Spinal Cord and Peripheral Nervous System (Vocabulary Flashcards)

Spinal Cord: Structure and Function

The spinal cord is an oval-shaped hollow cylinder of nervous tissue. It runs from the base of the brain and is protected by the vertebrae of the spine.
There are 31 pairs of nerves that originate from the spinal cord.
Each spinal nerve has a dorsal root of sensory neurons and a ventral root of motor neurons.
Therefore, all spinal nerves are mixed nerves.
The spinal cord serves as a link between body parts and the brain. It receives sensory information from receptors and sends out motor commands to the receptors.
The spinal cord acts as a coordinator and carries out reflex actions.
It also helps in better functioning of the brain.

The cross section of the brain and spinal cord shows that the central nervous system (CNS) has two distinct areas.
White matter consists of many myelinated fibers.
Gray matter consists mainly of cell bodies and non-myelinated fibers.
In the spinal cord, a butterfly-shaped gray matter is surrounded by white matter, while in the brain, gray matter makes up the outer layer with some white matter in the center.
These distinctions reflect the organization of information flow: white matter mainly for communication (myelinated axons), gray matter for processing (neuronal cell bodies).

The peripheral nervous system is composed of nerves that link the CNS with different body parts.
Nerves arising from the brain are called cranial nerves.
Nerves arising from the spinal cord are called spinal nerves.
There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in the human body.
Some cranial nerves are sensory, some motor, and some mixed, while all spinal nerves are mixed nerves.

The peripheral nervous system can be divided into two functional divisions: somatic and autonomic nervous systems.
The somatic nervous system generally consists of the cranial and spinal nerve fibers that connect the CNS to the skin and skeletal muscles.
The autonomic nervous system includes fibers that connect the CNS to visceral organs such as the heart, stomach, intestine, and various glands. It is concerned with unconscious activities.

The ANS is divided into the sympathetic and parasympathetic nervous systems.
Sympathetic system: it consists of only spinal nerves. It functions only in the state of emergency and prepares the body for a flight or fight response.
Parasympathetic system: it consists of a few cranial and spinal nerves.
It controls various autonomic nerves during the state of rest and, in short, returns body functions to normal after they have been altered by sympathetic stimulation.

Reflex actions: spinal cord involvement in reflexes allows rapid responses without direct brain involvement, illustrating its coordinating role.
CNS organization: distinction between white matter (myelinated fibers) and gray matter (cell bodies, non-myelinated fibers) underpins how signals are processed and transmitted.
Mixed nerves: spinal nerves being mixed (sensory and motor fibers) enable integrated sensory-motor pathways.
Autonomic balance: sympathetic vs parasympathetic actions illustrate how the body maintains homeostasis across stress and rest states.
Real-world relevance: understanding PNS pathways helps explain responses to environmental stimuli, autonomic regulation of heart rate and digestion, and how reflexes protect the body from harm.