Nerves

ANATOMY OF THE NERVOUS SYSTEM Part 2

Protection of the Brain

• Bones

• Meninges

• Cerebrospinal fluid

• Blood-brain barrier

Meninges 

• Meninges

  • 3 CT membranes that cover & protect CNS protect blood vessels & enclose venous sinuses contain cerebrospinal fluid form partitions within skull

Protection of the Brain Cranial Meninges

• Three layers

  • Dura mater

  • Arachnoid mater

  • Pia mater

• Meningitis

  • Inflammation of meninges

• Dura mater

  • Strongest meninx

  • Two layers of fibrous connective tissue (around brain)

    • outer periosteal layer 

    • inner meningeal layer

  • around brain, 2 layers fused except where they separate to form dural sinuses

  • dural septa to partition and anchor:  falx cerebri, falx cerebelli, tentorium cerebell

  • Spinal cord has only meningeal layer

Archanoid matter

• Middle layer with weblike extensions

• Separated from dura mater by subdural space

• Subarachnoid space contains CSF and largest blood vessels of brain

• Arachnoid villi protrude into superior sagittal sinus and permit CSF reabsorption

Pia mater

• delicate CT + tiny blood vessels – clings tightly to brain, follows convolutions

• Encephalitis: inflammation of the active brain tissue

Dura Septa

• limit excessive movement & partition brain

  • Falx cerebri—in longitudinal fissure; attached to crista galli

  • Falx cerebelli—along vermis of cerebellum

  • Tentorium cerebelli—horizontal dural fold over cerebellum and in transverse fissure

Protection of the Brain: Cerebrospinal Fluid

• Composition

  • Watery solution formed from blood plasma 

    • Less protein & different ion concentrations than plasma

  • Constant volume

• Functions

  • Gives buoyancy to CNS structures

  • Reduces weight by 97%

  • Protects CNS from blows and other trauma

  • Nourishes brain and carries chemical signals 

• Choroid Plexuses

  • Hang from roof of each ventricle; produce CSF at constant rate; keep in motion 

    • Clusters of capillaries enclosed by pia mater and layer of ependymal cells

• Ependymal cells use ion pumps to control composition of CSF and help cleanse CSF by removing wastes

• Normal volume ~ 150 ml; replaced every 8 hours

• Formation and circulation

  • CSF is produced by the choroid plexus of each ventricle.

  • 2 CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures.

  • Some CSF flows through the central canal of the spinal cord.

  • 3 CSF flows through the subarachnoid space.

  • 4 CS is absorbed into the dural venous sinuses via the arachnoid villi.

• Hydrocephalus: abnormal buildup of cerebrospinal fluid (CSF) in the brain's cavities (ventricles), putting harmful pressure on brain tissue

Protection of the Brain: Blood-Brain Barrier

• Why??  some hormones also act at NTs; ions can increase rate of neuronal firing

• Helps maintain stable environment for brain 

• Chemical variations could lead to uncontrollable neuron firings

• Exceptionally impermeable tight junctions keep brain separated from many blood borne substances

• Composed of 3 layers

  • Continuous endothelium of capillary walls

  • Thick basal lamina around capillaries

  • Feet of astrocytes surrounding neurons

• Selective barrier

  • Allows nutrients to move by facilitated diffusion

  • Metabolic wastes, proteins, toxins, most drugs, small nonessential amino acids, K+ denied

  • Allows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics 

  • Absent in some areas, e.g., vomiting center and hypothalamus, where necessary to monitor chemical composition of blood

Homeostatic Imbalances of the Brain

• Traumatic brain injuries

  • Concussion—temporary alteration in function

  • Contusion—permanent damage

  • Subdural or subarachnoid hemorrhage—may force brain stem through foramen magnum, resulting in death

  • Cerebral edema—swelling of brain associated with traumatic head injury

• Cerebrovascular accidents (CVAs or strokes)

  • Ischemia: Tissue deprived of blood supply; brain tissue dies, e.g., blockage of cerebral artery by blood clot

Degenerative Brain Disorders

• Degenerative brain disorders

• Alzheimer's disease (AD): a progressive degenerative disease of brain that results in dementia

• Memory loss, short attention span, disorientation, eventual language loss, irritable, moody, confused, hallucinations

• Plaques of beta-amyloid peptide form in brain

• Toxic effects may involve prion proteins

• Neurofibrillary tangles inside neurons kill them

• Brain shrinks

• Parkinson's disease

• Degeneration of dopamine-releasing neurons of substantia nigra

• Basal nuclei deprived of dopamine become overactive  tremors at rest

• Huntington's disease

• Fatal hereditary disorder

• Caused by accumulation of protein huntingtin

• Leads to degeneration of basal nuclei and cerebral cortex

• Initial symptoms wild, jerky "flapping" movements

• Later marked mental deterioration

Degenerative Brain Disorders

• Alzheimer's disease (AD): a progressive degenerative disease of brain that results in dementia

  • Memory loss, short attention span, disorientation, eventual language loss, irritable, moody, confused, hallucinations

  • Plaques of beta-amyloid peptide form in brain

    • Toxic effects may involve prion proteins

  • Neurofibrillary tangles inside neurons kill them

  • Brain shrinks

• Parkinson's disease

  • Degeneration of dopamine-releasing neurons of substantia nigra

  • Basal nuclei deprived of dopamine become overactive  tremors at rest

• Huntington's disease

  • Fatal hereditary disorder

  • Caused by accumulation of protein huntingtin

    • Leads to degeneration of basal nuclei and cerebral cortex

  • Initial symptoms wild, jerky "flapping" movements

  • Later marked mental deterioration

The Spinal Cord

Gross anatomy and Projection

• from the foramen magnum to L1 or L2 (below this is ideal spot for lumbar puncture)

• Contains spinal reflex centers

• Provides 2-way communication to & from brain

• 31 pairs that exit via  _________

• Enlargements at the cervical and lumbar regions

• Terminates in conus medullaris

• Filum terminale extends to coccyx

  • Fibrous extension of conus covered with pia mater 

  • Anchors spinal cord

• Denticulate ligaments

  • Extensions of pia mater that secure cord to dura mater

• Cauda quina: a bundle of nerves at the base of your spinal cord

• Protected by bone, meninges & CSF

  • Dural and arachnoid membranes extend to sacrum, beyond end of cord at L1 or L2

  • Epidural space: cushion of fat & network of veins in space between vertebrae & spinal dura mater  

  • CSF in subarachnoid space

Cross Sectional Anatomy - Gray Matter and Spinal Roots

• Two lengthwise grooves partially divide cord into right and left halves 

  • Ventral (anterior) median fissure 

  • Dorsal (posterior) median sulcus 

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• Gray matter

  • organized like butterfly wings. Three areas of gray matter:

    • Dorsal Horns: interneurons that receive somatic and visceral sensory input

    • Lateral horns associated with thoracic & superior lumbar regions of cord: sympathetic neurons

    • Gray commissure: connects masses of gray matter; encloses central canal

    • Ventral horns: some interneurons; bodies of somatic motor neurons

Spinal Cord - Gray matter and spinal roots

• Dorsal roots: Sensory input to the cord 

  • afferent fibers from periphery sensory receptors form the dorsal roots

• Spinal nerves: formed by fusion of dorsal and ventral roots

• Dorsal root (spinal) ganglia:

  • house cell bodies of associated sensory neurons- their axons enter cord to:

    • Synapse with interneurons in posterior horns at level they enter

    • Travel to higher cord/brain centers

• Ventral roots: bundles of motor neuron axons that exit the spinal cord

Organization of the Gray Matter of the Spinal Cord

• Gray matter is divided into four groups based on of somatic visceral innervation

  • SS Interneurons receiving input from somatic sensory neurons 

  • VS Interneurons receiving input from visceral sensory neurons

  • VM Visceral motor (autonomic) neurons

  • SM Somatic motor neurons

White Matter

• Myelinated & nonmyelinated nerve fibers allow communication between parts of spinal cord and brain

• Run in three directions

  • Ascending – up to higher centers (sensory inputs)

  • Descending – from brain to cord or lower cord levels (motor outputs)

  • Transverse – from one side to other (commissural fibers)

• divided into three white columns (funiculi) on each side

  • Dorsal (posterior)

  • Lateral

  • Ventral (anterior)

• Each spinal tract is composed of axons with similar destinations and functions

Neuronal Pathways

• Major spinal tracts are part of multi-neuron pathways

• Four key points about spinal tracts and pathways:

  • Decussation: Most cross from one side of CNS to other at some point

  • Relay: Consist of chain of two or three neurons

  • Somatotopy: precise spatial relationship in CNS correspond to spatial relationship in body

  • Symmetry: pathways are paired symmetrically (right and left)

Ascending Pathways

• Conduct sensory pathways upward through a chain of three neurons:

• First-order neuron

  • Conducts impulses from cutaneous receptors and proprioceptors

  • Synapses with second-order neuron 

• Second-order neuron

  • Interneuron

  • Cell body in dorsal horn of spinal cord or medullary nuclei

  • Axons extend to thalamus or cerebellum

• Third-order neuron

  • Also an interneuron

  • Cell bodies in thalamus 

  • Axon extends to somatosensory cortex

  • No third-order neurons in cerebellum

Descending Pathways

• Deliver efferent impulses from the brain to the spinal cord 

• Involve two neurons:

  • Upper motor neurons (Pyramidal cells in primary motor cortex)

  • Lower motor neurons (Ventral horn motor neurons /Innervate skeletal muscles

Peripheral Nervous System (PNS)

• Provides links from and to world outside body

• All neural structures outside brain

  • Sensory receptors

  • Peripheral nerves and associated ganglia

  • Efferent motor endings

Structure of  a nerve

• Cordlike organ of PNS

• Bundle of myelinated &  nonmyelinated peripheral axons enclosed by connective tissue

• Connective tissue coverings include

  • Endoneurium—loose connective tissue that encloses axons and their myelin sheaths

  • Perineurium—coarse connective tissue that bundles fibers into fascicles

  • Epineurium—tough fibrous sheath around a nerve

Classification of Nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

• Classified according to direction transmit impulses

  • Mixed nerves – both sensory and motor fibers; impulses both to and from CNS

  • Sensory (afferent) nerves – impulses only toward CNS

  • Motor (efferent) nerves – impulses only away from CNS

Cranial Nerves

• 12 pairs of cranial nerves pass through various foramina of the skull

• first two pairs attach to forebrain; rest originate from brain stem - pairs purely sensory

• Each numbered (I through XII) and named from rostral to caudal

• All except vagus nerve serve only head & neck structures

• Except for those serving special sense organs, most cranial nerves are mixed nerves

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Cranial Nerves 

Olfactors Nerves (S)

• Sensory nerves of smell

• Run from nasal mucosa to olfactory bulbs

• Pass through cribriform plate of ethmoid bone

• Fibers synapse in olfactory bulbs

• Pathway terminates in primary olfactory cortex

Optic Nerves (S)

• Arise from retinas; really a brain tract

• Pass through optic canals, converge and partially cross over at optic chiasma

• Optic tracts continue to thalamus, where they synapse

• Optic radiation fibers run to occipital (visual) cortex

Oculomotor Nerves (M)

• Fibers extend from ventral midbrain through superior orbital fissures to four of six extrinsic eye muscles

• Function in raising eyelid, directing eyeball, constricting iris (parasympathetic), and controlling lens shape

Trochlear Nerves (pulley) (M):

• Fibers from dorsal midbrain enter orbits via superior orbital fissures to innervate superior oblique muscle

• Primarily motor nerve that directs eyeball

Abducens Nerves (M):

• Fibers from inferior pons enter orbits via superior orbital fissures

• Primarily a motor, innervating lateral rectus muscle

• primarily motor fibers to lateral rectus muscle

Trigeminal Nerves (B):

• Largest cranial nerves; fibers extend from pons to face

• Three divisions

  • Ophthalmic (V1) passes through superior orbital fissure 

  • Maxillary (V2) passes through foramen rotundum

  • Mandibular (V3) passes through the foramen ovale

• Convey sensory impulses from various areas of face (V1) and (V2)

• Supply motor fibers (V3) for mastication

• The trigeminal nerve (Cranial Nerve V) is the most important nerve in dentistry

Facial Nerves

• VII

• large nerve; from pons to lateral face

• Chief motor nerves of face with 5 major branches

• Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands

• Sensory function (taste) from anterior two-thirds of tongue

• mixed nerve (primarily motor)

Vestibulocochlear Nerves (B)

• VIII

• Afferent fibers from hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from inner ear through internal acoustic meatuses, and enter brain stem at pons-medulla border

• Mostly sensory function; small motor component for adjustment of sensitivity of receptors

Glossopharyngeal Nerves (B)

• IX

• “tongue & pharynx”; mixed nerves to & from medulla

• Motor functions - innervate part of tongue and pharynx for swallowing, and provide parasympathetic fibers to parotid salivary glands

• Sensory functions - fibers conduct taste and general sensory impulses from pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors

Vagus Nerve (B)

• X

• “wanderer”

• only cranial nerve to extend beyond head-neck region

• mixed nerves; to & from medulla

• Most motor fibers are parasympathetic fibers that help regulate activities of heart, lungs, and abdominal viscera

• Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx 

Accessory Nerves (M)

• XI

• Formed from ventral rootlets from C1–C5 region of spinal cord (not brain) 

• Accessory nerves exit skull via jugular foramina to innervate trapezius and sternocleidomastoid muscles

Hypoglossal Nerves (M)

• Fibers from medulla exit skull via hypoglossal canal 

• Innervate extrinsic and intrinsic muscles of tongue that contribute to swallowing and speech

Spinal Nerves

• 31 pairs of mixed nerves named for point of issue from spinal cord

• Cervical (C1–C8);  (T1–T12); Lumbar (L1–L5); Sacral (S1–S5); Coccygeal (C0)

• Dorsal & ventral roots unite to form spinal nerves, which emerge from vertebral column via intervertebral foramina 

• Supply all body parts but head and part of neck

• 7 cervical vertebrae give rise to 8 pairs of cervical spinal nerves because:

  • Each of the first 7 pairs (C1 to C7) exits the vertebral canal superior to vertebra for which it is named

  • Last spinal nerve (C8) exits canal inferior to C7

    • So vertebra C7  has a nerve that leaves above it and one that leaves below it

• Each of the other spinal nerves exits inferior to vertebra for which it is named

• Spinal nerves quite short (~1-2 cm); each spinal nerve branches into a dorsal ramus & a ventral ramus 

  • Meningeal branch – tiny, reenters vertebral canal, innervates meninges and blood vessels

  • Rami communicantes (autonomic pathways) join ventral rami in thoracic region

• Understand the difference between a root and a ramus

Innervation of Specific Body Regions

• All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses (cervical, brachial, lumbar, and sacral)

• Fibers from different rami crisscross in plexuses

  • Each branch contains fibers from several spinal nerves

  • Fibers from ventral ramus go to body periphery via several routes

• Result: Each limb muscle innervated by more than one spinal nerve

• Damage to one does not → paralysis

Cervical Plexus and the Neck

• A

• Formed by ventral rami of C1–C4 with minor contribution from C5

• Deep to the SCM

• Most branches form cutaneous nerves

  • Innervate skin of neck, ear, back of head, and shoulders

  • Other branches innervate neck muscles

• Single most important nerve is 

• Phrenic nerve

  • Major motor and sensory nerve of diaphragm (receives fibers from C3–C5)

  • Irritation → hiccups

Brachial PLexus and Upper Limb

• B

• C5-C8 & most of T1;  partly in neck, partly in axilla – nerves supplying upper limbs

• Musculocutaneous nerve

  • to biceps brachii & brachialis to flex arm

• Median

  • flexor muscles in anterior forearm & into palm

    • pronate forearm, flex wrist/fingers, oppose thumb

• Axillary

  • nerve to shoulder (esp. deltoid muscle)

• Radial

  • largest - to humerus & dorsal part of hand

    • elbow extension, supination of forearm, extension of wrist & fingers, abduction of thumb

• Ulnar

  • medial to elbow ("funny bone") & follows ulna along medial forearm

    • wrist & finger flexion/ adduction & abduction of medial fingers

Anterolateral Thorax and Abdominal Wall

• C. Anterolateral Thorax

  • T2-T12;  simple & segmented as for innervation of back Intercostal nerves:  to intercostal muscles; anterolateral thorax

• D. The Back

  • Dorsal rami innervate posterior body trunk

• E. Lumbar PLexus

  • Arises from L1–L4: Innervates thigh, abdominal wall, and psoas muscle

  • Femoral Nerve: Arises from L1–L4 Innervates thigh, abdominal wall, and psoas muscle

  • Obturator Nerve: passes through obturator foramen to innervate adductor muscles

• E. Sacral PLexus

  • Arises from L4–S4

  • Immediately caudal to the lumbar plexus; serves the buttock, lower limb, pelvic structures & perineum

  • Sciatic Nerve:

    • longest & thickest nerve of body

    • Innervates hamstring muscles, adductor magnus, and most muscles in leg & foot

  • Common fibular nerve:

    • to knee joint, calf (anterolateral) & dorsum of foot 

    • Biceps femoris, tibialis anterior

  • Tibial nerve:

    • behind knee to posterior calf & sole of foot

    • Hamstrings, gastrocnemius, soleus, tibialis posterior

  • Superior and inferior gluteal nerves: 

    • To gluteal muscles

  • Pudendal nerve:

    • muscles & skin of perineum (e.g. erection, voluntary urination)

Innervation of Skin

• Dermatome

  • area of skin innervated by cutaneous branches of single spinal nerve

  • All spinal nerves except C1 participate in dermatomes

  • Extent of spinal cord injuries ascertained by affected dermatomes

  • Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness