Spinal cord is enclosed within the vertebral column from the foramen magnum to L 2Spinal cord is enclosed within the vertebral column from the foramen.

Chapter 12 & 13 Spinal Cord and Spinal Nerves

• Spinal cord is enclosed within the vertebral column from the foramen magnum to L2

• Provides two-way communication to and from the brain

• Protected by bone, meninges, and CSF

• Is composed of

– Cervical segments– Thoracic segments– Lumbar segments– Sacral segments

• Gives rise to 31 pairs of spinal nerves

• SC is not uniform in diameter throughout length

– Cervical enlargement: nerve fibers that supplies upper limbs enter & leave SC

– Lumbosacral enlargement: nerve fibers that supplies lower limbs enter & leave SC

• Conus Medullaris: terminal tapered portion of the spinal cord below Lumbosacral enlargement

• Cauda Equina: origins of spinal nerves from inferior end of lumbosacral enlargement and Conus Medullaris

• Spinal Cord is surrounded by connective tissue membranes – Meninges

– Dura mater: Outermost membrane– Continuous with epineurium of the

spinal nerves

– Arachnoid mater: Middle layer, thin and wispy

– Pia mater: Deepest layer, bound tightly to surface of spinal cord

– Filum terminale: fibrous extension of the pia mater, which anchors the spinal cord to the coccyx

– Denticulate ligaments: Are saw-

toothed shelves of pia mater; attach the spinal cord to the dura mater

• Spaces– Epidural: space between the vertebrae

and dura mater – anesthesia injected– Contains blood vessels, areolar

connective tissue and fat

– Subdural: space between dura mater and arachnoid mater

– Contains small amount of serous fluid

– Subarachnoid: space between arachnoid and pia mater

– Contains CSF, blood vessels

• Gray Matter

– Consists of neuron cell

bodies, dendrites and axon

– Is present in the interior of

the spinal cord

– Forms an ‘H’ shape

– Ventral (Anterior) Horns:• Two anterior

projections of gray matter

• Contain cell bodies of motor neurons

• Dorsal Horns– sensory neurons enter

and synapse with association neurons

• Lateral Horns– Only visible from T1

to L2– Contain autonomic

neuron cell bodies

• Gray commissure– Connects right and left halves

of gray matter

• External fissures– Anterior median fissure &

– Posterior median sulcus

– Are deep clefts partially separating left and right halves

• White matter

– Consists of myelinated

axons, forms nerve tracts

– Divided into columns

called columns or funiculi

– Anterior, lateral and

dorsal white columns or

funiculi

• White matter

– Consists of myelinated

axons, forms nerve tracts

in columns

• Ascending tracts: carry information from spinal cord to brain

• Descending tracts: carry information from brain to spinal cord

• Spinal cord provides means of communications between brain and various organs with the help of spinal nerves

• Conduction of sensory impulses upward – through ascending tracts to the brain

• Conduction of motor impulses from brain down

– through descending tracts to the efferent neurons

– To muscles or glands

• Spinal nerves arises from series of rootlets from the dorsal and ventral surface of the spinal cord

• 6-8 rootlets combine to form ventral root on ventral side and dorsal root on dorsal side of spinal cord

• Ventral and dorsal root extend laterally and join to form Spinal cord

• Dorsal root ganglion:

• Each dorsal root contains a ganglion called Dorsal root ganglion

• Dorsal root ganglion are collections of cell bodies of sensory neurons forming dorsal roots

• Axons of these neurons extend from various parts of body to spinal nerve to dorsal root ganglia to dorsal root to dorsal horn of spinal cord

• Axons synapse with interneurons in dorsal horn or pass into white matter

• Ventral root:– Axons of motor neurons form

ventral roots and pass into spinal nerves

– Motor neuron cell bodies are in anterior and lateral horns of spinal cord gray matter • Multipolar somatic motor

neurons in anterior (motor) horn

• Autonomic neurons in lateral horn

• Dorsal root contains sensory axons

• Ventral root contains motor axons

• Spinal nerves have both sensory and motor axons

• A reflex is a rapid, predictable motor response to a stimulus

• There are five components of a reflex arc– Receptor – site of stimulus– Sensory neuron – transmits the afferent impulse to CNS– Integration center – either monosynaptic or polysynaptic region within the

CNS– Motor neuron – conducts efferent impulses from the integration center to an

effector– Effector – muscle fiber or gland that responds to the efferent impulse

• Some integrated within spinal cord; some within brain

• Some involve excitatory neurons yielding a response; some involve inhibitory neurons that prevent an action

• Major spinal cord reflexes are:– Stretch Reflex– Golgi Tendon Reflex– Withdrawal Reflex

• In stretch reflex, muscles contract in response to a stretching force applied to them

• Sensory receptor of stretch reflex is muscle spindle

• Muscle spindle: Are composed of 3-10 specialized skeletal muscle cells that lack Myofilaments in their central regions, are non-contractile, innervated by Sensory neurons

• Cells are contractile only at the ends, innervated by Gamma Motor neurons

• Muscle spindle detect stretch of the muscle

• Sensory neurons of muscle spindle conduct AP to the spinal cord

• Sensory neurons of muscle spindle synapse with motor neurons of the spinal cord called alpha motor neurons

• Alpha motor neurons transmit AP to skeletal muscle

• And causes contraction of stretched muscle, opposes the stretch of muscle

• Eg. A person standing in the upright position begins to lean to one side– The postural muscles that are closely connected to the vertebral column on

the other side will stretch– Stretch reflexes are initiated– Then muscles contract to correct posture

• Eg. The patellar (knee-jerk) reflex– Tapping the patellar tendon stretches the quadriceps and starts the reflex

action– Quadriceps tendon stretched muscle spindles send impulse (muscle

stretching) spinal cord multipolar motor neuron quadriceps muscle contracts

– Extend the lower leg

• The opposite of the stretch reflex

• A tendon reflex operates as follows:

• As the tension applied to a tendon increases, the Golgi tendon organ (sensory receptor) is stimulated

• AP arise and propagate into the spinal cord via sensory neuron

• Within the spinal cord (integrating center), the sensory neuron activates an inhibitory interneuron that makes a synapse with a motor neuron

• The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer nerve impulses

• The muscle relaxes and relieves excess tension

• Example: weight lifter suddenly drops heavy weight

• Function is to remove a body limb or other part from a painful stimulus

• Reciprocal innervation: • Polysynaptic Reflexes

– Require 3 or more sets of neurons

• Causes relaxation of extensor muscle when flexor muscle contracts– Also involved in stretch reflex,

(eg. Quadriceps contract & hamstrings relax.)

• Crossed extensor reflex: • Polysynaptic Reflexes

– Require 3 or more sets of neurons

– Person steps on a sharp object– When a withdrawal reflex is

initiated in one lower limb, the crossed extensor reflex causes extension of opposite lower limb for balance

• Reflexes do not operate alone but as a whole

• Divergent and convergent pathways reflex activities are integrated with Nervous system

• Diverging branches of sensory neuron send AP along ascending nerve tracts to the brain

• e.g., pain stimulus, Initiates withdrawal reflex and enable to perceive pain

• Descending tracts from brain carry info to reflexes

• Neurotransmitters produce either EPSPs or IPSPs modifying the reflex

• Peripheral nerves consist of:– Axons – Schwann cells– Connective tissue

• Each axon and its Schwann cell sheath are surrounded by connective tissue layer:

• Endoneurium: surrounds individual neurons

• Perineurium: surrounds axon groups to form fascicles

• Epineurium: surrounds the entire nerve

• A dermatome is the area of skin supplied with sensory innervation by a pair of spinal nerves

• All spinal nerves except C1 participate in dermatomes

• Thirty-one pairs of spinal nerves

• First pair exit vertebral column between skull and atlas

• Last four pair exit via the sacral foramina

• Others exit through intervertebral foramina

• Eight pair cervical, twelve pair thoracic, five pair lumbar, five pair sacral, one pair coccygeal

• Spinal nerve is very short (1-2 cm) and divides into– Small dorsal ramus– Larger ventral ramus– Additional rami, called

Communicating rami is present at the base of the ventral rami in the thoracic and upper lumbar spinal cord regions

– Contain autonomic (visceral) nerve fibers

• Dorsal rami : innervate deep muscles of the dorsal trunk responsible for movements of the vertebral column

• Ventral rami : Distributed in 2 ways:– Thoracic region: Ventral rami

(T1-T12) form intercostal nerves that innervate the intercostal muscles and the skin over the thorax

– Remaining Spinal nerve Ventral rami : form five plexuses (interlacing nerve networks )

• Nerve Plexus: – Ventral rami of spinal

nerves C1-C4= cervical plexus

– Ventral rami of C5-T1= brachial plexus

– Ventral rami of L1-L4= lumbar plexus

– Ventral rami of L4-S4= sacral plexus

– Ventral rami of S4 and S5= coccygeal plexus

• The cervical plexus is formed by ventral rami of C1-C4

• Most branches are cutaneous nerves of the neck, ear, back of head, and shoulders

• The most important nerve of this plexus is the phrenic nerve, from C3-C5 (cervical and brachial plexuses)

• The phrenic nerve is the major motor and sensory nerve of the diaphragm

• Formed by C5-C8 and T1 (C4 and T2 may also contribute to this plexus)

• It gives rise to the nerves that innervate the upper limb

• C4 from cervical plus C5-T1

• “Robert Taylor Drinks Cold Beer”• “Roots Trunks Divisions Cords

Branches”• Five ventral rami (roots C5-T1) form three

trunks (upper/ middle/ lower) that separate into six divisions (ant./post.) then form 3 cords (lat./ med./ post.)from which five branches or nerves of upper limbs emerge

• Terminal branches/ nerves– Axillary innervates part of shoulder– Radial innervates post. arm– Musculocutaneous innervates ant. arm– Ulnar & Median innervates ant.

Forearm and hand– Smaller nerves such as pectoral, long

thoracic, thoracodorsal, subscapular, suprascapular innervates shoulder and pectoral muscles.

• Axillary nerve innervates the deltoid and teres minor

• Laterally rotate arm - teres minor

• Abducts arm – deltoid

• Cutaneous (Sensory) Innervation: inferior lateral shoulder

• Innervates essentially all extensor muscles

• Movements at elbow and wrist, thumb movements

• Cutaneous (Sensory) Innervation - posterior surface of arm and forearm, lateral 2/3 of dorsum of hand

• Movements at shoulder, elbow and wrist

• Sends fibers to the biceps brachii and brachialis

• Cutaneous (Sensory) Innervation - lateral surface of forearm

• Movements at wrist, fingers, hand

• Supplies the flexors (flexor carpi ulnaris and part of the flexor digitorum profundus)

• Cutaneous (Sensory) Innervation - medial 1/3 of hand, little finger, and medial 1/2 of ring finger

• Movement of hand, wrist, fingers, thumb

• Branches to most of the flexor muscles of arm.

• Cutaneous (Sensory) Innervation - lateral 2/3 palm, thumb, index and middle fingers; lateral 1/2 of ring finger and dorsal tips of same fingers

• Small nerves that innervate muscles acting on scapula and arm– Pectoral– Long thoracic– Thoracodorsal– Subscapular– Suprascapular

• Supply cutaneous innervation of medial arm and forearm

• Lumbar plexus: ventral rami of L1-L4

• Sacral plexus: ventral rami of L4-S4

• Usually considered together because of their close relationship

• Four major nerves exit and enter lower limb– Obturator innervates medial thigh– Femoral innervates ant. thigh– Tibial innervates post. thigh– Common fibular (peroneal) innervates

post. Thigh, ant & lateral leg and foot

• Obturator nerve supplies the muscle that adduct the thigh and knee

• Cutaneous (Sensory) Innervation - superior middle side of thigh

• Femoral nerve innervates iliopsoas, Sartorius, quadriceps femoris group muscles

• Movements of hip and knee

• Cutaneous (Sensory) Innervation: anterior and lateral thigh; medial leg and foot

• The Tibial and Fibular nerves together referred to as the sciatic (ischiadic) nerve

• Tibial– Movement of hip, knee, foot,

toes

– Cutaneous (Sensory) Innervation: none

• Common fibular– Anterior and lateral

muscles of the leg and foot

– Cutaneous (Sensory) Innervation: lateral and anterior leg and dorsum of the foot

• Nerves that innervate the skin of the suprapubic area, external genitalia, superior medial thigh, posterior thigh are:– Gluteal nerves – Pudendal nerve– Iliohypogastric nerve – Ilioingual nerve– Genitofemoral nerve– Cutaneous femoral

• Coccygeal plexus formed by S5 and coccygeal nerve (Co)

• Innervates muscles of pelvic floor

• Sensory Cutaneous Innervation : over coccyx