Neuroanatomy Chapter 3: Peripheral Nervous System – Upper Extremity

SECTION 1: BRACHIAL PLEXUS

Nerve Roots and Trunks

  • Usually formed from C5-T1 ventral rami.
  • C5 and C6 make up upper trunk (join at Erb’s point and shoulder injury here is called Erb’s palsy).
  • C7 makes up middle trunk.
  • C8 and T1 form lower trunk.

Trunks and Cords

  • Each trunk splits into anterior and posterior divisions.
  • Posterior divisions all join to form posterior cord.
  • Anterior division of upper trunk and anterior division of middle trunk form lateral cord.
  • Anterior division of lower trunk forms medial cord.
  • Cords are named by relationship to section part of axillary artery (lateral cord lies lateral to the artery, medial cord lies medial to it, and the posterior cord lies posterior to it).

Cords and Nerves

  • Lateral and medial cords combine to form median nerve.
  • Medial cord forms the ulnar nerve.
  • Posterior cord forms the radial nerve.

Miscellaneous

  • Supination is partially preserved in radial nerve injury when supinator becomes weakened due to musculocutaneous nerve innervation of biceps brachii.
  • Injury to either serratus anterior, rhomboids, or trapezius (innervated by cranial nerve 11) results in scapular wining.
  • Nerves to serratus anterior and rhomboids are derived directly from nerve roots (not from brachial plexus) so pure brachial plexopathy these two muscles are spared.
  • Medial brachial cutaneous and medial antebrachial cutaneous nerves – sensory nerves that cover medial aspect of upper arm and forearm, respectively.
  • In ulnar nerve injuries medial arm and forearm sensation is spared due to sparing of these nerves, in medial cordopathies these nerves are injured and medial upper arm and forearm sensation is damaged.
  • If brachial plexus is shifted one level up and receives innervation from C4 it is called prefixed plexus, if it is shifted down one level and receives innervation from T2 it is called a postfixed plexus. 

SECTION 2: MEDIAN NERVE

  • Median nerve does not innervate any upper arm muscles or provide sensory to them. 

Entrapment sites

  • Ligament of Struthers – all components of median nerve lie downstream of this ligament so entrapment here affects all median nerve components
  • Pronator teres heads – all components downstream become affected except pronator teres itself because innervation to pronator teres lies proximal to the site
  • Carpal tunnel at wrist – most common entrapment site, everything proximal to carpal tunnel unaffected

SECTION 3: ULNAR NERVE

Sensory innervation

  • Medial cutaneous nerves to arm and forearm (AKA medial brachial and medial antebrachial cutaneous nerves) are direct cords from medial cord (NOT part of the ulnar nerve).
  • Ulnar nerve sensory coverage is confined to the medial hand. 

Entrapment sites

  • Cubital tunnel – most common entrapment site, all motor/sensory components of ulnar nerve lie distal to tunnel so all components are affected
  • Guyon’s canal (AKA Guyon’s tunnel) – variations exist so different distal ulnar components are affected; palmar and dorsal ulnar cutaneous nerves are unaffected since lie upstream from it

SECTION 4: RADIAL NERVE

Sensory innervation

  • Proximal radial sensory nerves are posterior cutaneous nerves to arm and forearm and lower lateral cutaneous nerve of the arm. These cover the lower upper arm and midline posterior arm and forearm.
  • The superficial sensory radial nerve coming off the posterior interosseous nerve provides distal radial sensory coverage. 

SECTION 5: CERVICAL PLEXUS

Cervical plexus

  • From ventral rami of C1-C4

Infrahyoid muscles (AKA strap muscles)

  • Sternohyoid
  • Sternothyroid
  • Omohyoid
  • Thyrohyoid

Suprahyoid muscles

  • Geniohyoid
  • Mylohyoid
  • Stylohyoid
  • Digastric
Neuroanatomy Chapter 2: Meninges and Ventricular System

SECTION 1: MENINGES

Three Meningeal Layers

  1. Dura mater
  2. Arachnoid mater
  3. Pia mater (covers nervous system parenchyma)

Three Meningeal Spaces

  1. Epidural space - external to dura mater
  2. Subdural space - internal to dura mater
  3. Subarachnoid space - internal to arachnoid mater; external to pia mater

Epidural space

  • Cranial - potential space because dura tightly sticks to skull
  • Spinal - actual space because separates the vertebral column from the dura mater
  • **Epidural hematoma more common in spinal canal than cranial vault

Dura Mater Sublayers

  • Periosteal dural sublayer - tightly sticks to skull and ends within cranium
  • Meningeal dural sublayer - has inward reflections and continues within spinal canal
  • Dural border cell layer - loosely arranged layer that lies in between these two sublayers

Meningeal Sublayer Inward Reflections

  • Falx cerebri - separates the cerebral hemispheres; reflection in midline vertex
  • Tentorium cerebelli - separates cerebellum from overlying occipital lobes; reflection along lateral base of brain
  • Falx cerebelli - separates cerebellar hemispheres

Dural Venous Sinuses

  • Superior sagittal dural venous sinus - forms within flax cerebri
  • Transverse sinuses - form within tentorium cerebelli
  • Both function in CSF (cerebrospinal fluid) absorption and blood-flow return

Herniations

  • Tentorium cerebelli - divides cranial vault into supratentorial compartment and infratentorial compartment
  • Supratentorial compartment - contains cerebral hemispheres
  • Infratentorial compartment - contains cerebellum and brainstem
  • Supratentorial herniation - subfalcine herniation, uncal herniation, central herniation 
  • Infratentorial herniation – upward cerebellar herniation and tonsillar herniation
  • Subfalcine herniation – one cerebral hemisphere herniates underneath falx cerebri
  • Uncal herniation – medial temporal lobe (uncus) herniates over the tentorium cerebelli
  • Central herniation (AKA transtentorial herniation) – diencephalon herniates directly down through the tentorium cerebelli
  • Upward cerebellar herniation – cerebellum herniates upward into the supratentorial cavity
  • Tonsillar herniation – cerebellar tonsils undergo downward herniation through the foramen magnum 

Hemorrhages

  • Cranial epidural space hemorrhage (epidural hematoma) – occurs when a ruptured middle meningeal artery rips dura away from skull
  • Spinal epidural space hemorrhage (epidural hematoma) – epidural venous plexus injury either spontaneously like with use of anticoagulants or from trauma
  • Subdural space (subdural hematoma) – occurs from injury to bridging veins
  • Subarachnoid space (bathes nervous system) – contains cisternal fluid collections around brainstem/midline regions; CSF contained here so test CSF to evaluate for subarachnoid hemorrhage

Epidural hematoma

  • High pressure blood collects between dura and skull
  • Pushes aside the spongy brain parenchyma
  • Forms biconvex lens-shape
  • Not affected by dural reflections
  • Interrupted by cranial sutures

Subdural hematoma

  • Low pressure blood spreads out along border cell layer
  • Forms crescent shape
  • Interrupted by dural reflections
  • Not affected by cranial sutures

SECTION 2: CEREBRAL VENTRICLES & CEREBROSPINAL FLUID FLOW

Lateral ventricles (bilateral)

  • Frontal horn: head of caudate makes up majority of lateral wall
  • Occipital horn: extends deep into the occipital lobe
  • Temporal horn: hippocampus makes up anterior medial wall (amygdala sits in front of it and forms anterior border)
  • Body: long superior bend
  • Atrium (AKA Trigone): region where temporal and occipital horns and body come together

Flow: Lateral ventricles => 3rd ventricle => cerebral aqueduct (of Sylvius) => 4th ventricle => central canal of spinal cord

Choroid Plexus

  • highly vascularized secretory epithelial tissue that makes CSF
  • lie within central regions of cerebral ventricles: body, atrium, & temporal horn of lateral ventricle; and 3rd and 4th ventricles
  • tight junctions form an important blood-CSF barrier
  • NONE in frontal and occipital horns – good places for intraventricular drains

Cerebrospinal fluid (CSF) flow

  • CSF empties from lateral ventricles through paired foramina of Monro into 3rd ventricle, down the cerebral aqueduct, then into 4th ventricle, then through foramen of Magendie (midline) and bilateral foramina of Luschka (laterall) into subarachnoid space, and also empties down central canal.
  • CSF reabsorbed through arachnoid villi into dural venous sinuses.
  • Dural Venous channels mostly filled with blood since rate of CSF production/reabsorption is far slower than rate of blood entry and reabsorption into/out of cranial vault.

Lumbar Puncture

  • CSF produced at rate of ~0.35 mL/min (20 mL/hr)
  • In lumbar puncture 10-20 mL of CSF is withdrawn
  • This fluid is replaced within 30-60 mins
  • 150 mL of CSF in nervous system

Choroid plexus

  • Formed where invaginations of vascularized meninges (tela choroidea) merge with ventricular ependyma
  • Tela choroidea are either combinations of pia and ependyma or double plial layers

Arachnoid villus

  • Extends from subarachnoid space through the subdural space into a dural venous sinus
  • CSF passes into villus to be reabsorbed within dural venous sinus
  • Meningiomas formed from neoplastic arachnoid villi cells; greatest concentration of meningiomas = greatest concentration of arachnoid villi = at the cerebral convexity (falx cerebri) and base of skull

Obex

  • Inferior angle of the 4th ventricle (at level of gracile tubercle) 4th ventricle becomes the obex and descends as central canal of the spinal cord
  • Gracile tubercle = swelling formed by gracile nucleus in posterior wall of the medulla

Floor of 4th ventricle

  • Important anatomic site
  • Forms the posterior border of tegmentum of the pons and medulla
  • Many important anatomic structures lie within or near it

Other structures

  • Superior-posterior border of 4th ventricle = superior medullary velum (AKA anterior medullary vellum); **Medulloblastoma tumors often lie along here.
  • Inferior-posterior border of 4th ventricle = inferior medullary velum (AKA posterior medullary vellum)
  • Lateral borders of 4th ventricle formed by cerebellar peduncles
  • Rest of posterior border (roof) of 4th ventricle formed by cerebellum

SECTION 4: CISTERNS, SINUSES, & VEINS

Subarachnoid Cisterns (AKA Basal Cisterns)

  • Suprasellar cistern (AKA chiasmatic cistern) – above the sella turcica; optic chiasm lies within this cistern
  • Interpeduncular cistern – in front of midbrain, in between cerebral peduncles
  • Ambient cistern – along lateral midbrain
  • Quadrigeminal cistern (AKA cistern of the great vein) – behind the midbrain; contains the great cerebral vein (AKA vein of Galen)
  • Cistern of the velum interpositum – in between the thalamus and the splenium of the corpus callosum; actually lies between the tela choroidea that lines inferior surface of the fornix and splenium of corpus callosum and superior surface of 3rd ventricle and thalamus; contains internal cerebral veins
  • Prepontine cistern – in front of pons
  • Pontocerebellar cistern – lateral to pons
  • Premedullary cistern – in front of medulla
  • Lateral cerebellomedullary cistern – lateral to medulla
  • Posterior cerebellomedullary cistern (AKA cisterna magna) – underneat the cerebellum
  • **Posterior cerebellomedullary cistern = extraction site during cisternal puncture

Sinuses

  • Superior sagittal sinus – runs along superficial midline surface of the cerebrum
  • Confluence of sinuses (torcular Herophili) – lies at occiput; merges with bilateral transverse sinuses and receives the straight sinus
  • Transverse sinuses – wrap horizontally along the tentorium cerebelli; empties a sigmoid sinus
  • Straight sinus – drains the deep cerebral veins
  • Occipital sinus – drains inferiorly from confluence of sinuses
  • Inferior sagittal sinus – runs inferior to superior sagittal sinus along the same midline course just above corpus callosum; empties into straight sinus
  • Sigmoid sinus – forms an S-shaped curve along the intracranial surface of mastoid portion of temporal bone; drains into internal jugular vein at jugular bulb

Cerebral Veins

  • Vein of Galen (AKA great cerebral vein) – lies posterior to splenium of the corpus callosum and drains directly into the straight sinus
  • Bilateral basal veins of Rosenthal – pass around the midbrain to drain into vein of Galen
  • Bilateral internal cerebral veins – pass through the cistern of the velum interpositum to drain into veins of Galen
  • Superior cerebral veins – drain into superior sagittal sinus; divide into superior, middle, and inferior groups of veins which drain the superficial cerebrum
Neuroanatomy Chapter 1: General Organization

SECTION 1: OVERVIEW OF NEUROANATOMY

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Brain, Brainstem, & Cerebellum

  • Meninges—protect and nourish nervous system
  • Cerebral Cortex—outer cellular gray matter of brain
  • Subcortical White Matter—underlying axons
  • Basal Ganglia—motor function
  • Thalamus—relays information throughout cerebrum (mostly sensory)
  • Hypothalamus—center for autonomic nervous system
  • Cerebrospinal Fluid System—assists meninges in protecting and nourishing nervous system
  • Brainstem—cranial nerve nuclei, neuronal populations, and fiber tracts; composed of midbrain, pons, and medulla
  • Cerebellum—fundamental for coordination

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Spinal Cord and Peripheral Nervous System

  • Spinal Cord—intermediary between brain and periphery
  • Dorsal Nerve Root—comes from posterior; sensory
  • Ventral Nerve Root—comes from anterior; motor

Autonomic Nervous System

  • Parasympathetic Ganglion—rest
  • Sympathetic Chain—action

SECTION 2: ORIENTATIONAL TERMINOLOGY

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Planes

  • Anterior—front
  • Posterior—behind
  • Superior—top
  • Inferior—bottom

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Sagittal View

  • Dorsal (dorsal fin of shark)
  • Ventral (underbelly)
  • Rostral (beak)
  • Caudal (tail)
  • Cerebrum: dorsal = superior; ventral = inferior; rostral = anterior; caudal = posterior
  • Brainstem moves 180 degrees so rostral = superior; caudal = posterior; ventral = anterior; dorsal = posterior

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Coronal View

  • Radiographic - head is viewed from front
  • Anatomic - head is viewed from behind

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Axial View (AKA horizontal/transverse)

  • Radiographic - feet are coming out at you
  • Anatomic - head is coming up at you

SECTION 3: DIVISIONS & SIGNS

Central Nervous System

  • Brain
  • Brainstem
  • Cerebellum
  • Spinal Cord

Peripheral Nervous System

  • Cranial Nerves
  • Spinal Nerves

**Autonomic nervous system excluded for simplicity. 

Clinical Scenarios

  1. UMN injury - stroke with injury to cerebral cortex and underlying white matter
  2. LMN injury - peripheral polyneuropathy
  3. Mixed pattern injury - spinal cord injury (aka myelopathy)
  • above level of lesion - normal
  • below level of lesion - UMN injury pattern
  • at level of lesion - LMN injury pattern