Spinal Nerves

There are also autonomic nervous system pathways as well. Most of the sympathetic preganglionic neurons are found between T1 and L3, also known as the thoracolumbar junction. We will talk more about these ganglia when we talk about the autonomic nervous system. There are also sacral preganglionic neurons near S2 to S4. These control the male erection, urination, and defecation. There are somatic nerves, lumbar sympathetic nerves and sacral parasympathetic nerves that all play a role in these processes.

SPINAL NERVES

Spinal nerves, as mentioned, come from the spinal cord in a paired fashion. These are both sensory and motor fibers that connect the peripheral nervous system and the central nervous system. Autonomic nervous system fibers run along with these as well. These are specifically classified as parts of the peripheral nervous system rather than the central nervous system.

Out of the dorsal and ventral roots of the spinal cords, a fila radiculara is formed that forms the spinal nerve itself that exits a lateral space in the bone called the intervertebral foramina. The dorsal root is sensory and the ventral root is motor in nature. The sensory nerves travel up to the thalamus, while the motor nerves emanate downward from the thalamus. There are shorter pathways in the motor aspect of the spinal cord that involve the deep tendon reflexes, like the knee-jerk reflex. This does not need brain interventions.

All of the muscles that are innervated by the same cranial nerve is collectively called a myotome and all the skin areas innervated by the same cranial sensory nerve is called a dermatome. Basically, as each bundle exits the spinal cord, it is called a ramus. Figure 20 shows how they come together to make a spinal nerve:

Figure 20.

The parasympathetic branch of the ANS has preganglionic cells found in the cranial and sacral areas both. The cranial nerves III, VII, IX, and X along with S2 to S4 in the sacrum all have parasympathetic influences. These figures carry their impulse far from the spinal cord in order to have the parasympathetic visceral ganglia sitting very near to their target tissues.

There are several plexuses that represent a mixing of motor and sensory nerves of many different spinal levels to make peripheral nerves that are made from several spinal levels at once. There is a cervical plexus in the neck, a brachial plexus in the axilla, and a lumbosacral plexus in the lumbosacral area. The thoracic nerves do not make a plexus but generally innervate just one area per spinal nerve.