Lecture 20: Vertebrate Spinal Cord And PNS

Question 1

Give the gross anatomy of the human spinal cord

Answer 1

• Approx. ½ meter in length.
• Varies from 1 to 1.5 cm in diameter
• Enlargements:
• Cervical: C3 –T1
• Lumbar region: L1 –S2
• Conus medullaris: Ends at about the level of L2
• Cauda equina.
• Filum terminale: Thin filament of meningeal tissue extending from conus medullaris to the coccyx

Question 2

List the spinal nerve pairs

Answer 2

08 cervical 
 12 thoracic 
 05 lumbar 
 05 sacral 
 01 coccygeal
* Numbering System

Question 3

List the spinal nerve plexus

Answer 3

Cervical: C1 –C4
Brachial: C5 –T1
Lumbosacral: L1 –S4

Question 4

Describe Gray Matter

Answer 4

• Forms regions known as horns
• Equivalent of CNS nuclei
• Posterior (dorsal) horns:
• Receive incoming sensory fibers
• Anterior (ventral) horns:
• Site of cell bodies of alpha motor neurons to skeletal muscle fibers
• Lateral horns:
• Located only in thoracic and upper lumbar regions
• Site of cell bodies of ANS motor neurons

Question 5

Describe White Matter

Answer 5

• Forms longitudinal columns (funiculi)
• Consists of myelinated tracts
• Posterior (dorsal) columns
• Anterior (ventral) columns
• Lateral columns

Question 6

Describe Commisures

Answer 6

• White: Anterior and posterior
  Gray: Anterior and Posterior
• See Slide 6-7

Question 7

Describe Meninges

Answer 7

• Epidural space
• Dura mater: Tough outer layer
• Subdural space
• Arachnoid
• Subarachnoid space: Contains CSF
• Pia mater Delicate innermost layer
• Denticulate ligaments
• See Slide 8-11

Question 8

Describe Spinal Cord Anatomy in Cyclostomes, Fish, and Tetrapods

Answer 8

• Cyclostomes:
• No sharp distinction between white and gray matter
• Fish:
• Gray matter is triangular in cross section with a dorsal-pointing apex
• Paired ventral columns of gray matter
• Tetrapods:
• Spinal cord has a distinct separation between gray matter and white matter.
• The gray matter is centrally located and is organized into the shape of an “H” when viewed in cross-section.
• The arms of the “H” are called horns.
• There are a pair of dorsal horns and a pair of ventral horns.
• In humans, because of our upright posture, these are often referred to as posterior and anterior horns respectively.

Question 9

Describe the dorsal and ventral horns of the spinal cord, and the spinal canal

Answer 9

• Dorsal (posterior) horns:
• Serve as the entry point for incoming sensory neurons:
• May synapse here with secondary neurons traveling up the spinal cord to the brain
• May travel up the spinal cord in the white matter
• Ventral (anterior) horns:
• Serve as the location of the cell bodies of motor neurons
• Mark the exit of their axons from the spinal cord to the periphery
• Spinal canal:
• Located in the middle of the “H”
• Remnant of the original hollow cavity (lumen) of the neural tube
• Continuous with the ventricles of the brain
• Filled with cerebrospinal fluid.
• See Slide 14

Question 10

How is the white matter of the spinal cord organized?

Answer 10

• The white matter of the spinal cord is organized into columns or funiculi.
• Dorsal columns and carry myelinated fibers up the spinal cord to the brain.
• • These, therefore, would be made up of afferent or sensory tracts.
• Lateral columns are found on either side of the gray “H”.
• • Lateral columns carry both sensory and motor tracts.
• Ventral (anterior) columns are found between the ventral arms of the gray matter “H”.
• • Ventral columns carry mostly motor tracts.
• See Slide 15

Question 11

Describe the Spinal Nerve

Answer 11

• The horns and columns are continuous throughout the length of the spinal column.
• Externally the spinal cord has a segmented appearance.
• Due mostly to the presence of paired nerves that enter and leave the spinal cord.
• • Spinal nerves
• • Approximately one pair of spinal nerves for each vertebra.
• • Each spinal nerve is attached to the spinal cord by means of two branches, or roots.
• • Dorsal root: Carry sensory fibers into the spinal cord
• • Ventral root: Carry motor fibers from the spinal cord
• • Lateral to the spinal cord the two roots unite to form the spinal nerve itself.
• Each spinal nerve divides near the spinal cord into two branches called rami (sing. ramus).
• The dorsal ramus of each spinal nerve supplies epaxial musculature.
• The ventral ramus, the hypaxial musculature.
• This pattern is retained in all vertebrates, including humans.
• Cell bodies for the motor neurons are located:
• Within the spinal cord in masses of gray matter called columns
• Within the brain within masses of gray matter called nuclei.
• Cell bodies of the sensory neurons are located outside the central nervous system in masses of gray matter called ganglia.
• These ganglia are located along the dorsal roots and are called spinal or cranial nerve ganglia.
• See Slide 18 - 20

Question 12

Describe the autonomic nervous system

Answer 12

• The autonomic nervous system is sometimes considered to be part of the peripheral nervous system.
• It is strictly a motor system.
• Depends on sensory input from the viscera.
• It can be thought of as a visceral motor system.
• It controls all visceral activity, usually as a subconscious level.
• Two subsystems:
• • Sympathetic: Motor neurons originate in the thoracic and lumbar areas of the spinal cord. Referred to as the thoracolumbar system
• • Parasympathetic systems: Motor neurons arise from the brain and sacral regions of the spinal cord. Referred to as the craniosacral system

Question 13

Autonomic pathways consist of two neurons. Describe each neuron

Answer 13

• First neuron originates in the central nervous system, either the brain (cranial nerves) or the spinal cord.
• Cell bodies are located either in the gray matter of the spinal cord or specific nuclei in the brain.
• The axons are myelinated and are called preganglionic axons.
• Axons follow typical cranial and spinal nerves to the periphery.
• In the periphery the preganglionic axons synapse with a second group of neurons, the postganglionic neurons.
• The cell bodies of the postganglionic neurons are found in peripheral ganglia.
• Their axons are never myelinated.
• Axons terminate in smooth muscle of the viscera
• See Slide 24, 26-29

Question 14

What are basic characteristics of cranial nerves?

Answer 14

• Traditional pattern:
• Anamniotes have ten pairs of cranial nerves.
• Amniotes have twelve pairs of cranial nerves.
• Realistic pattern:
• May be up to 25 pairs of cranial nerves.
• Characteristics:
• Superficial origin: Site where a cranial nerve emerges from the surface of the brain.
• Deep origin: Site where motor fibers arise and sensory fibers terminate in nuclei within the brain stem.

Question 15

List the 12 Traditional Cranial Nerves (In order)

Answer 15

I Olfactory 
II Optic
III Oculomotor 
IV Trochlear 
V Trigeminal 
VI Abducens 
VII Facial 
VIII Vestibulocochlear 
IX Glossopharyngeal 
X Vagus 
XI Spinal  accessory 
XII Hypoglossal

Question 16

What are additional nerves sometimes cited?

Answer 16

* Additional  nerves: 
0 Terminal 
VN Vomeronasal 
E Epiphyseal 
P  Profundus(= V1) 
ALL Anterior  lateral  line 
PLL Posterior  lateral  line 

Note that the taste components of VII, IX, and X are considered to be separate cranial nerves by some authors.

Question 17

List the 7 modalities

Answer 17

• SSS: Special Sensory:
• Special senses derived from ectoderm:
• Sight, sound, balance.
• SVS: Special Visceral Sensory:
• Special senses derived from endoderm:
• Taste
• SVM: Special Visceral Motor:
• Muscles derived from pharyngeal arches.
• GVS: General Visceral Sensory: • General sensation from viscera. • GVM: General Visceral Motor:
• To smooth muscles of gut tract.
• Autonomic motor
• GSS: General Somatic Sensory:
• General senses from ectoderm (skin).
• GSM: General Somatic Motor:
• Skeletal muscles.

Question 18

What are some generalizations about the cranial nerves?

Answer 18

• Cranial nerves have same basic structure as spinal nerves.
• Dorsal root and ventral root.
• Dorsal root ganglion.
• Motor components of cranial nerves begin within brain within motor nuclei.
• Comparable to spinal cord anterior horns.
• Cell bodies of sensory fibers are located in dorsal root (cranial) ganglia.
• Special sensory cranial nerves:
• No motor nuclei and no sensory ganglia.
• Motor cranial nerves:
• Begin in cranial motor nuclei in basal plate.
• Resemble spinal nerves with only ventral roots.
• Include III, VI, and XII.
• Have no sensory roots and no sensory root ganglia.
• Mixed cranial nerves:
• Have both cranial motor nuclei and sensory ganglia.
• Sensory ganglia are usually given specific names.

Question 19

Describe the olfactory nerve (CN I)

Answer 19

• Olfactory nerve fibers pass from:
• Olfactory receptor cells (bipolar neurons)
• Through: Cribriform plate
• To: Olfactory bulbs
• Modality: SVS

Question 20

Describe the Optic Nerve (CN II)

Answer 20

• Begins in ganglionic layer of retina (not rods and cones).
• Exits orbit through optic canal.
• 50% decussation in optic chiasma in mammals but complete decussation in most other vertebrates:
• Near pituitary gland
• Near internal carotid
• Modality: SSS

Question 21

Describe the occulomotor nerve (CN III)

Answer 21

• Supplies four of the six extrinsic muscles of the eye.
• Has a parasympathetic component.
• Will be discussed in a group with other parasympathetic nerves.

Question 22

Describe the Trochlear Nerve (CN IV)

Answer 22

• Enters orbit through superior orbital fissure.
• Innervates superior oblique muscle of the eye.
• Modality: GSM.
• THE ONLY cranial nerve that emerges dorsally.

Question 23

Describe the abducens nerve (CN VI)

Answer 23

• Enters orbit through superior orbital fissure.
• Lies on medial aspect of lateral rectus muscle.
• Innervates lateral rectus muscle of the eye.
• Modality: GSM.

Question 24

Describe the vestibulocochlear nerve (CN VIII)

Answer 24

• Leaves cranial cavity via internal acoustic meatus:
• Accompanied by facial nerve.
• Modality: SSS
• Auditory (cochlear) component:
• Cell bodies in spiral ganglion of cochlea.
• Function: Hearing.
• Vestibular component:
• Cell bodies in vestibular ganglion.
• From receptors for balance and equilibrium.
• Function: Balance and equilibrium.

Question 25

Describe the Spinal Accessory Nerve (CN XI)

Answer 25

• Arises from C1-3 (4).
• Single trunk enters cranial cavity through foramen magnum.
• Temporarily joins cranial root.
• Cranial roots leave with vagus nerve.
• Spinal accessory trunk exits cranial cavity via jugular foramen.
• Supplies: Sternocleidomastoid and trapezius.
• Modality: SVM.

Question 26

Describe the Hypoglossal Nerve (CN XII)

Answer 26

• Exits cranial cavity via hypoglossal foramen.
• Descends anteriorly in neck between internal carotid and internal jugular.
• In neck gives rise to superior root of ansa cervicalis and a nerve to thyrohyoid muscle.
• Supplies extrinsic and intrinsic muscles of tongue.
• Modality: GSM.

Question 27

List the parasympathetic cranial nerves

Answer 27

• Oculomotor (III)
• Facial (VII)
• Glossopharyngeal (IX)
• Vagus(X)

Question 28

Describe the oculomotor nerve

Answer 28

• Modalities:
• GSM:
• • From: oculomotor complex.
• • To: 4 of 6 extrinsic muscles of the eye and levator palpebrae superioris.
• GVM:
• • Preganglionics: From Edinger-Westphal nucleus. To: ciliary ganglion.
• • Postganglionics: To: sphincter pupillae.
• See Slide 46

Question 29

Describe the Trigeminal Nerve

Answer 29

• Three branches:
  Ophthalmic, Maxillary, Mandibular.
• Major sensory nerve to face.
• (Also supplies tongue, but this is general sensory, not special sensory for taste.)
• Motor to muscles of mastication:
• Muscles derived from first pharyngeal arch (mandibular arch).
• Not parasympathetic.
• Modalities:
• SVM:
  – From: masticator nucleus.
  – To: muscles of mastication, tensor veli palatini, tensor tympani, mylohyoid, and anterior belly of digastric.
• GSS:
  – Cell bodies of sensory neurons in trigeminal (semilunar) ganglion.
  – Sensory to face and anterior scalp, conjunctiva, paranasal sinuses, nasal and oral cavities, anterior 2/3 of tongue, external surface of eardrum, and dura of middle and anterior cranial fossae.
• See slide 48

Question 30

Describe the facial nerve

Answer 30

• Primary motor nerve to muscles of facial expression.
• Supplies muscles derived from second pharyngeal arch (hyoid arch).
• Branches:
• • Temporal. Zygomatic. Buccal. Mandibular. Cervical.
• Carries taste from anterior 2/3 of tongue.
• Carries motor to submandibular and sublingual salivary glands.
• Carries motor to lacrimal gland and mucous glands in nose.
• Modalities:
• GVM:
• • Preganglionics: From superior salivatory and lacrimal nuclei. To: submandibular/sublingual ganglia.
• • Postganglionics: To: lacrimal gland, submandibular gland, sublingual gland, and mucous glands of mouth and nose.
• GSS:
• • Cell bodies of sensory neurons in geniculate ganglion.
• • Supplies sensory to part of external ear.
• SVS:
• • Cell bodies of sensory neurons in geniculate ganglion.
• • Carries taste from anterior 2/3 of tongue.
• • Carries taste from hard and soft palates.
• See slide 51

Question 31

Describe the glossopharyngeal nerve

Answer 31

• Motor to parotid gland.
• Carries taste from posterior 1/3 of tongue.
• Supplies general sensation to posterior 1/3 of tongue.
• Supplies muscles derived from third pharyngeal arch.
• Modalities:
• SVM:
• • From: Nucleus ambiguous.
• • To: Stylopharyngeus.
• GVS:
• • From: Carotid body and carotid sinus.
• GVM:
• • Preganglionics: From inferior salivatory nucleus. To: otic ganglion.
• • Postganglionics: To parotid gland.
• GSS: From: Posterior 1/3 of tongue, external ear, middle ear cavity.
• SVS: From: Posterior 1/3 of tongue (taste).
• See slide 56

Question 32

Describe the vagus nerve

Answer 32

• Motor to muscles of larynx:
• Muscles of fourth and fifth pharyngeal arches.
• Important in swallowing and speaking.
• Major parasympathetic nerve to thoracic and abdominal organs:
• Parasympathetic supply distally to left colic (splenic) flexure.
• Modalities:
• SVM:
• • From: Nucleus ambiguous. To: Almost all muscles of pharynx and larynx.
• GVS:
• • From: Larynx and pharynx and from thoracic and abdominal viscera.
• GSS:
• • From: External ear and pharynx.
• GVM:
• • Preganglionics: From dorsal motor nucleus. To myenteric plexus of wall of gut tube.
• • Postganglionics: smooth muscle of pharynx and larynx and smooth muscle of most of wall of gut tube.