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 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 The spinal cord consists of white matter surrounding a core of grey matter. 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
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
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) • 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
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.
Ascending Tracts 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 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 Tracts 1.
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 • Coordinate muscles & controls posture
Descending Tracts
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