The human spinal cord is located in the vertebral canal. It is a cylindrical cord 43–45 cm long and weighing about 30 g. The spinal cord is washed by cerebrospinal fluid, which protects it from shocks.
Spinal cord structure
At the top, the spinal cord is connected to the medulla oblongata (part of the brain). At the bottom, it continues to the lumbar spine. The spinal cord is divided into two symmetrical halves by the anterior and posterior longitudinal grooves.
In the center of the spinal cord is a spinal canal filled with cerebrospinal fluid. Around it is concentrated gray matter, formed by the bodies of neurons, in the cross section having the shape of a butterfly. In the gray matter, horns are distinguished: anterior, posterior and lateral.
The anterior horns are formed by the bodies of motor neurons (motoneurons). The axons of these neurons conduct excitation to the skeletal muscles of the trunk and limbs.
In the posterior horns, there are mainly bodies of intercalary neurons, which transmit nerve impulses from the processes of sensory neurons to the bodies of motor neurons. Interneurons also transmit information to other parts of the spinal cord and to the brain.
The lateral horns contain neurons that form the centers of the sympathetic part of the autonomic nervous system.
The outer layer of the spinal cord is formed by a white matter, consisting of neuronal outgrowths that form pathways.
The spinal cord is divided into sections – segments. Spinal nerves branch off from each segment. In total, 31 pairs of mixed spinal nerves depart from the spinal cord, each of which begins with two roots: anterior (motor) and posterior (sensory). The anterior roots also contain autonomic nerve fibers. Nerve nodes are located on the posterior roots – clusters of bodies of sensitive neurons. When connected, the roots form mixed nerves. Each pair of spinal nerves innervates a specific area of the body.
Example: The cervical and upper thoracic segments control the organs of the thoracic cavity, the muscles of the neck and the arms of the organs. Nerves extending from the lower thoracic and upper lumbar segments innervate the abdominal organs and trunk muscles. From the lower lumbar and sacral segments there are nerves that control the pelvic organs and leg muscles.
Spinal cord functions
The main functions of the spinal cord:
- reflex – carried out by the somatic and autonomic nervous systems;
- conductive – carried out by the white matter of the ascending and descending pathways.
The reflex function of the spinal cord is that the arcs of unconditioned reflexes are closed here, which regulate the somatic and autonomic functions of the body. Reflex arcs pass through the spinal cord, which are associated with contractions of all skeletal muscles of the body (except for the muscles of the head).
Example: An example of the simplest motor reflex would be the knee reflex, which manifests itself in the extension of the leg when striking the tendon of the muscle below the patella.
The conductive function of the spinal cord is to conduct impulses from the brain to the organs and vice versa. Information from receptors of the skin, muscles, blood vessels, and organs of the genitourinary system enters the brain along the ascending pathways.
In descending paths, nerve impulses come from the brain to the motor neurons of the anterior horns, and from them to the organs.
The white matter of the spinal cord provides communication and coordinated work of all parts of the central nervous system, carrying out the conductive function. All reflexes of the spinal cord are under the control of the brain.