• Spinal cord is enclosed within the vertebral column from the foramen magnum to L
2
• 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 sawtoothed 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 noncontractile , 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 C
1 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
(T
1
-T
12
) 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 C
1
-C
4
• 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 C
5
-C
8 and T
1
(C
4 and T
2 may also contribute to this plexus)
• It gives rise to the nerves that innervate the upper limb

• C4 from cervical plus C
5
-T
1
• “R obert T aylor D rinks C old B eer ”
• “R oots T runks D ivisions C ords
B ranches ”
• Five ventral rami (roots C
5
-T
1
) 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

• 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
over coccyx