Chapter 11, Section 2

Spinal Cord

Spinal Cord

The spinal cord is continuous with

the brain and extends downward

through the vertebral canal.

The spinal cord begins at the foramen

magnum, and terminates between the

first and second lumbar vertebra.

Two regions of the spinal cord are thickened

1. Cervical Enlargement – nerves to upper limbs

2. Lumbar enlargement – nerves to lower limbs

Structure of the

Spinal Cord

31 pairs of spinal nerves arise from the

spinal cord.

• 8 cervical nerves

• 12 thoracic nerves

• 5 lumbar nerves

• 5 sacral nerves

• 1 coccygeal nerve

Figure 11.29 The thirty one

pairs of spinal nerves are

grouped according to the level

from which they arise.

Terminal end of the

Spinal Cord

The spinal cord tapers at its terminal

end into the conus medullaris

Filum terminale – thin cord of connective

tissue arising from the conus medullaristissue arising from the conus medullaris

Cauda Equina “Horse’s tail” – spinal nerves

at the conus medullaris fan outward, creating

a structure that resembles a horse’s tail.

Cross Section of the

Spinal Cord

Anterior Median Fissure &

Posterior Median Sulcus

• Grooves that divide the spinal cord into left and right halves

Central Canal – continuous with ventricles in the brain

The spinal cord consists of white matter

surrounding a core of grey matter.

Central Canal – continuous with ventricles in the brain

Structures of the

Spinal Cord Grey Matter

• Posterior horns – located towards the dorsal surface

• Anterior horns – located towards the ventral surface

• Lateral horns – located in some regions of the spinal cord,

• Grey commissure – connects the left and right grey matter

White Matter

• Posterior funiculus

• Anterior funiculus

• Lateral funiculus

The funiculi (sing. funiculus)

are columns that provide

pathways for axons, called

nerve tracts.

Spinal Nerves

Dorsal Root – Branch of spinal

nerves that carry sensory impulses

towards spinal cord

Dorsal Root Ganglion – Mass of

sensory neuron cell bodies in the

dorsal root of the spinal cord

Ventral Root – Branch of spinal

nerves that carry motor impulses

away from the spinal cord

dorsal root of the spinal cord

Functions of Spinal Cord

• The spinal cord is a conduit for

nerve impulses to and from the

brain and brainstem

• The spinal cord is a center

for spinal reflexes

Reflex Arcs

•Reflexes are automatic, subconscious responses to stimuli within or

outside the body

• Reflexes are the simplest response to a stimulus

• Simple reflex arc (sensory – motor)• Simple reflex arc (sensory – motor)

• Most common reflex arc (sensory – interneuron – motor)

Figure 10.7a. A schematic of a reflex arch.

Reflex Arcs

Five components of a reflex arc.

1. Receptor - detects changes in environment

• Dendritic end of a sensory neuron, or specialized receptor cell

2. Sensory neuron – conveys stimulus to spinal cord

• Passes through Dorsal Root• Passes through Dorsal Root

3. Interneuron – conducts impulse from sensory neuron to motor neuron

4. Motor Neuron – transmits impulse to effector

• Passes through Ventral Root

5. Effector – muscle or gland that produces the reflex

Figure 10.7b the five components of a reflex arc.

Reflex Arcs

Table 11.2 summarizes the components of a reflex arc.

Tracts of the Spinal Cord

• Ascending tracts conduct sensory impulses to the brain

• Descending tracts conduct motor impulses from the brain to

motor neurons reaching muscles and glands

Figure 11.11 Major

ascending and descending

tracts in a cross section of

the spinal cord.

1. Fasciculus gracilis & fasiculus cuneatus

• Within the posterior funiculus of spinal cord

• Conducts sensory information from skin, muscles, and joints

• Sense touch, pressure, and body movement

• Fibers decussate (cross over) in medulla oblongata of brain

2. Lateral Spinothalamic tract

• Conducts sensations of pain and

Ascending Tracts

• Conducts sensations of pain and

temperature to thalamus of the brain

• Fibers decussate in spinal cord

3. Anterior Spinocerebellar tract

• Conducts impulses from

muscles of lower limbs and

trunk to cerebellum

• Coordinate muscle movements

Ascending Tracts

Figure 11.12 sensory impulses originating in skin

touch receptors ascend in the fasciculus cuneatus

tract and cross over in medulla oblongata of the

brain. Pain and temperature information ascends

in the lateral spinothalamic tract, which crosses

over in the spinal cord.

Descending Tracts1. Corticospinal tracts

• Lateral and anterior tracts

• Motor impulses from cerebral cortex to spinal

nerves and outward to various skeletal muscles

• Fibers decussate in medulla oblongata

• Voluntary movement

2. Rubrospinal tract

• Passes through lateral funiculi

• Motor impulses from brain to skeletal muscles• Motor impulses from brain to skeletal muscles

• Coordinate muscles & controls posture

Descending Tracts

Figure 11.13 Most fibers of the corticospinal tract Figure 11.13 Most fibers of the corticospinal tract

originate in the cerebral cortex, cross over in the

medulla, and descend in the spinal cord, where they

synapse with neurons whose fibers lead to spinal

nerves supplying skeletal muscles. Some fibers

cross over in the spinal cord.

End of Chapter 11, Section 2