AP 2 2.4: Spinal Nerves Flashcards
Spinal Nerves
A nerve is found in the peripheral nervous system and consists of hundreds of thousands of axons wrapped together in a connective tissue. The axons are parts of different neurons, but they are bundled together carrying information from the same location in the body. Cell bodies of neurons are grouped together in masses called ganglia, which are part of a single nerve.
ganglia
Spinal Nerves
Cell bodies of neurons are grouped together in masses called ganglia, which are part of a single nerve.
thirty-one pairs of spinal nerves
There are thirty-one pairs of spinal nerves, originating from the spinal cord, which branch out to innervate the entire body (Figure 2.35). There are eight pairs of cervical (cranial) nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves.
roots
Each spinal nerve emerges from the spinal cord by two short branches, or roots, which lie within the vertebral column
dorsal root
The dorsal root contains the axons of afferent sensory neurons, which conduct impulses to the cord.
after emerging from the vertebral column
Almost immediately after emerging from the vertebral column, a spinal nerve divides into branches called the dorsal ramus and ventral ramus (plural rami).
ventral root
The ventral root contains the axons of efferent motor neurons, which conduct impulses away from the cord. These two roots join just before a spinal nerve leaves the vertebral column. Therefore, all spinal nerves are mixed nerves that take impulses to and from the spinal cord.
dorsal ramus
The smaller dorsal ramus contains nerves that serve the dorsal portions of the trunk carrying visceral motor, somatic motor, and sensory information to and from the skin and muscles of the back
ventral ramus
The larger ventral ramus contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs carrying visceral motor, somatic motor, and sensory information to and from the body surface, structures in the body wall, and the limbs
plexus
Some ventral rami merge with adjacent ventral rami to form a nerve plexus, a network of interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to some target location. Major plexuses include the cervical, brachial, lumbar, and sacral plexuses
Sensory and motor information is carried by spinal nerves in a systematic method
Sensory and motor information is carried by spinal nerves in a systematic method. Sensory (afferent) information is received by spinal nerves distributed throughout the body. The signal passes through the dorsal roots into the spinal cord and then finally up to the brain for processing. Motor commands are initiated in the central nervous system, where a decision by the brain is made to move. The command signal is sent down the spinal cord to the appropriate level, then out the ventral root of the spinal cord. The signal is carried via a spinal nerve to its appropriate destination to the muscle fiber to enable a voluntary muscular contraction.
dermatome
A dermatome is a region of skin that carries sensory information through a specific pair of spinal nerves to the spinal cord and up to the brain. The dermatome regions from each spinal nerve are color coded
Cervical Plexus
The cervical plexus is a combination of the ventral rami of spinal nerves C1-C5
phrenic nerve
The phrenic nerve is the most important nerve of the cervical plexus and supplies both motor and sensory fibers to the diaphragm. Irritation of this nerve causes hiccups. Severing this nerve would cause paralysis of the diaphragm.
Paralysis
phrenic nerve
Paralysis means motor loss of function. Paralysis of the diaphragm would require use of a ventilator, a mechanical respirator. The saying, “three, four, five keeps the diaphragm alive,” is an easy way to remember that the phrenic nerve arrives from the ventral rami of C3-C5.
