms three areas of adult brain

Prosencephalon (forebrain)

Mesencephalon (midbrain)

Rhombencephalon (hindbrain)

Secondary brain vesicles

5th week: three primary vesicles further develop into future adult brain regions

Telencephalon: forms cerebrum; develops from proencephalon and grows rapidly

Diencephalon: forms thalamus, hypothalamus, epithalamus; develops from proencephalon, becomes enveloped by telencephalon

Mesencephalon: midbrain; only primary brain vesicle that does not form new vesicles

Metencephalon: forms pons and cerebellum; develops from rhombencephalon

Myelencephalon: medulla oblongata; develops from rhombencephalon

Gray matter

Derived from color of CNS cell bodies and dendrites; designated as cortex (superficial), center (cluster) or nucleus (CNS center with discrete anatomic boundaries)

Synapses allow for integration and processing

White matter

Derives color form bundles of myelinated axons located close to superficial or deep which relay nerve impulses between different regions of brain, spinal cord, body

Tracts in CNS share common origin and destination

Funiculus in specific area of spinal cord

Peduncle in spinal cord connects two brain regions (stalklike appearance)

Cranial meninges

Three consecutive tissue layers protecting and supporting brain and blood vessels, separating bones, and aiding CSF circulation

Pia mater

Thin later of delicate areolar CT tightly adhering to brain & following contours of surface; covers small blood vessels entering brain and helps form CSF

Innermost cranial meninx

Arachnoid mater

Contains arachnoid trabeculae, delicate web of collagen & elastic fibers, superior to subarachnoid space which contains CSF; both support cerebral arteries/veins in subarachnoid space

Middle cranial meninx

Dura mater

Tough, dense, irregular CT forming meningeal layer superficial to arachnoid and periosteal layer forming periosteum on internal cranial bone surface

Dural venous sinuses: large spaces filled with blood that drain blood from brain between layers

Epidural space contains arteries & veins for meninges & bones; subdural space is below meningeal layer; either may become actual space if blood/fluid accumulates (hematoma)

Hematoma

Pooling of blood outside vessel; requires surgical treatment to relieve pressure

Epidural usually from blow to head

Subdural from ruptured vein due to fast/violent rotation of head (slower)

Cranial dura septa

Meningeal layer of dura matter extending as four flat partitions in four areas:

Falx cerebri, tentorial notch, falx cerebelli and diaphragma sellae

Sinuses on margins: superior & inferior sagittal, transverse, straight, occipital

Ventricles

Four cavities within brain filled with CSF, lined with ependymal cells and derived from neural canal; site of CSF production as fluid circulates

Cerebrum: Laterals are separated by septum pellucidum and connect to third (slender, medial) through interventricular foramen

Cerebellum/brain stem: Third connects via cerebral aqueduct to fourth (pyramidal, medial) which opens to subarachnoid space via paired lateral and single medial apertures and merges with central canal of spinal cord

Cerebrospinal fluid

Clear, color liquid which circulates in ventricles and subarachnoid space for buoyancy (>95% weight reduction), protection (movement buffer), environmental stability (nutrition, signaling, waste removal)

1/2L per day produced by ventricle ependymal cells (30%), choroid plexus (30%), interstitial fluid (40%)

Falx cerebri

Sickle-shaped vertical fold in midsagittal plane projecting into longitudinal fissure

Largest cranial dura septa

Tentorium cerebelli

Horizontal fold of dura mater separating occipital and temporal lobes from cerebellum; contains tentorial notch to allow for passage of brainstem

Falx cerebelli

Sickle shaped vertical partition separating left & right cerebellar hemispheres inferior to falx cerebri

Diaphragma sellae

Forms roof over sella turcica of sphenoid bone with small opening of infundibulum attaching pituitary gland to base of hypothalamus

Smallest cranial dural septa

Choroid plexus

Region of specialized tissue in each ventricle composed of ependymal cells and capillaries of pia mater

Blood plasma filtered across capillary wall, pia mater then is modified by ependymal cells

arachnoid villi

Fingerlike extensions of arachnoid mater projecting through dura mater into dural venous sinuses group to form arachnoid granulation.

CSF volume increases due to production increases pressure which forces fluid across vili to reach dural venous sinuses for one-way flow of excess CSF to be returned to blood

Hydrocephalus

Excessive CSF leading to brain distorion, neurologic damage when untreated often due to CSF flow obstruction or impaired absorption

Treated with shunts

blood-brain barrier

Regulates which substances can and cannot be filtered from blood to enter interstitial fluid of brain preventing exposure to harmful agents (drugs, waste, ion/hormone fluctuations.

Capillaries:

1) Contain tight junctions requiring membrane transport processes regulating transfer

2) Thickened basement membrane restricts passage

3) Capillaries wrapped in perivascular feed of astrocytes

Can be crossed by lipid-soluable substances (nicotine, alcohol, anesthetics, etc)

Missing in choroid plexus, hypothalamus, pineal gland due to production of CSF and hormones for blood transport

Olfactory nerve (CN I)

Special sensory nerve that conducts olfactory (smell) sensation from the nose to brain

Origin bipolar neurons in olfactory epithelium of nasal cavity

Nerves extend through ethmoid cribiform foramina and synapses in olfactory bulbs, which extend to primary olfactory cortex (temporal lobe) and other brain locations

Nerve damage causes Anosmia

Optic nerve (CN II)

Special sensory nerve that conducts visual info from retina to brain

Origin in retina relayed by optic nerve through optic canal of sphenoid and left/right nerves unite at optic chiasm and optic tract extends to lateral geniculae nucleus of thalamus; axons project to primary visual cortex (occipital lobe)

Nerve damage causes Anopsia

Oculomotor nerve (CN III)

Motor nerve that innervates intrinsic and most extrinsic eye muscles, upper eyelid muscle

Somatic: contracts muscles to move eye and elevate lid

Parasympathetic: contracts sphincter pupillae of iris to contract pupil and smooth muscle of ciliary body to make lens more spherical for near vision

Origin in Oculomotor and Edinger Westphal nuclei within midbrain; leaves crainium via superior orbital fissure, extending to eye/eyelid

Nerve damage causes ptosis, strabismus (deviation/not parallel), diplopia, focusing difficulty, dilated pupil

Trochlear nerve (CN III)

Motor nerve that innervates superior oblique muscle that loops through trochlea at superior eye orbit

Somatic: contracts muscle to move eye inferiorly and laterally

Origin in trochlear nucleus of midbrain; leaves cranium via superior orbital fissure extending to eye muscle

Nerve damage causes strabismus and diplopia

Trigeminal nerve (CN V)

Mixed nerve consisting of ophtalmic (V1), maxillary (V2) and mandibular (V3) divisions for touch, temperature, pressure sensation; also controls muscles of mastication muscles

V1: cornea, nose, forehead, anterior scalp, meninges

V2: nasal mucosa, palate, gums, cheek, meninges

V3: anterior 2/3 of tongues, meninges, chin skin, lower jaw, lower teeth, 1/3 sensory axons of ear auricle

Somatic: innervates temporalis, masseter, lateral & medial pterygoids, mylogyoid, anterior belly of digastric, tensor tympani and tensor veli palatini

Extends from nuclei in pons or sensory receptors through openings to trigeminal ganglia

Trigemnial neuralgia: inflammation causes minutes to hours of intense, pulsating pain

Abducens nerve (CN VI)

Motor nerve innervating lateral rectus to move eye laterally

Originates in pontine (abducens) nucleus in pons leaving cranium through superior orbital fissure to extend to muscle

Nerve damage causes limits on lateral eye movement, diplopia

Facial nerve (CN VII)

Mixed nerve for taste sensations from anterior 2/3 of tongue (sweet, salty, sour, umami), facial expression, lacrimal and inferior salivary glands (submandibular, sublingual)

Sensory: taste from anterior tongue

Somatic: Facial expression from 5 motor branches (temporal, zygomatic, buccal, mandibular, cervical)

Parasympathetic: increases lacrimal and submandibular/sublingual salivary secretions

Origin in pons

Nerve damage causes dry eye, dry mouth, loss of taste (anterior), bells palsy (facial droop)

Vestibulocochlear nerve (CN III)

Sensory nerve transmitting equilibrium (vestibular branch) and auditory (cochlear branch) info from inner ear to brain

Sensory bodies in vestibular ganglion and spiral ganglion with branches merging and entering cranium through internal acoustic meatus to pons and medulla oblongata

Damage causes loss of balance, nausea, vomiting, dizziness, deafness

Glossopharygneal nerve (CN IX)

Mixed nerve for taste and touch from posterior 1/3 of tongue; innervates one pharynx muscle, parotid salivary gland

Sensory: taste of posterior third of tongue (bitter), pharynx sensation, carotid cheoreceptors and baroreceptors

Somatic: contracts stylopharyngeus for swallowing

Parasypathetic: increases parotid salivary secretions

Extends from posterior glossal taste buds and carotid bodies through inferior/superior ganglion into jugular foramen to pons or from cranium via jugular foramen to stylopharyngeus

Damage causes dry mouth, loss of posterior third taste (bitter)

Vagus nerve (CN X)

Mixed nerve that innervates structures in head, neck, thorax and abdomen

Sensory: relays visceral info from heart, lungs, abdominal organs; general sensory info from external acoustic meatus, tympanic membrane (eardrum), inferior throat and voice box

Somatic: controls most of pharynx and all of layrnx

Parasympathetic: controls smooth muscle and glands of thoracic and most abdominal organs and cardiac muscle

Origin in motor nuclei of medulla oblongata; leaves via jugular foramen, branching in neck, thorax, abdomen with sensory neuron cell bodies in superior and inferior ganglia

Damage leads to variety of layrnx problems, difficulty in swallowing, impaired GI motility

Accessory nerve (CN XI)

Spinal root:

Innervates trapezius and sternocleidomastoid

Cranial root:

Assists CN X in pharynx innervation

Origins nucleus in spinal cord and nucleus in medulla merge in magnum foramen to travel with CN X to muscles

Nerve damage causes paralysis of affected muscles

Hypoglossal nerve (CN XII)

Innervates tongue muscles from medulla oblongata via hypoglossal canal

Nerve damage causes swallowing and speech difficulties