The Nervous System and the Brain

The Human Nervous System consists of the Central Nervous System, which relates to the Brain and the Spinal Cord, and the Peripheral Nervous System, which relates to the functions of the body and how the body responds and acts to these functions.

The Central Nervous System is divided into two parts, which are the brain and the spinal cord. On average the adult human brain weighs in at 1.3 to 1.4 kg, or about 3 pounds. The brain itself contains about 100 billion nerve cells that are referred to as neurons, and the brain has trillions of supporting cells called glia that support the neuron cells. The second part of the Central Nervous System is the Spinal Cord. The spinal cord is approximately 43 cm long in the adult woman and approximately 45 cm long in the adult mail and weighs in at about 35 to 40 grams. It is also relevant to point out that the vertebral column that houses the spinal cord is approximately 70 cm long. This tells us that the spinal cord is much shorter than the vertebral column, and that it extends from the skull (foramen magnum) to the first lumbar vertebra. But none the less the spinal cord has extended fibers that branch out in a wide array of webbing.

The Peripheral Nervous System is made up of 12 pair of cranial nerves and their related branches are made up of 31 pair of spinal nerves that are further subdivided into their correlated branches. There are two main divisions to the Peripheral Nervous System. The first is the Somatic Nervous System, which supplies and receives information from fibers connected to neurons throughout the body, to and from the skin, joints, tendons, and skeletal muscles. The somatic nervous system also consists of peripheral nerve fibers that act as sensory senders of information to the central nervous system. It also consists of motor nerve fibers that send information to the skeletal muscles. It is well to point out that at times these fibers are referred to as Axons, the delivery link to the cell body that is either located in the brain or spinal cord and to the skeletal muscles, which in turn sends information out to the tendons, joints, and skin. The second division of the Peripheral Nervous System is referred to as the Viscera, which are motor fibers. These fibers supply the cardiac muscles, smooth muscles, and the glands. The glands, smooth muscles and the cardiac muscles make up the Autonomic Nervous System. The Autonomic Nervous System is then made up of two divisions. The first is the Parasympathetic Division, which is important for the control of normal bodily functions, such as the heart, lungs, bladder, liver, kidneys, and other bodily related functions. The second is referred to as the Sympathetic Division and is also referred to as the Fight or Flight division, which in itself helps the body cope with both internal and external stress factors.

The nervous system is embedded within the cranium and its effect reaches all the way down to the smallest nerve endings in our toes. For our purpose here we focus on an overview of the nervous system and how it effects/ties into the function of the bodies well being. By doing so we will be able to clearly see the point of contact where subdexcation can take place and how it affects the entire skeletal structure.

Like the brain the spinal nervous system consists of both gray and white matter. The gray matter (Anterior gray horn) is made up of cell bodies and is centrally located and surrounded by Myelinated Axons, the white matter. The Myelinated Axons/white matter of the spinal cord is made up of descending and ascending fiber tracks that carry messages to and from the brain. The ascending fiber tracks transmit sensory information from receptors in the tendons, joints, skin, and skeletal muscles, the Somatic Nervous System.

The descending fiber tracks transmit motor function information to the glands and to the skeletal cardiac, and the smooth muscle system of the body, the Peripheral Nervous System. These spinal column nerve fibers are also responsible for reflex toward the outward environment. In this description reflex refers to a multiple rapid unconscious response to changes in the external and internal environment of ones body. These reflexes are the neural pathways that information and impulses flow/travel in any given response to stimuli within the bodies system and also to response to outward interaction. We can then conclude that these reflexes are the super fiber highway that carries impulse signals to the spinal column and to and from the brain. There are 5 components to these reflexes. The first is the receptor, which responds to the inner or outer stimulus. The second is the efferent pathway/sensory neuron, which transmits impulses into the spinal cord. The third is the central nervous system, the part of the nerves that are intertwined within the spinal cord and where information is processed. The forth reflex is the efferent pathway, which is the motor neuron that transmits impulses and information out of the spinal cord. The fifth reflex is the effector, which can either be a muscle or take the form of a gland. This effector receives the impulse/information from the motor neuron/efferent pathway and then carries out the desired response to either the outside element or an internal response to the bodies many functions.

There are 31 pair of spinal nerves and each one of them has a dorsal root and a ventral root. The dorsal root is a sensory nerve that conducts impulses into the spinal cords central nervous system. The dorsal root has a ganglion, which is a tissue mass that makes up and provide relay points for the neurological functions of the body. These ganglions contain the cells bodies of the sensory neurons. These sensory neuron fibers pass through the dorsal root. Each single spinal nerve includes multiple sensors/efferent neurons. Many of these sensory/efferent neurons conduct impulses to form somatic structures. Those structures are the joints, tendons, skin, and skeletal muscles. The ventral root is motor related and conducts impulses of information out of the spinal cords central nervous system. The ventral structures are made up of glands, cardiac muscles and smooth muscles.

The spinal nerves and the peripheral nervous system can be divided into four categories. The Somatic afferent, the Somatic efferent, the Visceral afferent, and the Visceral efferent. Somatic afferent neurons are sensory indicators that conduct impulses and send information to and from receptors in the skin, tendons, joints, and the skeletal muscles. The Neurons/receptors that are located in the skin are responsible for sensing touch, pain, pressure, and temperature. These seniority receptors are referred to as exteroceptors. Neurons/receptors that are located in the joints, tendons and the skeletal muscles provide the brain with information relating to the bodies position and movement. These seniority receptors are referred to as proprioceptors. These somatic afferent neurons are uni-polar and enter the spinal cord through the dorsal root. The cell bodies for these fibers are located in the dorsal root ganglia. The somatic efferent neurons are motor related and conduct impulses originating from the spinal cord to the skeletal muscles. The somatic efferent neurons are multi-polar and have cell bodies located strategically within the gray matter of the spinal cord. These somatic efferent neurons leave the spinal cord through the vertral root and through the spinal nerves.

The visceral afferent neurons are mainly sensory neurons that indeed conduct impulses originated in receptors within the smooth muscle and the cardiac muscle. These visceral neurons are collectively referred to as visceroceptors or as interceptors. Visceral neurons are uni-polar and they enter the spinal cord through the dorsal root and their cell bodies are housed and located in the dorsal root ganglia, much like the somatic neurons.

Visceral efferent neurons conduct motor impulses to the smooth muscle, the cardiac muscle, and to various glands. Some of these visceral neurons begin in the brain others are located and begin in the spinal cord. It is well to mention that it takes two visceral neurons to conduct and impulse, these impulses are then broken down into two categories, the visceral efferent 1 and the visceral efferent 2. The visceral efferent 1 is also referred to as the preganglionic neuron. It is a multi-polar neuron that begins in the spinal cords gray matter. The preganglionic neuron leaves the spinal cord through the ventral root then leaves the spinal nerve through the white ramus and ends up in an autonomic ganglion. The ramus acts as a gate/doorway for impulse signals to travel on. In the ganglion the visceral efferent 1 neuron connects/sparks with the visceral efferent 2 neuron. The visceral efferent 2 neuron is also referred to as the postganglionic neuron and is also considered to be multi-polar and begins in the sympathetic ganglion where its cell body is located. Visceral efferent 2 neurons exit the ganglion through the gray matter of the spinal cord then precede to the cardiac muscle, the smooth muscle, or to various glands.

The autonomic nervous system is entirely motor related, controls the smooth muscle of the internal organs. It also controls the glands and consists of three divisions. The Sympathetic Division, the Parasympathetic Division, and the Enteric Nervous System. The rhythmic impulses, from these divisions always travels along two neurons, the preganglionic (visceral efferent 1) and the postganglionic (visceral efferent 2).

The Sympathetic Division leaves the central nervous system through a series of spinal nerves/fibers and extends into the thoracic and lumbar regions of the spine. The sympathetic neurons also prepare the body for intense physical activity in the case of stressful situations. The sympathetic is comprised of two different neurons, the preganglionic neurons/fibers, which are rather short, and the postganglionic neurons/fibers, which are rather long. The parasympathetics neurons help regulate the body’s functions such as in digestion and the slow down/relaxation after a given stressful situation. These neurons leave the central nervous system through cranial nerves as well as spinal nerves located in the sacral region of the spinal cord.
The Parasympathetic Division is important for the control of normal bodily functions, such as the heart, lungs, bladder, liver, kidneys, and other bodily related functions and its preganglionic neurons/fibers are rather long and its postganglionic
neurons/fibers are short.

The Enteric Nervous System is the third division of the autonomic nervous system. It is a collective mass of nerve fibers that interact with the viscera, which connect and operate the gastrointestinal track, the pancreas, and the gall bladder.
We now turn our focus towards identifying some slight differences between the Central Nervous System and the Peripheral Nervous System. In the central nervous system groups of neurons/fibers are referred to as nuclei. In the peripheral nervous system groups of neurons/fibers are referred to as ganglia. As well groups of axons are referred to as tracks in the central nervous system, and in the peripheral nervous system they are referred to as nerves.

In the peripheral nervous system neurons/fibers can be divided into three groups. The sensory (afferent) group that carry information into the center of the nervous system from sensory organs and the motor (efferent) group carry the information back out and away from the central nervous system towards the muscle to give it the command and control desired. The second of these peripheral divisions is referred to as the cranial division. It connects the brain to the periphery fibers and to the spinal cord. The third of these is the somatic, which connects the skin and muscles with the central nervous system as well as providing connections to the internal organs from the central nervous system.