The Human Vertebrae
The Anatomy of the Spine – The Bones
The human spine is composed of 26 individual bony masses, 24 of those are bones called vertebrae. The vertebrae are stacked one on top of the other and form the main part of the spine running from the base of the skull to the pelvis. At the base of the spine, is a bony plate called the sacrum which is made of 5 fused vertebrae. The sacrum forms the back part of the pelvis. At the bottom of the sacrum is a small set of 4 partly fused vertebrae, the coccyx or tailbone. Adding the fused and partly fused bones of the sacrum and coccyx to the 24 vertebrae, the spine has 33 bones all together.
The spine is labeled in 3 sections: the cervical spine, the thoracic spine and the lumbar spine. Starting from the top there are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae.
Spinal Cord Overview
The Spinal Cord is connected to the brain and is about the diameter of a human finger. From the brain the spinal cord descends down the middle of the back and is surrounded and protected by the bony vertebral column. The spinal cord is surrounded by a clear fluid called Cerebral Spinal Fluid (CSF), that acts as a cushion to protect the delicate nerve tissues against damage from banging against the inside of the vertebrae.
The anatomy of the spinal cord itself, consists of millions of nerve fibres which transmit electrical information to and from the limbs, trunk and organs of the body, back to and from the brain. The nerves which exit the spinal cord in the upper section, the neck, control breathing and the arms. The nerves which exit the spinal cord in the mid and lower section of the back, control the trunk and legs, as well as bladder, bowel and sexual function.
The nerves which carry information from the brain to muscles are called Motor Neurones. The nerves which carry information from the body back to the brain are called Sensory Neurones. Sensory Neurones carry information to the brain about skin temperature, touch, pain and joint position.
Cross-sectional of spinal cord
Reflex Action and Reflex Arc
Reflex action is the involuntary functioning or movement of any organ or body part in response to a particular stimulus. The function or action occurs immediately, without the involvement of the will or consciousness.
A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs.
The knee-jerk reaction
The pathway for this reflex arc starts at a stretch receptor within the tendon. Hitting this receptor stimulates it, which causes it to send a nerve impulse along a sensory neuron to the spinal cord. Within the spinal cord, the nerve impulse passes from the sensory neuron to a motor neuron and travels back to the thigh muscle. When the impulse arrives at the thigh muscle, it causes it to contract and jerk the lower part of the leg upward. The person is aware that this is happening, so sensory impulses do travel from the spinal cord to the brain, but there is nothing that can be done to stop it happening.