2.4 Spinal Nerves and Reflexes Flashcards
Describe the spinal cord
The spinal cord is a delicate structure made up of nervous tissue. It is surrounded and protected by the spinal (vertebral) column which is made up of bone. The spinal cord is not bone. The spinal cord begins at the foramen magnum of the skull as a continuation of the medulla oblongata, and it ends at the L1/L2 vertebrae in adults. The inferior end of the spinal cord is known as the conus medullaris which then continues as the filum terminale (anchors the lower part of the cord).
Explain the difference in lengths between the spinal cord and spinal column.
In the embryo (at 8 weeks) the spinal cord and vertebral canal are at equal lengths, so the spinal segment level is equal to the vertebrae level. But then the spinal cord grows more slowly compared to the bone landmarks and the vertebral column. In adults, the spinal cord exists in the upper 2/3rds of the vertebral column (from the foramen magnum to L1/2)
How many pairs of spinal nerves are there and from where do they emerge
There are 31 pairs of spinal nerves emerge from intervertebral foramina, each nerve exits from a specific foramina
How are spinal nerves classified
Each pair of spinal nerves relates to a spinal segment.
8 cervical nerves
12 thoracic nerves
5 lumbar nerves
5 sacral nerves
1 coccygeal nerve
The cervical spinal nerves are numbered according to the vertebra below (other than C8 as there are only 7 cervical vertebra), all the other spinal nerves are numbered according to the vertebra above
Why dont spinal segments correspond to the bone landmarks (vertebrae)
As there are unequal numbers. All the spinal nerves emerge between adjacent vertebrae except for C1 which emerges between the occipital bone of the skull and C1 atlas
Describe the cross section of a spinal cord section
the dorsal area where we find the sensory afferent and ventral area for motor efferent
rootlets converge to become roots (dorsal or ventral), and the roots converge to become a spinal nerve
-the spinal nerves then divide into ventral and dorsal rami (which are mixed motor and sensory nerve fibres)
-the grey matter is composed of a dorsal horn, lateral horn and ventral horn on each side, with the central canal running through the middle
What is a lumbar puncture
Also known as a spinal tap, a lumbar puncture is used to diagnose many conditions can be performed between L3/L4 or L4/L5 in adults, this line is between the highest points of iliac crest.
What are myotomes
Functionally related groups of skeletal muscles in a specific area that are supplied by a specific single spinal nerve.
What are dermatomes
Localised areas of skin that are supplied by a single spinal nerve, however they can overlap
Describe a virus that targets dermatomes
chickenpox virus (VZV) lies dormant in a dorsal root ganglion, the virus can then be reactivated later in life to cause shingles- this rash develops in the specific skin area of the dermatome that the nerve correlates to
What is the basic circuit of a spinal reflex arc
sensory receptor, sensory neuron (afferent), integration centre (one or more synapses in CNS), motor neuron (efferent) & effector
Describe a withdrawal/flexion reflex
A withdrawal/flexion reflex is a protective reflex in response to a noxious stimulus causing general flexion of the entire limb. There is then recipcrocal inhibition causing the limb to be withdrawn from the stimulus. This reflex circuitry is polysynaptic with excitatory and inhibitory interneurons.
Describe the crossed extension reflex
where collateral axons cross the spinal cord midline, facilitating extensors and inhibiting flexors
-this allows for a smooth transfer of weight from one leg to the other
