2. 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 โ nerves to upper limbs
2.Lumbar enlargement โ nerves to lower limbs
3. 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.
4. Terminal end of the spinal cord
The spinal cord tapers at its terminal
end into the conus medullaris
Filum terminale โ thin cord of connective
tissue arising from the conus medullaris
Cauda Equina โHorseโs tailโ โ spinal nerves
at the conus medullaris fan outward, creating
a structure that resembles a horseโs tail.
5. 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
โข Grooves that divide the spinal cord into left and right halves
Central Canal โ continuous with the ventricles in the brain
6. structures of the spinal cord
Grey Matter โ unmyelinated tissue
Posterior horns โ located towards the dorsal surface
Anterior horns โ located towards the ventral surface
Lateral horns โ located in some regions of the spinal cord
Grey commissure โ connects the left and right grey matter
White Matter โ myelinated axons
The white matter can be separated into 3 columns
Posterior funiculus
Anterior funiculus
Lateral funiculus
The funiculi (sing. funiculus) are columns that
provide pathways for axons, called nerve tracts.
7. Spinal Nerves
Dorsal Root โ Branch of spinal
nerves that carry sensory impulses
towards spinal cord
Dorsal Root Ganglion โ Mass of
sensory neuron cell bodies in the
dorsal root of the spinal cord
Ventral Root โ Branch of spinal
nerves that carry motor impulses
away from the spinal cord
8. Functions of the 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.
Most reflexes occur at the
level of the spinal cord.
9. Reflex Arcs
Reflexes are automatic, subconscious responses to
stimuli within or outside the body
Reflexes are the simplest response to a stimulus
In a simple reflex arc a sensory neuron is directly
connected to a motor neuron within the spinal cord.
Most reflex arcs also involve interneurons that
connect the sensory neuron to a motor neuron.
Figure 10.7a. A schematic of a reflex arch.
Figure 10.7a. A schematic of a reflex arch.
10. Reflex Arcs
Five components of a reflex arc.
1.Receptor - detects changes in environment
A receptor may be the dendritic end of a sensory neuron, or it
may be a specialized cell
2.Sensory neuron โ conveys stimulus to spinal cord
sensory neurons pass through the dorsal root
โขInterneuron โ connects the sensory neuron to the motor neuron
โขMotor Neuron โ transmits the impulse to the effector
motor neurons pass through the ventral root
5. Effector โ muscle or gland that produces the reflex
12. Reflex Arcs
Table 11.2 summarizes the components of a reflex arc.
Table 11.2 summarizes the components of a reflex arc.
13. Tracts of the spinal cord
Ascending tracts conduct sensory impulses up the
spinal cord to the brain
Descending tracts conduct motor impulses from the
brain down the spinal cord to motor neurons reaching
muscles and glands
Figure 11.11 Major ascending
and descending tracts in a cross
section of the spinal cord.
14. Examples of ascending tracts
1.
Fasciculus gracilis & fasiculus cuneatus are located within the posterior
funiculus of the spinal cord.
โข
They transmit sensory information from skin, muscles, and joints
โข
Sense touch, pressure, and body movement
โข
Fibers decussate (cross over) in medulla oblongata of brain
2.
Lateral Spinothalamic tract
โข
Conducts sensations of pain and
temperature to thalamus of the brain
โข
Fibers decussate in spinal cord
3.
Anterior Spinocerebellar tract
โข
Conducts impulses from
muscles of lower limbs and
trunk to cerebellum
โข
Coordinate muscle movements
15. Ascending Tracts
Figure 11.12
Sensory impulses originating touch receptors
of the skin 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.
16. Examples of descending tracts
1. Corticospinal tracts
โข Lateral and anterior tracts
โข Transmits motor impulses from the cerebral cortex to spinal
nerves and outward to various skeletal muscles
โข Fibers decussate in medulla oblongata
โข Corticospinal tracts are responsible for voluntary movement
2. Rubrospinal tract
โข Passes through the lateral funiculi
โข Transmits motor impusles that coordinate
muscles and controls posture.
17. Examples of descending tracts
3.
Reticulospinal tracts
transmits motor impulses responsible for sweat
glands and muscle tone.
18. Descending Tracts
Figure 11.13
Most fibers of the corticospinal tract originate
in the cerebral cortex.
They cross over in the medulla, and descend
in the spinal cord
Corticospinal tracts 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
