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SPINAL CORD & SPECIAL NERVES CHAP 13
***Spinal cord and special nerves provides quick, reflexive responses to many stimuli ***Spinal cord – pathway for sensory input to the brain & motor output from the brain ***100 million neurons and neuroglia compose the spinal cord
Spinal Cord Anatomy
- Protective Structures o Vertebral column provides a bony covering of the spinal cord ▪ Spinal cord is located within the vertebral canal o 3 Meninges (Meninx) – CT coverings that encircle the spinal cord and brain ▪ Dura mater – outermost layer Thick strong layer, dense irregular CT Continuous w the epineurium (outer covering of spinal & cranial nerves) Epidural space – space between dura mater and wall of vertebral canal ▪ Arachnoid mater – middle layer
Thin avascular covering Thin loosely arranged collagen & elastic fibers Subdural space – space between dura mater & arachnoid mater filled w/ interstitial fluid ▪ Pia mater – inner layer Thin transparent CT Thin squamous to cuboidal cells Adheres to surface of the spinal cord & brain Denticulate ligaments – thickenings of pia mater that suspend the spinal cord in the middle of its dural sheath ➢ Project laterally & fuse w/ arachnoid mater and inner surface of dura mater ➢ Protect spinal cord against sudden displacement that my result from shock
Subarachnoid space – space between arachnoid and pia materfilled w/ CSF
Spinal meninges surround the spinal cord& are continuous w/ the cranial meninges that encircle the brain
CC: Spinal tap - needle penetrates the subarachnoid space to collect a sample of CSF for diagnostic purposes; ABT; contrast media for myelography; anesthetics; administer chemotherapy; measure CSF pressure
➢ Normally performed between L3 & L4 or L4 & L
Spinal nerves T1-L5 exit below correspondin gvertebrae
S1-S5 roots and Co1 enter sacralcanal S1-S4 exit via 4 pairs of anterior/posterio rsacral foramina S5-Co1 exit via sacral hiatus
- Roots – two bundles of axons that connect each spinal nerve to a segment of the cord by smaller bundles
(rootlets)
o Posterior (dorsal) root – contains only sensory axons ▪ Posterior root ganglion – swelling that contains cell bodies of sensory neurons o Anterior (ventral) root – contain axons of motor neurons
C1-C7 exit vertebral canal above corresponding vertebrae C8 exit between C7 &T
o Posterior median sulcus – narrow furrow
- Gray matter – consist primarily of neurons & glia, unmyelinated axons & dendrites of association and motorneurons o Horns – regions that divides gray matter ▪ Posterior gray horn – contain cell bodies & axons of interneurons and axons of incoming sensory neurons ▪ Anterior gray horn – contain somatic motor nuclei ▪ Lateral gray horn – present only in thoracic & upper lumbar segments Contain autonomic motor nuclei
- White matter – consist of bundles of myelinated axons of motor & sensory neurons o Columns – regions in white matter (anterior, posterior, lateral white columns) ▪ Each column contains distinct bundles of nerve axons that have a common origin or destination &carry similar information These bundles are called tracts ➢ Ascending (sensory tracts) – conduct impulses toward the brain ➢ Descending (motor tracts) – conduct impulses from the brain
- Gray commissure – forms the bar of the H-shape o Central canal – is in the center of the graycommissure & circulates CSF; extends length of spinal cord
- Anterior white commissure – connects white matter of R & L sides of the spinal cord
COMPARISON OF VARIOUS SPINAL CORD SEGMENTS
Segment Distinguishing Characteristics
Cervical Relatively large diameter, large amount of white matter, oval C1-C4 posterior gray horn=large; anterior gray horn=small C5-C8 posterior gray horn are enlarged & anterior gray horns well developed
entire
Thoracic Small diameter, small amount of gray matter Anterior & posterior gray horns=small (except T1) Small lateral gray horn
Lumbar Circular Very large anterior & posterior gray horns Small lateral gray horn in upper segments Less white matter than cervical segments
Sacral Small
Large amounts of gray matter; small amount of white matter Anterior & posterior gray horns are large and thick
Coccygeal Resembles lower sacral spinal segments but smaller
Distribution of Spinal Nerves
- Branches (rami) – divides spinal nerves o Posterior ramus – serves deep muscles and skin of the posteriorsurface of the trunk o Anterior ramus – serves muscles and structures of upper & lower limbs and skin of lateral & anterior surfaces of the trunk ▪ Except T2-T12, form networks of nerves called plexus (Fig 13.2 & Exhibit 13.1-13.4) o Meningeal branch – reenters the vertebral cavity through the intervertebral foramen & suppliesvertebrae, vertebral ligaments, blood vessels of spinal cord and meninges o Rami communicantes – components of autonomic nervous system
- Plexuses – networks of axons that do not go directly to the body structures they supply (except T2-T12)
o Cervical Plexus (C1-C4) – supplies the skin & muscles of the head, neck & upper part of shoulders and chest & diaphragm ▪ Connects w/ some cranial nerves ▪ Supplies the diaphragm
CC: Damage to the spinal cord above the origin of the phrenic nerve (C3-C5) causes respiratory arrest
- Breathing stops because the phrenic nerve is no longersending impulses to the diaphragm
o Brachial Plexus (C5-C8) – constitutes the nerve supply for the upper extremities & a number of neck and shoulder muscles
R isk T akers D on’t C autiously B ehave
R oots T runks D ivisions C ords B ranches
▪ Roots – the anterior rami of spinal nerves unite to form trunks in inferior part of the neck (superior, middle, inferior trunks) diverge into divisions (anterior & posterior divisions) unite to form cords (lateral, medial, posterior cords) ▪ Branches – form the principal nerves of the brachial plexus 5 large terminal branches ➢ Axillary nerve – supplies deltoid and teres minor muscles ➢ Musculocutaneous nerve – supplies anterior muscles of arm ➢ Radial nerve – supplies muscle on the posterior aspect of the arm and forearm ➢ Median nerve – supplies most anterior forearm and some muscle of the hand ➢ Ulnar nerve – supplies anteromedial muscles of forearm and most of muscles ofhand
Nerve Origi n
Distribution
Superficial (sensory) Branches Lesser occipital C2 Skin of scalp posterior & superior to ear Great auricular C2-C3 Skin anterior, inferior, over ear, over parotid glands Transverse cervical C2-C3 Skin over anterior & lateral aspect of neck Supraclavicular C3-C4 Skin over superior portions of chest & shoulder Deep (largely motor) Branches Ansa cervicalis - Divides into superior & inferior roots
C1 Infrahyoid & geniohyoid muscles of neck
C2-C3 Infrahyoid muscles of neck
Phrenic C3-C5 Diaphragm Segmental branches C1-C5 Prevertebral (deep) muscles of neck, levator scapulae, middle scalene muscles
Thoracodorsal C6-C8 Latissimus dorsi muscle
Lower subscapular C5-C6 Subscapularis & teres major muscle
Axillary C5-C6 Deltoid & teres minor muscles; skin over deltoid & superior posterior aspect of arm
Median C5-T1 Flexors of forearm, except flexor carpi ulnaris; ulnar half of flexor digitorum profundus & some muscles of hand (lateral palm); skin of lateral 2/3 of palm of hand & fingers
Radial C5-T1 Triceps brachii, anconeus, extensor muscles of forearm; skin of posterior arm & forearm, lateral 2/3 of dorsum of hand, fingers over proximal & middle phalanges
Medial pectoral C8-T1 Pectoralis major & pectoralis minor muscles
Medial cutaneous nerve of arm C8-T1 Skin of medial & posterior aspects of distal third of arm
Medial cutaneous nerve of forearm C8-T1 Skin of medial & posterior aspects of forearm
Ulnar C8-T1 Flexor carpi ulnaris, ulnar half of flexor digitorum profundus, most muscles of hand; skin of medial side of hand, little finger, medial half of ring finger proximal interphalangeal joints of
all digits. Wrist flexion is weak w/ adduction, weak thumb movements
- Ulnar nerve palsy – inability to abduct/adduct fingers, atrophy of interosseous muscles of hand, hyperextension of metacarpophalangeal joints, flexion of the interphalangeal joints called claw hand. Loss of sensation over little finger
- Winged scapula – injury to long thoracic nerve. Arm cannot be abducted beyond horizontal position
- Thoracic outlet syndrome – compression of brachial plexus on one or more of its nerves. Results in spasm of scalene/pectoralis minor muscles. Pain, numbness, weakness; tingling in upper limb, across upper thoracic area, overscapula of affected side. Exaggerated during physical/emotional stress
CC: Injury to the femoral nerve - indicated by an
inability to extend the leg & by loss of sensation in the skin
over the anteromedial portion aspect of the thigh. May result from stab/gunshot wound.
CC: Obturator nerve injury - common
complication of child birth & results in paralysis of the adductor muscles of the leg and loss of sensation over the medial aspect of the thigh. Result in pressure on nerve by fetal head
• Sacral Plexus (L4-L5 & S1-S4) & Coccygeal Plexus (S4-S5, Co1)
- are situated in the anterior sacrum o They supply the buttocks, perineum & part of the lower extremities o Sciatic nerve (largest in body) – the largest nerve arising from the sacral plexus o Anococcygeal nerves – arises from coccygeal plexus supply small area of skin in coccygeal region Nerve Origin Distribution
Superior gluteal L4-L5 & Gluteus minimus and medius, tensor fasciae latae muscles
S
Inferior gluteal L5-S2 Gluteus maximus
Nerve to piriformis S1-S2 Piriformis muscle
Nerve to quadratus femoris & inferior gemellus
L4-L5 & S
Quadratus femoris & inferior gemellus muscles
Nerve to obturator internus & superior gemellus
L5-S2 Obturator internus & superior gemellus muscles
Perforating cutaneous S2-S3 Skin over inferior medial aspect of buttock
Posterior cutaneous nerve of thigh S1-S3 Skin over anal region, inferior lateral aspect of buttock, superior posterior aspect of thigh, superior part of calf, scrotum, labia majora Pudendal S2-S4 Muscles of perineum; skin of penis & scrotum; clitoris, labia majora & minora, vagina
- Area of skin that provides sensory input to the CNS via one pair of spinal nerves or trigeminal (V) nerve
- All spinal nerves (except C1) innervate specific, constant segments of the skin, those skin segments are called dermatomes
- Knowledge of dermatome help clinician determine which segment of the spinal cord/spinal nerve is malfunctioning o Ex: Shingles ▪ Varicella – Zoster
Chicken poxShingles
Spinal Cord Physiology
- White matter tracts – highways for nerve impulse propagation o Sensory input travels along these tracts toward the brain & motor output tracts from the brain toward skeletal muscles/effector tissues
- Gray matter – receives and integrates incoming & outgoing information
• Sensory & Motor Tracts
o Sensory information from ® travels up the spinal cord along 2 main route son each side of the cord ▪ Spinothalamic tract – conveys nerve impulses sensing pain, warmth, coolness,, itching, tickling, deep pressure, crude touch ▪ Posterior column tract – consist of two tracts gracile fasciculus & cuneate fasciculus Convey nerve impulses discriminative touch, light pressure, vibration, conscious proprioception (awareness of position and movements of body parts) o Motor output information travels from the brain down the spinal cord to effectors along 2 types of descending tracts ▪ Direct pathways (lateral & anterior corticospinal and corticobulbar tracts) – convey nerve impulses originated in cerebral cortex and destines to cause voluntary movements ▪ Indirect pathways (rubrospinal, tectospinal, vestibulospinal, lateral reticulospinal, medial
reticulospinal tracts) – convey nerve impulses from brain stem to cause autonomic movements & help coordinate body movements w/ visual stimuli; maintain skeletal muscle tone, sustain contraction of postural muscles, major role in equilibrium
