Lecture

An Introduction to the Spinal Cord, Spinal Nerves and Spinal Reflexes Spinal reflexes are rapid, automatic nerve responses triggered by specific stimuli They are controlled by the spinal cord; they are not controlled by the brain

What is part of the CNS? What is part of the PNS? An Overview of the CNS and PNS The Brain (CNS) Motor output over cranial nerves (PNS) Sensory input over cranial nerves Effectors Reflex centers in brain Sensory receptors Muscles The Spinal Cord (CNS) Glands Sensory input over spinal nerves Motor output over spinal nerves (PNS) Reflex centers in spinal cord Sensory receptors Adipose tissue What is part of the CNS? What is part of the PNS? 3

Spinal Cord Structure and Function Gross Anatomy of the Spinal Cord The cord is about 18 inches (45 cm) long and a 1/2 inch (14 mm) wide It ends between the first and second lumbar vertebrae (L1 and L2) It has bilateral symmetry (grooves divide the spinal cord into left and right sides) Posterior median sulcus – on posterior side Anterior median fissure – on anterior side Cord enlargements are caused by the amount of gray matter in the segment, and the involvement with sensory and motor nerves of the limbs The cervical enlargement serves nerves of the shoulders and upper limbs The lumbar & sacral enlargements serve nerves of the pelvis and lower limbs

There are 30 vertebrae with 31 pairs of spinal nerves in the PNS There are 30 vertebrae with 31 pairs of spinal nerves in the PNS. Spinal nerves are based on the vertebrae where the spinal nerves originate Cervical nerves are named for the vertebra superior to the nerve’s exit (except for the 8th nerve; it is inferior). All other nerves are named for the vertebra inferior to the nerve’s exit The first pair of spinal nerves pass between the skull and C1. The rest pass through the intervertebral foramina

Mention that Cranial section has 7 bones and 8 nerves Cranial - superior radiation of nerves Thoracic - inferior radiation of nerves C7, T12, L5, S5, C1 = 30 bones C8, T12, L5, S5, C1 = 31 nerves HaHa!

On the the distal end of the cord… The Conus medullaris is the tapered terminus of the cord (connective tissue), located below the lumbar enlargement (L1-L2) The Filum terminale is a thin thread of fibrous tissue (pia mater) at the end of the conus medullaris. It helps anchor the conus medullaris to the coccyx (attaches to coccygeal ligament) The Cauda equina are nerve roots extending inferiorly below the conus medullaris (L2-S5) that innervates the pelvic organs and lower limbs

Roots are the two branches of spinal nerves The ventral root (anterior side) contains axons of motor neurons (efferents) The dorsal root (posterior side) contains axons of sensory neurons (afferents) Dorsal root ganglion contains many cell bodies of sensory neurons Spinal nerves are on each side of the spine; the dorsal and ventral roots join to form a spinal nerve Mixed Nerves carry both afferent (sensory) and efferent (motor) fibers

The Spinal Meninges The spinal meninges are specialized membranes that isolate the spinal cord from their surroundings They protect the spinal cord, provide continuity within the CNS (brain and spinal cord) and channel the vasculature (blood supply) Meningitis is a deadly viral or bacterial infection of these tissues (inflammation and damage will kill the nervous system!)

There are three meningeal layers of the spinal cord The dura mater is the dense, outer layer, composed of collagen. It is contiguous with the cranial dura mater, and serves as a longitudinal anchor, terminating with the filum terminale (coccygeal ligament – shown in previous slide) The arachnoid mater is the middle, avascular layer The pia mater is the delicate, inner vascular layer, composed of elastin. Dendiculate ligaments are a latitudinal anchor (next slides)

The epidural space is between the dura mater and the walls of the vertebral body. It contains blood vessels, loose connective and adipose tissue; this is the site of epidural injection of anesthetic – commonly, it is done wrong (damage)! The subdural space is between the dura mater and the arachnoid mater (little or no space) The subarachnoid space is between the arachnoid mater and pia mater. It is filled with cerebrospinal fluid (CSF) that carries dissolved gases, nutrients and wastes; this is the site of lumbar puncture (spinal tap) to withdraw CSF for diagnostic purposes (L3 to L5) – this is a very delicate procedure!

Paired denticulate ligaments are found along the length of the spinal cord They extend from the pia mater to the dura mater They help to laterally suspend and anchor the spinal cord

Gray Matter and White Matter Spinal tissue is structurally divided into two types of matter Inside gray matter (shaped like H) is the location of cell bodies and unmyelinated axons. There is a greater amount of gray matter in the spinal cord segments serving the limbs. It also has projections (horns) Outside white matter is the location of myelinated axons organized in ascending and descending columns (funiculi)

of white matter. The right half indicates the Posterior white column Posterior gray horn Lateral white column Lateral gray horn Dorsal root ganglion Anterior gray horn Anterior white column a The left half of this sectional view shows important anatomical landmarks, including the three columns of white matter. The right half indicates the functional organization of the nuclei in the anterior, lateral, and posterior gray horns.

Structural Organization of Gray Matter POSTERIOR Structural Organization of Gray Matter Posterior median sulcus The projections of gray matter toward the outer surface of the spinal cord are called horns. Posterior gray horn Lateral gray horn Dorsal root Anterior gray horn Dorsal root ganglion Ventral root ANTERIOR A micrograph of a section through the spinal cord, showing major landmarks in and surrounding the cord. 15

Gray Matter is Functionally Organized in Anatomical Horns Posterior gray horns contain somatic and visceral sensory nuclei Anterior gray horns contain somatic motor nuclei Lateral gray horns are in thoracic and lumbar segments; they contain visceral motor nuclei Commissures are axons that cross laterally from one side of the cord to the other before reaching gray matter (anterior and posterior sides)

We can pinpoint damage in the spinal cord to functional deficits which are observed…

Functional Organization of White Matter White matter is organized in tracts (fasciculi); these are bundles of myelinated axons that relay information Ascending tracts carry information to the brain Descending tracts carry motor commands to spinal cord

Spinal Nerves and Plexuses Anatomy of Spinal Nerves Every spinal cord segment is connected to a pair of spinal nerves Every spinal nerve is surrounded by three connective tissue layers that support structures and contain blood vessels

Three Connective Tissue Layers of Spinal Nerves Epineurium Outer layer Dense network of collagen fibers Perineurium Middle layer Divides the nerve into fascicles (axon bundles) Endoneurium Inner layer Surrounds Schwann cells that envelope individual axons in the fascicle Is continuous onto peripheral nerves Hmmm, looks a lot like muscle tissue, eh?

Perineurium (around one fascicle) (around myelin sheath) Blood vessels Perineurium (around one fascicle) Endoneurium (around myelin sheath) 21

Peripheral Distribution of Spinal Nerves Spinal nerves form lateral to intervertebral foramen, where dorsal and ventral roots unite, and then branch, to form pathways leading to and from their destination There are two classified branches; sensory or motor

Somatic sensory nuclei Visceral sensory nuclei From interoceptors of back From exteroceptors, proprioceptors of back The dorsal root of each spinal nerve carries sensory information to the spinal cord. The dorsal ramus carries sensory information from the skin and skeletal muscles of the back. Somatic sensory nuclei The ventral ramus carries sensory information from the ventrolateral body surface, structures in the body wall, and the limbs. Dorsal root ganglion From exteroceptors, proprioceptors of body wall, limbs From interoceptors of body wall, limbs Rami communicantes Visceral sensory nuclei Ventral root  Somatic sensations  Visceral sensations The sympathetic nerve carries sensory information from the visceral organs. From interoceptors of visceral organs 23

Motor 24 To skeletal muscles of back Postganglionic fibers to smooth muscles, glands, etc., of back The spinal nerve forms just lateral to the intervertebral foramen, where the dorsal and ventral roots unite. The dorsal ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles of the back. Dorsal root ganglion Dorsal root The axons in the relatively large ventral ramus supply the ventrolateral body surface, structures in the body wall, and the limbs. The ventral root of each spinal nerve contains the axons of somatic motor and visceral motor neurons. To skeletal muscles of body wall, limbs Visceral motor nuclei Somatic motor nuclei Rami communicantes Postganglionic fibers to smooth muscles, glands, etc., of body wall, limbs  Somatic motor commands Sympathetic ganglion  Visceral motor commands The white ramus is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion. Because these preganglionic axons are myelinated, this branch has a light color and is therefore known as the white ramus. Postganglionic fibers to smooth muscles, glands, visceral organs in thoracic cavity A sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity. The gray ramus contains postganglionic fibers that innervate glands and smooth muscles in the body wall or limbs. These fibers are unmyelinated and have a dark gray color. Preganglionic fibers to sympathetic ganglia innervating abdominopelvic viscera 24

C2C3 N V A specific bilateral region of the skin surface monitored by a single pair of spinal nerves is known as a dermatome (The pair of spinal nerves are sensory and motor) Shingles is caused by a herpes virus that attacks dorsal roots of spinal nerves and cranial ganglia C2C3 C2 C3 C3 C4 T2 C4 C5 T3 T1 T2 T4 T5 T3 C5 T6 T4 T7 T5 T8 T6 T9 T2 T2 T7 T10 T8 T11 T12 T9 L1 C6 T10 L2 T11 L4 L3 T1 C6 T12 L5 C7 L1 S L2 4 S S2 3 C8 C8 T1 L3 L1 C7 S5 S1 L5 L4 L2 S2 L5 L3 S1 L4 ANTERIOR POSTERIOR 25

If you ever had chicken pox, beware If you ever had chicken pox, beware! The virus that caused it is within you and can express itself (unknown why) at a later stage in life  It follows a dermatome pattern of expression

Nerve Plexuses Nerve plexuses (trunks) are a complex, interwoven network of nerve fibers They are formed from blended fibers of ventral rami of adjacent spinal nerves The trunks form cords that control the skeletal muscles of the neck and limbs (arms and legs)

There are four major Plexuses of Ventral Rami Cervical plexus (C1 – C5) Brachial plexus (C4 – T2) Lumbar plexus (T12 – L4) Sacral plexus (L4 – S4)

Lesser occipital nerve C2 Great auricular nerve C3 Cervical plexus C3 Transverse cervical nerve C4 C5 Supraclavicular nerve C6 C7 Phrenic nerve Brachial plexus C8 T1 T2 T3 Axillary nerve T4 T5 T6 T7 Musculocutaneous nerve T8 T9 Thoracic nerves T10 T11 29

Iliohypogastric nerve S2 S3 S4 Ilioinguinal nerve Radial nerve Lumbar plexus L2 Ulnar nerve L3 Median nerve L4 L5 Sacral plexus S1 Iliohypogastric nerve S2 S3 S4 Ilioinguinal nerve S5 Co1 Lateral femoral cutaneous nerve Genitofemoral nerve Femoral nerve Obturator nerve Superior Gluteal nerves Inferior Pudendal nerve Saphenous nerve Sciatic nerve 30

Extra Stuff…

The Cervical Plexus Includes ventral rami of spinal nerves C1–C5 It innervates the neck, thoracic cavity and diaphragmatic muscles Major nerves: Phrenic n. (diaphragm mvt) Great auricular n. (ear mvt) Cervical n. (neck mvt)

Redundant slide… Cranial Nerves Nerve Roots of Cervical Plexus Accessory nerve (XI) Hypoglossal nerve (XII) Lesser occipital nerve Nerve Roots of Cervical Plexus C1 C2 C3 C4 C5 Supraclavicular nerves Clavicle 33

The Brachial Plexus Includes ventral rami of spinal nerves C4–T2 It innervates the pectoral girdle and upper limbs Nerves that form the brachial plexus originate from: Superior, middle, and inferior trunks; these are large bundles of axons from several spinal nerves Lateral, medial, and posterior cords; these are smaller branches that originate at trunks

Major nerves of the Brachial Plexus Musculocutaneous (lateral cord) and Ulnar n. (medial cord) flexes the arm Median n. (lateral and medial cords) flexes and pronates the arm Radial n. (posterior cord) elongates and supinates the arm

Major nerves of the Brachial Plexus Pectoral n. pectoralis Thoracic n. serratus anterior Subclavius n. subclavius

Major nerves of the Brachial Plexus Axillary n. deltoid and teres minor Thoracodorsal n. latissimus dorsi Subscapular n. subscapular and teres major Suprascapular n. supraspinatus and infraspinatus Dorsal Scapular n. rhomboid and levator scapulae

The Lumbar Plexus Includes ventral rami of the spinal nerves T12–L4 Iliohypogastric nerve Ilioinguinal nerve Genitofemoral nerve The Lumbar Plexus Includes ventral rami of the spinal nerves T12–L4 Major nerves: Genitofemoral n. skin of genitals Lateral femoral cutaneous n. skin of upper pelvic area Femoral n. flex the thigh and hip Sciatic n. flex the knee Fibular n’s (from Sciatic branch) flex the knee (to walk) Lateral femoral cutaneous nerve Femoral nerve Obturator nerve Superior gluteal nerve Inferior gluteal nerve Pudendal nerve Posterior femoral cutaneous nerve (cut) Sciatic nerve Saphenous nerve Common fibular nerve Superficial fibular nerve Deep fibular nerve Nerves of the lumbar and sacral plexuses, anterior view

The Sacral Plexus Includes ventral rami of spinal nerves L4–S4 Superior gluteal nerve Inferior gluteal nerve The Sacral Plexus Includes ventral rami of spinal nerves L4–S4 Major nerves: Pudendal n. controls urinary and anal sphincters (control urination and defacation) Sciatic n. Two branches of the sciatic nerve Fibular n. flex the knee (to walk) Tibial n. flex the knee, extend the ankle, extend the toes (to walk) Pudendal nerve Posterior femoral cutaneous nerve Sciatic nerve Tibial nerve Common fibular nerve Sural nerve Nerves of the sacral plexus, posterior view

Cutaneous distribution of the nerves in the foot and ankle Saphenous nerve Sural nerve Sural nerve Saphenous nerve Sural nerve Fibular nerve Tibial nerve Sural nerve Saphenous nerve Fibular nerve Tibial nerve Cutaneous distribution of the nerves in the foot and ankle 40