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The Central and Peripheral Nervous System

Although the brain often gets most of the credit for being the "central processing unit" of the body, the spinal cord also plays a crucial role in what is known as the central nervous system (CNS). The CNS controls most functions of the body, including movement, organ functions, and behavior. 

The spinal cord descends from the brain down the middle of the back, and acts as the primary information pathway between the brain and all the other nervous systems of the body. It receives sensory information from the skin, joints, and muscles of the trunk, arms, and legs, which it then relays upward to the brain. It also contains motor neurons, which direct voluntary movements and adjust reflex movements. Because of the central role it plays in coordinating muscle movements and interpreting sensory input, any kind of spinal cord injury can cause significant problems throughout the body.


The CNS communicates with the limbs, heart, skin, muscles, and other organs of the body through a network of nerves collectivly known as the peripheral nervous system (PNS). Most of the nerves of the PNS connect to the spinal cord at various locations up and down the back.  The PNS controls the somatic nervous system, which regulates muscle movements and the response to sensations of touch and pain, and the autonomic nervous system, which provides nerve input to the internal organs and generates automatic reflex responses. The autonomic nervous system is itself divided into the sympathetic nervous system, which mobilizes organs and their functions during times of stress and arousal ("fight or flight"), and the parasympathetic nervous system, which conserves energy and resources during times of rest and relaxation ("rest and digest").

Spinal Anatomy

Because the soft, jelly-like spinal cord is so important for nearly every function of the body, it is protected by the boney spinal column. The spinal column is made up of 33 bones called vertebrae, each with a circular opening similar to the hole in a donut. The bones are stacked one on top of one another, and the spinal cord runs through the hollow channel created by the holes in the stacked bones. 

The vertebrae can be organized into sections, and are named and numbered from top to bottom according to their location along the backbone:

Spine Vertebrae


  • Cervical vertebrae (1-7) located in the neck
  • Thoracic vertebrae (1-12) in the upper back (attached to the ribcage)
  • Lumbar vertebrae (1-5) in the lower back
  • Sacral vertebrae (1-5) in the hip area
  • Coccygeal vertebrae (1-4 fused) in the tailbone

The spinal cord is also cushioned by a fluid called Cerebral Spinal Fluid (CSF), which protects the delicate nerve tissues from being damaged by rubbing against the inside of the vertebrae.

Although the hard vertebrae and CSF protect the soft spinal cord from injury most of the time, the spinal column is not all hard bone. Between the vertebrae are discs of semi-rigid cartilage, and in the narrow spaces between them are passages through which the spinal nerves exit to the rest of the body. These are places where the spinal cord is vulnerable to direct injury.

Spinal Cord Anatomy

Like the vertebrae, the spinal cord is also organized into segments and named and numbered from top to bottom. Each segment marks where spinal nerves emerge from the cord to connect to specific regions of the body. Locations of spinal cord segments do not correspond exactly to vertebral locations, but they are roughly equivalent.
  • Cervical spinal nerves (C1 to C8) control signals to the back of the head, the neck and shoulders, the arms and hands, and the diaphragm.
  • Thoracic spinal nerves (T1 to T12) control signals to the chest muscles, some muscles of the back, and parts of the abdomen.
  • Lumbar spinal nerves (L1 to L5) control signals to the lower parts of the abdomen and the back, the buttocks, some parts of the external genital organs, and parts of the leg.
  • Sacral spinal nerves (S1 to S5) control signals to the thighs and lower parts of the legs, the feet, most of the external genital organs, and the area around the anus.
  • The single coccygeal nerve carries sensory information from the skin of the lower back.

Spinal Canal

The functions of these nerves are determined by their location in the spinal cord. They control everything from body functions such as breathing, sweating, digestion, and elimination, to gross and fine motor skills, as well as sensations in the arms and legs. An injury to the spinal cord may inhibit the functioning of any nerves located below the site of injury.

Spinal Cord Physiology

The interior of the spinal cord is made up of neurons, their support cells called glia, and blood vessels. The neurons and their dendrites (branching projections that help neurons communicate with each other) reside in an H-shaped region called "grey matter."

The H-shaped grey matter of the spinal cord contains motor neurons that control movement, smaller interneurons that handle communication within and between the segments of the spinal cord, and cells that receive sensory signals and then send information up to centers in the brain.

Neurons

Surrounding the grey matter of neurons is white matter. Most axons are covered with an insulating substance called myelin, which allows electrical signals to flow freely and quickly. Myelin has a whitish appearance, which is why this outer section of the spinal cord is called "white matter." A number of diseases damage the myelin and the white matter, including multiple sclerosis and postinfectious encephalomyelitis

Axons carry signals downward from the brain (along descending pathways called efferent nerves) and upward toward the brain (along ascending pathways called afferent nerves) within specific tracts. Axons branch at their ends and can make connections with many other nerve cells simultaneously. Some axons extend along the entire length of the spinal cord.

The descending motor tracts control the smooth muscles of internal organs and the striated (capable of voluntary contractions) muscles of the arms and legs. They also help adjust the autonomic nervous system's regulation of blood pressure, body temperature, and the response to stress. These pathways begin with neurons in the brain that send electrical signals downward to specific levels of the spinal cord. Neurons in these segments then send the impulses out to the rest of the body or coordinate neural activity within the cord itself.

The ascending sensory tracts transmit sensory signals from the skin, extremities, and internal organs that enter at specific segments of the spinal cord. Most of these signals are then relayed to the brain. The spinal cord also contains neuronal circuits that control reflexes and repetitive movements, such as walking, which can be activated by incoming sensory signals without input from the brain.

The circumference of the spinal cord varies depending on its location. It is larger in the cervical and lumbar areas because these areas supply the nerves to the arms and upper body and the legs and lower body, which require the most intense muscular control and receive the most sensory signals.

The ratio of white matter to grey matter also varies at each level of the spinal cord.  In the cervical segment, which is located in the neck, there is a large amount of white matter because at this level there are many axons going to and from the brain and the rest of the spinal cord below. In lower segments, such as the sacral, there is less white matter because most ascending axons have not yet entered the cord, and most descending axons have contacted their targets along the way.  To pass between the vertebrae, the axons that link the spinal cord to the muscles and the rest of the body are bundled into 31 pairs of spinal nerves, each pair with a sensory root and a motor root that make connections within the grey matter. Two pairs of nerves - a sensory and motor pair on either side of the cord - emerge from each segment of the spinal cord.

Injury

Spinal cord injuries can be caused by trauma to the spinal column (stretching, bruising, applying pressure, severing, laceration, etc.). The vertebral bones or intervertebral disks can shatter, causing the spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will suffer loss of feeling in certain parts of their body. In milder cases, a victim might only suffer loss of hand or foot function. More severe injuries may result in paraplegia, tetraplegia, or full body paralysis (called Quadriplegia) below the site of injury to the spinal cord.

Damage to upper motor neuron axons in the spinal cord results in a characteristic pattern of ipsilateral deficits. These include hyperreflexia, hypertonia and muscle weakness. Lower motor neuronal damage results in its own characteristic pattern of deficits. Rather than an entire side of deficits, there is a pattern relating to the myotome affected by the damage. Additionally, lower motor neurons are characterized by muscle weakness, hypotonia, hyporeflexia and muscle atrophy.

Spinal shock and neurogenic shock can occur from a spinal injury. Spinal shock is usually temporary, lasting only for 2448 hours, and is a temporary absence of sensory and motor functions. Neurogenic shock lasts for weeks and can lead to a loss of muscle tone due to disuse of the muscles below the injured site.

The two areas of the spinal cord most commonly injured are the cervical spine (C1-C7) and the lumbar spine (L1-L5). (The notation C1, C7, L1, L5 refer to the location of a specific vertebra in either the cervical, thoracic, or lumbar region of the spine.)





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© 2010 Vivacare. Last updated April 12, 2011.

Reference: National Institute of Neurological Disorders and Stroke

www.consultantsinneurology.com

Raymond Rybicki, MD

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