بسم الله الرحمن الرحيم
Lecture 5 Neurophysioloy Dr. Noor
2nd stage 2021
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Spinal cord
Objective
What is the physiologic anatomy of spinal cord?
Organization of spinal cord for motor function?
Anatomy
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells and is the most important structure between the body and the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. It is a vital link between the brain and the body, and from the body to the brain.
The spinal cord is 40 to 50 cm long and 1 cm to 1.5 cm in diameter. Two consecutive rows of nerve roots emerge on each of its sides. These nerve roots join distally to form 31 pairs of spinal nerves.
The spinal cord is a cylindrical structure of nervous tissue composed of white and gray matter, is uniformly organized and is divided into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), each of which is comprised of several segments. The spinal nerve contains motor and sensory nerve fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome.
Internal Structure of the Spinal Cord
A transverse section of the adult spinal cord shows white matter in the periphery, gray matter inside, and a tiny central canal filled with CSF at its center. Surrounding the canal is a single layer of cells, the ependymal layer. Surrounding the ependymal layer is the gray matter – a region containing cell bodies – shaped like the letter “H” or a “butterfly”.
The two “wings” of the butterfly are connected across the midline by the dorsal gray commissure and below the white commissure .
Gray matter
The spinal cord plays a key role in integration of multiple peripheral and central inputs via the system of neurons in the gray matter. On cross-section, the gray matter in the spinal cord includes the dorsal, ventral, and an intermediolateral horns or columns.White matter
The white matter includes ascending and descending tracts that are composed of axons. The white matter also has glial cells.
Functions
The spinal cord works a bit like a telephone switchboard operator, helping the brain communicate with different parts of the body, and vice versa. Its three major roles are:To relay messages from the brain to different parts of the body (usually a muscle) in order to perform an action
To pass along messages from sensory receptors (found all over the body) to the brain
To coordinate reflexes (quick responses to outside stimuli) that don't go through the brain and are managed by the spinal cord alone
ORGANIZATION OF THE SPINAL CORD FOR MOTOR FUNCTIONS
The cord gray matter is the integrative area for the cord reflexes. Sensory signals enter the cord almost entirely through the sensory roots, also known as the posterior or dorsal roots.After entering the cord, every sensory signal travels to two separate destinations: (1)One branch of the sensory nerve terminates almost immediately in the gray matter of the cord and elicits local segmental cord reflexes and other local effects, and (2)another branch transmits signals to higher levels of the nervous system—that is, to higher levels in the cord itself, to the brain stem, or even to the cerebral cortex.
Each segment of the spinal cord (at the level of each spinal nerve) has several million neurons in its gray matter, Aside from the sensory relay neurons, the other neurons are of two types: (1) anterior motor neurons and (2) interneurons.
Anterior Motor Neurons.
Located in each segment of the anterior horns of the cord gray matter are several thousand neurons that are 50 to 100 percent larger than most of the others and are called anterior motor neurons . They give rise to the nerve fibers that leave the cord by way of the anterior roots and directly innervate the skeletal muscle fibers. The neurons are of two types, alpha motor neurons and gamma motor neurons.Alpha Motor Neurons.
The alpha motor neurons give rise to large type A alpha (Aα) motor nerve fibers, averaging 14 micrometers in diameter; these fibers branch many times after they enter the muscle and innervate the large skeletal muscle fibers. Stimulation of a single alpha nerve fiber excites anywhere from three to several hundred skeletal muscle fibers, which are collectively called the motor unit.Gamma Motor Neurons.
Along with the alpha motor neurons, which excite contraction of the skeletal muscle fibers, about one half as many much smaller gamma motor neurons are located in the spinal cord anterior horns. These gamma motor neurons transmit impulses through much smaller type A gamma (Aγ) motor nerve fibers, averaging 5 micrometers in diameter, which go to small, special skeletal muscle fibers called intrafusal fibers.
Interneurons
These fibers constitute the middle of the muscle spindle, which helps control basic muscle “tone,”Interneurons. Interneurons are present in all areas of the cord gray matter—in the dorsal horns, the anterior horns, and the intermediate areas between them.
These cells are about 30 times as numerous as the anterior motor neurons. They are small and highly excitable, often exhibiting spontaneous activity and capable of firing as rapidly as 1500 times per second. They have many interconnections with one another, and many of them also synapse directly with the anterior motor neurons.
The interconnections among the interneurons and anterior motor neurons are responsible for most of the integrative functions of the spinal cord
Only a few incoming sensory signals from the spinal nerves or signals from the brain terminate directly on the anterior motor neurons. Instead, almost all these signals are transmitted first through interneurons, where they are appropriately processed.
Thus, the corticospinal tract from the brain is shown to terminate almost entirely on spinal interneurons, where the signals from this tract are combined with signals from other spinal tracts or spinal nerves before finally converging on the anterior motor neurons to control muscle function.