1. SPINAL CORD
will kleinelp
Associate Professor
Department of Biology

2. Coverings Location • bony vertebra
• connective tissue meningeal membranes
• cushion of cerebral spinal fluid
Location
• located within the vertebral canal of the vertebral column
• provides protection in addition to the vertebral ligaments, meninges and CSF

5. External Anatomy cervical enlargement
extends from the medulla to the superior border of L2
elongation of the cord proceeds until 4 or 5 but the growth of the vertebral column continues.
the cord does not extend the length of the vertebral column
there are two enlargments
cervical enlargement
lumbar enlargement
the cord terminates as a tapering conical structure called the conus medullaris
the cord extends in a filamentous fashion from this point forming the cauda equina
the cauda equina terminates as the filum terminale at S5 Co1
cervical enlargement
lumbar enlargement
conus medullaris
cauda equina
filum terminale

6. External Anatomy
Spinal nerves are the paths of communication between the spinal cord and the nerves supplying specific regions of the body.
Spinal cord organization appears to be segmented because the 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina
Each pair of spinal nerves arises from a spinal segment.
The groups are
cervical C1 - C8: 8 pairs
thoracic T1 - T12: 12 pairs
Lumbar L1 - L5: 5 pairs
Sacral S1 - S5: 5 pairs
Coccygeal Co1: 1 pair
cervical
thoracic
lumbar
sacral
coccygeal

7. External Anatomy
Two bundles of axons, called roots, connect each spinal nerve to a segment of the cord.
The posterior (dorsal) root contains only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system.
Each posterior root has a swelling, the posterior (dorsal) root ganglion , which contains the cell bodies of sensory neurons.
The anterior (ventral) root contains axons of motor neurons, which conduct nerve impulses from the CNS to effector organs and cells.
dorsal root ganglion
posterior (dorsal) root
dorsal rootlets
spinal nerve
dorsal gray horn
posterior median sulcus
lateral white column
dorsal white column
gray commissure
central canal
sensory neuron axon
anterior gray horn
white commissure
lateral gray horn
ventral white column
cell body of neuron
cell body of motor neuron
anterior median fissure
ventral rootlets
axon of motor neuron
afferents
efferents

8. Comparison of Spinal Segments

9. Spinal Nerves
Spinal nerves and the nerves that branch from them are part of the peripheral nervous system (PNS).
They connect the CNS to sensory receptors, muscles, and glands in all parts of the body.
The 31 pairs of spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge.
The first cervical pair emerges between the atlas (first cervical vertebra) and the occipital bone. All other spinal nerves emerge from the vertebral column through the intervertebral foramina between adjoining vertebrae.
Not all spinal cord segments are aligned with their corresponding vertebrae. Recall that the spinal cord ends near the level of the superior border of the second lumbar vertebra, and that the roots of the lumbar, sacral, and coccygeal nerves descend at an angle to reach their respective foramina before emerging from the vertebral column. This arrangement constitutes the cauda equina
As noted earlier, a typical spinal nerve has two connections to the cord: a posterior root and an anterior root.
The posterior and anterior roots unite to form a spinal nerve at the intervertebral foramen.
Because the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is classified as a mixed nerve.
The posterior root contains a posterior root ganglion in which cell bodies of sensory neurons are located.

10. Spinal Nerve Structure
Each spinal nerve and cranial nerve consists of many individual axons and contains layers of protective connective tissue coverings.
Individual axons within a nerve, whether myelinated or unmyelinated, are wrapped in endoneurium, the innermost layer
Groups of axons with their endoneurium are arranged in bundles called fascicles , each of which is wrapped in perineurium, the middle layer.
The outermost covering over the entire nerve is the epineurium.
Extensions of the epineurium also occur between fascicles.
The dura mater of the spinal meninges fuses with the epineurium as the nerve passes through the intervertebral foramen.
epineurium
perineurium
fascicle
endoneurium around axons

11. Spinal Nerve Distribution
A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches. These branches are known as rami.
The posterior (dorsal) ramus serves the deep muscles and skin of the dorsal surface of the trunk
The anterior (ventral) ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and ventral surfaces of the trunk
In addition to posterior and anterior rami, spinal nerves also give off a meningeal branch. This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges.
Other branches of a spinal nerve are the rami communicantes, components of the autonomic nervous system
posterior ramus
anterior ramus
denticulate ligament
meningeal branch
rami communicantes
sympathetic
ganglion

12. Spinal Nerve Plexuses
Axons from the anterior rami of spinal nerves, except for thoracic nerves T2–T12, do not go directly to the body structures they supply.
Instead, they form networks on both the left and right sides of the body by joining with various numbers of axons from anterior rami of adjacent nerves.
Such a network of axons is called a plexus.
The principal plexuses are the
cervical plexus
brachial plexus
lumbar plexus and
sacral plexus .
A smaller coccygeal plexus is also present.
Emerging from the plexuses are nerves bearing names that are often descriptive of the general regions they serve or the course they take. Each of the nerves, in turn, may have several branches named for the specific structures they innervate.

13. Cervical Plexus
The cervical plexus is formed by the roots (anterior rami) of the first four cervical nerves (C1–C4), with contributions from C5. There is one on each side of the neck alongside the first four cervical vertebrae.
The cervical plexus supplies the
skin and muscles of the head, neck, and superior part of the shoulders and chest.
The phrenic nerves arise from the cervical plexuses and supply motor fibers to the diaphragm.
Branches of the cervical plexus also run parallel to two cranial nerves, the accessory (XI) nerve and hypoglossal (XII) nerve.

14. Brachial Plexus
The roots (anterior rami) of spinal nerves C5–C8 and T1 form the brachial plexus which extends inferiorly and laterally on either side of the last four cervical and first thoracic vertebrae. It passes above the first rib posterior to the clavicle and then enters the axilla.
The roots are the anterior rami of the spinal nerves. The roots of several spinal nerves unite to form trunks in the inferior part of the neck.
The brachial plexus provides the entire nerve supply of the shoulders and upper limbs.
Five important nerves arise from the brachial plexus:
(1) The axillary nerve supplies the deltoid and teres minor muscles.
(2) The musculocutaneous nerve supplies the flexors of the arm.
(3) The radial nerve supplies the muscles on the posterior aspect of the arm and forearm.
(4) The median nerve supplies most of the muscles of the anterior forearm and some of the muscles of the hand.
(5) The ulnar nerve supplies the anteromedial muscles of the forearm and most of the muscles of the hand.

15. Lumbar Plexus
The roots (anterior rami) of spinal nerves L1–L4 form the lumbar plexus. Unlike the brachial plexus, there is no intricate intermingling of fibers in the lumbar plexus.
On either side of the first four lumbar vertebrae, the lumbar plexus passes obliquely outward, posterior to the psoas major muscle and anterior to the quadratus lumborum muscle. It then gives rise to its peripheral nerves.
The lumbar plexus supplies the anterolateral abdominal wall, external genitals, and part of the lower limbs.
Lumbar Plexus Injuries
The largest nerve arising from the lumbar plexus is the femoral nerve. Femoral nerve injury, which can occur in stab or gunshot wounds, is indicated by an inability to extend the leg and by loss of sensation in the skin over the anteromedial aspect of the thigh.
Obturator nerve injury results in paralysis of the adductor muscles of the leg and loss of sensation over the medial aspect of the thigh. It may result from pressure on the nerve by the fetal head during pregnancy.

16. Sacral Plexus
The roots (anterior rami) of spinal nerves L4–L5 and S1–S4 form the sacral plexus. This plexus is situated largely anterior to the sacrum. The sacral plexus supplies the buttocks, perineum, and lower limbs.
The largest nerve in the body—the sciatic nerve—arises from the sacral plexus.
The roots (anterior rami) of spinal nerves S4–S5 and the coccygeal nerves form a small coccygeal plexus, which supplies a small area of skin in the coccygeal region.
Sciatic Nerve Injury
The most common form of back pain is caused by compression or irritation of the sciatic nerve, the longest nerve in the human body. Injury to the sciatic nerve and its branches results in sciatica , pain that may extend from the buttock down the posterior and lateral aspect of the leg and the lateral aspect of the foot.
The sciatic nerve may be injured because of a herniated (slipped) disc, dislocated hip, osteoarthritis of the lumbosacral spine, pressure from the uterus during pregnancy, inflammation, irritation, or an improperly administered gluteal intramuscular injection.
Damage to the common fibular nerve causes the foot to be plantar flexed, a condition called footdrop, and inverted, a condition called equinovarus.

17. Cord Physiology
The spinal cord has two principal functions in maintaining homeostasis:
nerve impulse propagation and
integration of information.
The white matter tracts in the spinal cord are highways for nerve impulse propagation.
Sensory input travels along these tracts toward the brain, and
motor output travels from the brain along these tracts toward skeletal muscles and other effector tissues.
The gray matter of the spinal cord receives and integrates incoming and outgoing information.
One of the ways the spinal cord promotes homeostasis is by conducting nerve impulses along tracts.
Often, the name of a tract indicates its position in the white matter and where it begins and ends. For example, the anterior spinothalamic tract is located in the anterior white column; it begins in the spinal cord and ends in the thalamus (a region of the brain).
Notice that the location of the axon terminals comes last in the name.
This regularity in naming allows you to determine the direction of information flow along any tract named according to this convention.
Because the anterior spinothalamic tract conveys nerve impulses from the spinal cord toward the brain, it is a sensory (ascending) tract.

18. Sensory or Ascending Tracts
Posterior Column - conveys impulses for touch, conscious proprioception, kinesthesia, weight discrimination & vibration
Lateral spinothalamic - conducts impulses for pain and thermal reception
Anterior spinothalamic - conducts impulses for itch, tickle, presure and crude consciousness and poorly localized touch sensations.
Anterior & Posterior Spinocerebellar - conducts impulses for itch, tickle, presure and crude consciousness and poorly localized touch sensations.
gracile fasciculus
cuneate fasciculus
posterior spinocerebellar
lateral spinothalamic
anterior spinocerebellar
anterior spinothalamic

19. Motor or Descending Tracts
Rubrispinal - conveys impulses from cerebral cortex and cerebellum to contralateral muscles that govern precise movements of the distal parts of limbs
Tectospinal - conveys impulses from superior colliculus to contralateral skeletal muscles that move head and eyes in response to visual stimuli
Vestibulospinal - conveys impulses from the vestibular nucleus that receives information about hed mvements from the inner ear to regulate ipsilateral muscle tone for maintaining balance in response to head movements
Lateral Reticulospinal - conveys impulses from the reticular formation to facilitate flexor reflexes, inhibit extensor reflexes and decrease muscle tone in muscles of the axial skeleton and proximal parts of limbs
Medial Reticulospinal - conveys impulses from the reticular formation to facilitate extensor reflexes, inhibit flexor reflexes and increae muscle tone in muscles of the axial skeleton and proximal parts of limbs
rubrispinal
medial reticulospinal
lateral reticulospinal
vestibulospinal
tectospinal

20. Reflex Arcs
The second way the spinal cord promotes homeostasis is by serving as an integrating center for some reflexes.
A reflex is a fast, automatic, unplanned sequence of actions that occurs in response to a particular stimulus.
Some reflexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot.
Other reflexes are learned or acquired. For instance, you learn many reflexes while acquiring driving expertise. Slamming on the brakes in an emergency is one example.
When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex. An example is the familiar patellar reflex (knee jerk).
If integration occurs in the brain stem rather than the spinal cord, the reflex is called a cranial reflex. An example is the tracking movements of your eyes as you read this sentence.
You are probably most aware of somatic reflexes, which involve contraction of skeletal muscles.
Equally important, however, are the autonomic (visceral) reflexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands. Body functions such as heart rate, digestion, urination, and defecation are controlled by the autonomic nervous system through autonomic reflexes.
Nerve impulses propagating into, through, and out of the CNS follow specific pathways, depending on the kind of information, its origin, and its destination. The pathway followed by nerve impulses that produce a reflex is a reflex arc.

21. Components of a Reflex Arc
Sensory receptor
The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a sensory receptor. It responds to a specific stimulus —a change in the internal or external environment—by producing a graded potential called a generator potential. If a generator potential reaches the threshold level of depolarization, it will trigger one or more nerve impulses in the sensory neuron.
Sensory neuron
The nerve impulses propagate from the sensory receptor along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem.
Integrating center
One or more regions of gray matter within the CNS act as an integrating center. In the simplest type of reflex, the integrating center is a single synapse between a sensory neuron and a motor neuron. A reflex pathway having only one synapse in the CNS is termed a monosynaptic reflex arc. More often, the integrating center consists of one or more interneurons, which may relay impulses to other interneurons as well as to a motor neuron. A polysynaptic reflex arc involves more than two types of neurons and more than one CNS synapse.
Motor neuron
Impulses triggered by the integrating center propagate out of the CNS along a motor neuron to the part of the body that will respond.
Effector
The part of the body that responds to the motor nerve impulse, such as a muscle or gland, is the effector. Its action is called a reflex. If the effector is skeletal muscle, the reflex is a somatic reflex. If the effector is smooth muscle, cardiac muscle, or a gland, the reflex is an autonomic (visceral) reflex.

22. Components of a Reflex Arc

23. Stretch Reflex
A stretch reflex causes contraction of a skeletal muscle (the effector) in response to stretching of the muscle. This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur by activation of a single sensory neuron that forms one synapse in the CNS with a single motor neuron. Stretch reflexes can be elicited by tapping on tendons attached to muscles at the elbow, wrist, knee, and ankle joints.
Slight stretching of a muscle stimulates sensory receptors in the muscle called muscle spindles. The spindles monitor changes in the length of the muscle.
In response to being stretched, a muscle spindle generates one or more nerve impulses that propagate along a somatic sensory neuron through the posterior root of the spinal nerve and into the spinal cord.
In the spinal cord (integrating center), the sensory neuron makes an excitatory synapse with and thereby activates a motor neuron in the anterior gray horn.
If the excitation is strong enough, one or more nerve impulses arise in the motor neuron and propagate along its axon, which extends from the spinal cord into the anterior root and through peripheral nerves to the stimulated muscle.
The axon terminals of the motor neuron form neuromuscular junctions (NMJs) with skeletal muscle fibers of the stretched muscle.
Acetylcholine released by nerve impulses at the NMJs triggers one or more muscle action potentials in the stretched muscle (effector), and the muscle contracts. Thus, muscle stretch is followed by muscle contraction, which relieves the stretching.

24. Tendon Reflex

25. Flexor Reflex

26. Crossed Extensor
Reflex