The Anatomy of the Nervous System
The larger part of the nervous system is tissue that falls into two classes or cells: called neurons and neuroglia.
Neurons also referred to as nerve cells; communicate via transmitting electrochemical signals throughout the body. Neurons appearance is very different from other cells within the body, as it has long cellular processes that extend from its central cell body. The cell body is round portion of a neuron, and it contains the nucleus, the majority of cellular organelles and mitochondria. Dendrites, which look like small tree-like structures, extend from the cell body to gather up stimuli from the environment, sensory receptors or other neurons. Axon, long transmitting processes, extend from the cell body as well, sending a signal on to different neurons or effector cells in the body.
There three fundamental classes of neurons, efferent neurons, afferent neurons, and interneurons.
1. Efferent Neurons, also called motor neurons, transmit signals from the CNS to effectors in the body to muscles, glands, and other organs.
2. Afferent neurons, known as sensory neurons, transmit sensory signals to CNS from receptors within the body.
3. Interneurons form complex networks within the CNS to combine information delivered from afferent neurons and to guide the function of the body through efferent neurons.
Neuroglia called glial cells, is an assistant cell within the nervous system. 6 to 60 neuroglia surround each neuron in the body to protect, feed and insulate the neuron. Neurons almost never reproduce, and since they are extreme;u specialized cells, that at are essential to the body, Neuroglia is critical to maintaining a working nervous system.
The brain is a soft, gray, crinkled organ that weighs near 3 pounds, located within the cranial cavity. The skull surrounds and protects the brain. The brain holds about the approximately 100 billion neurons, which creates for the main control center of the body. The brain along with the spinal cord together form the central nervous system (CNS). Within the CNS information is processed, and responses begin. Higher mental functions such as memory, consciousness, planning, voluntary actions as well as lower body functions, that keeps the body operating, such as respiration maintenance, blood pressure, heart rate, and digestion.
The spinal cord is a long thin portion of nerve cells, bundle together that carries information through the body via the vertebral cavity of the spine to the start of the medulla oblongata of the brain on the superior end and continuing inferiorly to the lumbar section of the spine. The Lumbar section. Spinal cord divides into a bundle of individual nerves called cauda equina ( as it resembles a horse tail). The Cauda equina continues inferior to the sacrum and coccyx. The substantia alba of the neural structure functions because the main passage of nerve signals to the body from the brain. The gray matter of the neural structure integrates reflexes to stimuli.
Nerves are bunches of axons in the peripheral nervous system (PNS) that serve as knowledge transmitters to relay signals between the brain, spinal cord and the remainder of the body. Each axon is covered in a connective tissue sheathing called the endoneurium. Unique axons of the nerve are combined into collections of axons called fascicles, encased in a sheath of connective tissue called the perineurium. Conclusively, many fascicles are enclosed together in another layer of connective tissue called the epineurium to make a whole nerve. The encasing of nerves with connective tissue helps to protect the axons and to enhance the speed of their communication inside the body.
Afferent, Efferent, and Mixed Nerves. Ther are several nerves in the body that are specialized for transmitting information in only one direction, similar to a one-way avenue. Nerves that send information from sensory receptors to the central nervous system only are described as afferent nerves. Different neurons, identified as efferent nerves, transfer signals only from the central nervous system to effectors, like muscles and glands. Lastly, some nerves are mixed nerves that hold both afferent and efferent axons. Mixed nerves operate similarly to 2-way streets where afferent axons act as lanes heading to the central nervous system and efferent axons operate as lanes traveling away from the central nervous system.
Cranial Nerves. Stretching from the lower side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is recognized by a Roman numeral 1 to 12 based upon its position on the anterior-posterior axis of the brain. Each nerve likewise has a representative name (e.g. olfactory, optic, etc.) that classifies its purpose or position. The cranial nerves provide a direct contact to the brain for the special sense organs, muscles of the neck, head, and heart, the shoulders, and the GI tract.
Spinal Nerves. Reaching from the right and left sides of the spinal cord there are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that transmit both sensory and motor signals connecting the spinal cord and specific regions of the body. The 31 spinal nerves are classified into five groups named for the five areas of the vertebral column. Therefore, there are 12 pairs of thoracic nerves, eight pairs of cervical nerves, five pairs of sacral nerves, five pairs of lumbar nerves, and one pair of coccygeal nerves. Every spinal nerve exits from the spinal cord via the intervertebral foramen connecting a pair of vertebrae or connecting the C1 vertebra and the occipital bone of the skull.
Meninges are the protective casings of the central nervous system (CNS). They comprise three layers: the dura mater, arachnoid mater, and pia mater.
Dura mater. The dura mater, which means “tough mother,” is the toughest, thickest and the most exterior layer of meninges. Consisting of dense irregular connective tissue, it holds many blood vessels and strong collagen fibers. Dura mater protects the CNS from external damage, holds the cerebrospinal fluid that encloses the CNS, and gives blood to the nervous tissue of the CNS.
Arachnoid mater. Which means “spider-like mother,” is much thinner and more sensitive than the dura mater. It pads the inside of the dura mater and contains numerous thin fibers that connect it to the underlying pia mater. These fibers cross a fluid-filled space called the subarachnoid space between the arachnoid mater and the pia mater.
Pia mater. The pia mater means “tender mother,” is a slim and sensitive layer of tissue that sits upon the outside of the brain and spinal cord. It contains numerous blood vessels that support the nervous tissue of the CNS, the pia mater inserts into the channels of the sulci and fissures of the brain as it comprises the entire exterior of the CNS.
The area encompassing the organs of the CNS is filled with a clear fluid distinguished as cerebrospinal fluid (CSF). CSF is developed from blood plasma by special formations called choroid plexuses. The choroid plexuses hold many capillaries filled with epithelial tissue that filters blood plasma and enables the filtered fluid to enter the area surrounding the brain.
Recently created CSF passes through the inside of the brain in unfilled places called ventricles and through a tiny cavity in the middle of the spinal cord called the central canal. CSF also circulates through the subarachnoid space throughout the outside of the brain and spinal cord. CSF is continually produced at the choroid plexuses and is reabsorbed into the bloodstream at structures named arachnoid villi.
Cerebrospinal fluid contributes several essential functions to the central nervous system:
CSF absorbs shocks between the brain and skull and between the spinal cord and vertebrae. This shock absorption guards the CNS against knocks or sudden changes in velocity, such as during an automobile collision.
The brain and spinal cord rest within the CSF, reducing their actual weight through buoyancy. The brain is a rather large but delicate organ that needs a high volume of blood to function efficiently. The decreased weight in cerebrospinal fluid allows the blood vessels of the brain to remain accessible and helps shield the nervous tissue from becoming damaged under its own weight.
CSF helps to control biochemical homeostasis within the central nervous system. It contains oxygen, nutrients, ions and albumins that support the osmotic and chemical balance of nervous tissue. CSF also eliminates waste products that accumulate as byproducts of cellular metabolism within nervous tissue.
All of the body’s sensory organs are parts of the nervous system. These abilities are called special senses—smell, taste, hearing, vision and balance—are all identified by specialized organs such as the olfactory epithelium, eyes, and taste buds. Sensory receptors for the general senses like touch, temperature, and pain are found everywhere most of the body. Every sensory receptor of the body is joined to afferent neurons that send their sensory information to the CNS to be integrated.