The brain and spinal cord make up the central nervous system. The spinal cord is about 16-18 inches long and and is basically a uniform structure throughout it’s length. The spinal cord is contained in the center cavity of the vertebral column (back bone) which protects the spinal cord from injury.
It has an inner mass of gray matter and an outer covering of white matter. It carries messages that coordinate movement and sensation. The cord is an ovoid shaped column of nerve tissue that extends from the medulla at the underside of the brain down through cavities in the spinal column to the second lumbar vertebrae.
The spinal cord is protected by the bones of the spinal cord and enclosed in the protective tissue of the meninges (mater) and cerebrospinal fluid. The center of the cord is gray matter and shaped like an H. The white matter is arranged in tracts around the gray matter and consists of axons that transmit impulses to and from the brain or between levels of gray matter in the spinal cord.
The spinal cord has two basic functions. It can act as a never center and can work without the brain. The spinal cord carries sensory impulses to the brain and motor impulses from the brain. The spinal cord also controls stretch reflexes, bowel and bladder control. Thirty-one pairs of nerves exit from the spinal cord and innervate our body and limbs. The spinal cord also acts as a nerve center between the brain and the rest of our body.
Labeled Cross Section of Spinal Cord
- Anterior Fissure
- Deep groove along the front of the spinal cord
- This membrane is the middle layer of the three meninges that covers and protects the spinal cord and brain.
- Central Canal
- Cerebrospinal fluid fills the narrow central canal and protects the neurons
- Dura Mater
- The outermost, toughest, and most fibrous of the three membranes that cover and protect the spinal cord and brain
- Gray Matter
- Gray matter is shaped like a butterfly. Gray matter is divided into three functional zones— the dorsal horns are sensory, the ventral horns are involved in motor functions and the middle zone carries out functions between sensory and motor zones. Gray matter is made up of both large and small neurons. The large neurons are either motor or sensory. The fibers of the motor neurons (output neurons), located in the ventral horns, go to the voluntary skeletal muscles. These motor neurons are grouped in clusters and each serves a different muscle. Autonomic neurons are clustered separately.
- Three membranes that cover and protect the spinal cord and brain—dura mater (outer), arachnoid (middle) and pia mater (inner)
- Pia Mater
- This thin vascular membrane of collagen fibers is the innermost layer of the three meninges that covers and protects the spinal cord and brain.
- Sensory Root
- Ganglion Cell bodies of sensory nerves cluster in ganglia
- Spinal Nerve
- Sensory and motor nerve rootlets merge to form a spinal nerve
- Subarachnoid Space
- The area between the arachnoid membrane and the pia mater
- White Matter
- This outer band surrounds the gray matter throughout the length of the spinal cord. Made up of axons of neurons grouped in bundles called funiculi that contain nerve fibers that travel between the spinal cord and the brain. These bundles form pathways that carry signals to and from the brain. Pathways to the brain are usually sensory, and pathways from the brain to the spinal cord are usually motor.
A spinal nerve is any of the 31 pairs of nerves that arise from the spinal cord. The spinal nerves correspond to where it emerges and passes through the spinal vertebrae: there are 8 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 5 sacral (sacrum bone) and one coccygeal (tailbone) nerve(s). Each spinal nerve is attached to the spinal cord by two roots: a dorsal or posterior sensory root and a ventral or anterior motor root. The fibers of the sensory root carry sensory impulses to the spinal cord—pain, temperature, touch and position sense (proprioception)—from tendons, joints and body surfaces. The motor roots carry impulses from the spinal cord. The spinal nerves exit the spinal cord and pass through the intervertebral foramen, then divides into four branches.
A plexus is a network. It can be a network of blood vessels or nerves.
- Cervical plexus
- A network formed by the first 4 cervical spinal nerves. It innervates parts of the face, neck, shoulder and chest and gives rise to the phrenic nerve to the diaphragm.
- Brachial plexus
- A network of the last 4 cervical and first thoracic spinal nerves supplying the arm, forearm and hand.
- Lumbar plexus
- A network of the first 4 lumbar nerves.
- Sacral plexus
- A network of the 4th and 5th lumbar nerves and the first four sacral nerves, from which the sciatic nerve originates.
Touch tells us about temperature, pressure, texture, movement and bodily location. Pain seems to be a part of touch, but it has its own receptors and sensory pathways. Each muscle in the body is supplied with nerves (innervated) by a particular segment of the spinal cord and by its corresponding spinal nerve. A myotome is the group of muscles supplied by a single nerve root. A dermatome is an area of the skin supplied by the nerve fibers from a single sensory nerve root. Each area is named from the spinal nerve that supplies it. These areas on the trunk resemble horizontal bands; on the arms and legs the areas are elongated, vertical strips. There is a good bit of overlap between dermatomes. If sensory function is lost in one spinal nerve, sensation isn’t completely lost in that area because of the overlap of the nearby spinal nerve.
The levels of the spinal cord segments don’t relate exactly to the levels of the vertebral bodies because there are 7 vertebrae and 8 cervical nerve roots coming from the spinal cord. Damage to a vertebrae at a particular level doesn’t mean there is damage to the spinal cord at that same level.
Map of Dermatomes
A dermatome is a band or region of skin supplied by a single sensory nerve. Sensory nerves carry sensory impulses to the spinal cord—pain, temperature, touch and position sense (proprioception)—from tendons, joints and body surfaces. The face is supplied by the cranial nerves.
Key to Spinal Nerve Regions
Each pair of spinal nerves links to one of four regions of the body.
- Cervical Region (green): 8 pairs of nerves supply the skin covering the back of the head, the neck, shoulders, arms and hands.
- Thoracic Region (blue): 12 pairs of thoracic nerves supply the skin on the chest, back and under arms
- Lumbar Region (pink): 5 pairs of lumbar nerves supply the skin on the lower abdomen, thighs and fronts of the legs
- Sacral Region (yellow): 6 pairs of sacral nerves supply the skin on the rear of the legs, the feet and genial areas
Levels of principal dermatomes
- C5 — clavicles
- C5, C6, C7 — lateral parts of the upper limb
- C8, T1 — medial sides of the upper limb
- C6 — thumb
- C6, C7, C8 — hand
- C8 — ring and little fingers
- T4 — level of nipples
- T10 — level of umbilicus
- T12 — inguinal or groin regions
- L1 L2 L3 L4 — anterior and inner services of lower limb
- L4, L5, S1 — foot
- L4 — medial side of big toe
- S1. S2, L5 — posterior and outer surfaces of lower limbs
- S1 — lateral margin of foot and little toe
- S2, S3, S4 — perineum
Nerves can be divided into two types – motor nerves and sensory nerves. Motor nerves control movement by carrying messages from the brain to the muscles. Sensory nerves carry messages from the sensory receptors in the body to the brain. In many places the motor nerves and sensory nerves run in pairs along side each other. There are 31 pairs of nerves that branch off the spinal cord.
Spinal nerve pairs
cervical nerve pairs – 8
thoracic nerve pairs – 12
lumbar pairs – 5
sacral pairs – 5
coccyx – 1
In the nervous system there is a “closed loop” system of sensation (sensory), decision (brain), and reactions (motor). This process is carried out through the activity of afferent neurons (sensory), interneurons (spinal cord), and efferent (motor) neurons. Nerves are made of very specialized cells called neurons.
Afferent neurons (otherwise known as sensory or receptor neurons) are receptors that receive the stimulus then carry nerve impulses from receptors or sense organs towards the central nervous system communicating sensory information to the spinal cord and brain. Sensory neurons respond to the senses of touch, sound, light, smell and taste. An example of a sensory response would is when your skin is stuck with a pin, the afferent neuron communicates pain or discomfort to the spinal cord and then to the brain. The brain processes the pain information, decides how your body should react and then sends information back through the efferent neuron to the muscle to contract which moves the area (finger, arm, etc.) away from the cause of the pain. This process is called a receptor or reflex arc.