Spinal Cord Anatomy Quiz

Chapter 13: The Spinal Cord, Spinal Nerves, and Somatic Reflexes

• Spinal cord is a cylinder of nervous tissue arising from the brainstem at the foramen magnum of the skull.
• It occupies the upper two-thirds of the vertebral canal.
• The inferior margin ends at L1 or slightly beyond.
• It averages 1.8 cm thick and 45 cm long.
• It gives rise to 31 pairs of spinal nerves.
• A segment is part of the spinal cord supplied by each pair of spinal nerves.
• The spinal cord has longitudinal grooves on its anterior and posterior sides.
• It has an anterior median fissure and a posterior median sulcus.
• The cord is divided into cervical, thoracic, lumbar, and sacral regions.
• Two areas are thicker than others: cervical and lumbosacral enlargements; they give rise to nerves for upper limbs and pelvic region/lower limbs, respectively.
• The medullary cone (or conus medullaris) is the inferior point of the spinal cord.
• The cauda equina is a bundle of nerve roots that occupy the vertebral canal from L2 to S5, resembling a horse's tail.
• The spinal cord is enclosed by three fibrous membranes (meninges): dura mater, arachnoid mater, and pia mater.
• Dura mater is a tough, thick membrane forming a loose-fitting sleeve around the spinal cord.
• Epidural space is between the dura and the vertebral bones. It contains blood vessels, adipose tissue, and loose connective tissue.
• Arachnoid mater is a thin membrane adhering to the dura that's separated from the pia by fibers spanning the subarachnoid space, which contains cerebrospinal fluid.
• Pia mater is a delicate, transparent membrane that closely follows the spinal cord's contours, continuing inferiorly as a fibrous terminal filum.
• It fuses with the dura to form the coccygeal ligament.
• Denticulate ligaments are extensions of the pia that pass through the arachnoid to the dura, anchoring the cord.

13.1d Cross-Sectional Anatomy 1

• Spinal cord has gray and white matter.
• Gray matter is dull in color (no myelin). It contains neuron cell bodies, dendrites, and proximal axons.
• White matter is bright, pearly white due to myelin and contains axon bundles coursing up and down the cord.

13.1d Cross-Sectional Anatomy 2

• Gray matter is shaped like a butterfly.
• It has two posterior (dorsal) horns receiving sensory nerve fibers and synapsing with interneurons.
• It has two anterior (ventral) horns with cell bodies of motor neurons.
• Additional lateral horns occur in segments T2-L1 and are associated with the sympathetic nervous system.
• Gray matter on each side is connected by a gray commissure.
• The central canal is at the center of the commissure.

13.1d Cross-Sectional Anatomy 3

• White matter surrounds the gray matter.
• It has three large bundles of axons-posterior, lateral, and anterior funiculi-on each side. 
• Each funiculus is subdivided into fasciculi or tracts.

13.1e Spinal Tracts 1

• Fibers in a tract have similar origin, destination, and function.
• Ascending tracts carry sensory information up.
• Descending tracts carry motor information down.
• Decussation is the crossing of the midline that occurs in many tracts.
• The brain senses and controls the contralateral side of the body.
• Contralateral-origin and destination are on opposite sides of the body.
• Ipsilateral-origin and destination are on the same side; it does not decussate.

13.1e Spinal Tracts 2

• Ascending tracts involve three neurons.
• First-order neuron detects stimulus and transmits signal to spinal cord or brainstem.
• Second-order neuron continues to thalamus.
• Third-order neuron carries signal to sensory region of the cerebral cortex.

13.1e Spinal Tracts 3

• Posterior funiculus consists of cuneate and gracile fasciculi.
• Gracile fasciculus carries signals from midthoracic and lower parts of the body.
• Cuneate fasciculus carries signals from chest and upper limbs.
• First-order nerve fibers travel up the ipsilateral side of the spinal cord.
• They terminate at the medulla; second-order neurons cross to the other side of the brain.
• It carries signals for vibration, visceral pain, deep and discriminative touch, and proprioception from lower limbs and lower trunk.
• Proprioception is the nonvisual sense of position and movements of the body.

13.1e Spinal Tracts 4

• Spinothalamic tract is part of the anterolateral system.
• It passes up the anterior and lateral funiculi of the spinal cord.
• It carries signals for pain, pressure, temperature, light touch, tickle, and itch.
• The tract is composed of second-order neuron axons.
• First-order neurons end in the posterior horn of the spinal cord.
• Second-order neurons start in the posterior horn, decussate, and form the spinothalamic tract.
• Third-order neurons continue to the cerebral cortex.
• Signals are sent to the cerebral hemisphere opposite to the site of the stimulation.

13.1e Spinal Tracts 5

• Spinoreticular tract travels up the anterolateral system.
• It carries pain signals resulting from tissue injury.
• It is composed of axons of second-order neurons.
• First-order neurons immediately synapse with second-order neurons in the posterior horn.
• Second-order neurons decussate to the opposite system.
• They ascend the cord and end in the reticular formation.
• This loosely organized core of gray matter is in the medulla and pons.

13.1e Spinal Tracts 6

• Posterior, and anterior spinocerebellar tracts go through the lateral funiculus.
• They carry proprioceptive signals from limbs and trunk to the cerebellum.
• They are composed of second-order neuron axons.
• First-order neurons originate in muscles and tendons and end in the posterior horn of the spinal cord.
• Second-order neurons ascend spinocerebellar tracts and end in the cerebellum for feedback needed to coordinate movements.

13.1e Spinal Tracts 7

• Descending tracts involve two motor neurons.
• Upper motor neuron—originates in cerebral cortex or brainstem and terminates on a lower motor neuron.
• Lower motor neuron—cell body in brainstem or spinal cord; axon leads to the muscle or other target organ.

13.1e Spinal Tracts 8

• Lateral, and anterior corticospinal tracts carry signals from cerebral cortex.
• They are for precise, finely coordinated movement.
• Pyramids are ridges on the anterior surface of the medulla oblongata formed from fibers of this system.
• Most fibers decussate in the lower medulla, forming the lateral corticospinal tract. 
• The tract is on the contralateral side of the spinal cord.
• Some fibers form the anterior (ventral) corticospinal tract that descends in the ipsilateral side of the spinal cord.
• It decussates inferiorly and ultimately controls contralateral muscles.

13.2 The Spinal Nerves

• Spinal nerves are 31 pairs.
• 8 cervical (C1-C8)
• 12 thoracic (T1-T12)
• 5 lumbar (L1-L5)
• 5 sacral (S1-S5)
• 1 coccygeal (C01)

13.2 Spinal Nerves 2

• Each spinal nerve is formed by two roots.
• The posterior (dorsal) root is for sensory input to the spinal cord.
• The posterior root ganglion has sensory neurons' cell bodies, carrying signals to the spinal cord.
• Six to eight rootlets enter the posterior horn of the cord.
• The anterior (ventral) root is motor output from the spinal cord.
• Six to eight rootlets leave the spinal cord and converge to form the anterior root.
• Cauda equina is formed from roots arising from L2-Co1.

13.2.b Spinal Nerves 1

• Spinal nerves are 31 pairs.
• 8 cervical (C1-C8)
• 12 thoracic (T1-T12)
• 5 lumbar (L1-L5)
• 5 sacral (S1-S5)
• 1 coccygeal (Co1)
• First cervical nerve exits between the skull and atlas.
• Other nerves exit at intervertebral foramina.
• Some nerves form five plexuses.

13.3 Somatic Reflexes

• Reflexes are fast, involuntary, stereotyped reactions of glands or muscles to stimulation
• They require stimulation, not spontaneous.
• These reactions involve a quick response due to the minimal number of interneurons, and minimal synaptic delay.
• Reflexes are difficult to suppress.
• Occur in a stereotyped way, essentially the same every time.
• Some reflexes involve glandular secretion or muscle contraction of somatic, visceral nerves.
• Somatic reflexes involve the somatic nervous system (innervating skeletal muscle).
• Visceral reflexes involve organs such as the heart and intestines.
• A somatic reflex involves a reflex arc with the following pathway: somatic receptors, afferent nerve fibers, integrating center, efferent nerve fibers, and effectors.
• Somatic receptors are in the skin, muscles, and tendons.
• Afferent nerve fibers carry information from receptors to the posterior horn of the spinal cord.
• The integrating center is a point of synaptic contact between neurons; it determines whether efferent neurons signal muscles.
• Efferent nerve fibers carry motor impulses to the muscles.
• Effectors carry out the response (the muscles).

13.3 The Muscle Spindle 1

• Many somatic reflexes involve muscle spindles—stretch receptors embedded in skeletal muscles.
• They are proprioceptors- specialized sense organs to monitor position and movement of body parts.
• They inform the brain about muscle length and body movement.
• They enable the brain to send motor commands back to the muscles (corrective reflexes); muscle tone and posture maintain.

13.3 The Muscle Spindle 2

• Muscle spindle structure includes intrafusal fibers- modified muscle fibers within the spindle.
• Outside fibers are extrafusal fibers and produce force for movement
• Gamma motor neuron is to innervate the ends of intrafusal fibers and keep it taut.
• Alpha motor neuron supplies extrafusal fibers.
• The midportion of the intrafusal fiber contains sensory nerve fibers.
• Primary afferent fibers monitor fiber length and speed of length changes.
• Secondary afferent fibers monitor length only.
• Example: spindle function: help you keep upright when standing on a boat.

13.3c. The Stretch Reflex 1

• Stretch (myotatic) reflex: when a muscle is stretched, it "fights back" and contracts.
• Helps maintain equilibrium and posture.
• The reflex is mediated primarily by brain but spinal component is pronounced if a muscle is suddenly stretched by a tendon tap (knee jerk).
• It stabilizes joints by balance in tension of extensors and flexors, making muscle action smooth.

13.3e The Crossed Extension Reflex

• Crossed extension reflex: when one limb withdraws, the opposite limb extends to maintain balance.
• Flexor reflex works ipsilaterally (on the same side), and crossed extension reflex is contralateral (opposite side)
• Intersegmental reflex- input and output at different levels of spinal cord
• Pain in foot can contract abdominal muscles.

13.3f The Tendon Reflex

• Tendon organs are proprioceptors in a tendon near its junction with a muscle.
• They are involved in the tendon reflex.
• 0.5 mm long, encapsulated bundles of collagen fibers and one or more nerve fibers.
• Tendon reflex-response to excessive tension on the tendon.
• Inhibits muscle from contracting strongly.
• Moderates muscle contraction before tearing or pulling it loose from bone or muscle.

Spinal Cord Trauma

• Complete transection: complete severance of the cord.
• Immediate loss of motor control below injury level.
• Above C4 results in threat of respiratory failure.
• Spinal shock.
• Paralysis: 
  • Paraplegia—paralysis of both lower limbs.
  • Quadriplegia—paralysis of all four limbs.
  • Hemiplegia—paralysis on one side of the body.
• Paresis—partial paralysis or weakness of the limbs.

Test your knowledge on the anatomy of the spinal cord with this quiz. Questions cover various structural and functional aspects of the spinal cord, including its regions, functions, and unique features. This is an excellent resource for students studying human anatomy or related fields.