Sunday, April 25, 2010

Spinal Nerves (sensory and motor )

There are 31 pairs of spinal nerves and each has a dorsal root and a ventral root. The dorsal root is sensory (all neurons conduct impulses into the spinal cord) while the ventral root is motor ( all neurons conduct impulses out of the spinal cord). The dorsal root has a ganglion that contains the cell bodies of the sensory neurons that pass through the dorsal root. Each spinal nerve includes numerous sensory, or afferent, and motor, or efferent, neurons. Some of these neurons are classified as somatic, and this conduct impulses to or form the "somatic" structures ( skin, skeletal muscles, tendons, and joints). Other neurons are "visceral", and these conduct impulses to or from the visceral structures (smooth muscle, cardiac muscle, and glands). Therefore, all neurons in spinal nerves and the peripheral nervous system are placed in one of the four categories somatic afferent, somatic efferent, visceral afferent, visceral efferent. Somatic affernt neurons are sensory neurons that conduct impules initiated in receptors in the skin, skeletal muscles, tendons, and joints. Receptors in the skin are responsible for sensing things such as touch, temperature, pressure and pain called exteroceptors. Receptors in skeletal muscles, tendons, and joints, provide info about body position and movement and are called propioceptors.They are unipolar. Somatic efferent neurons are motor neurons that conduct impulses from the spinal cord to skeletal muscles. These neurons are multipolar. Visceral afferent are sensory neurons that conduct impulses initiated in the receptors in the smooth and cardiac muscle. They are reffered to as enteroceptors or visceroceptors. They are unipolar. Visceral efferent are motor neurons that conduct impulses to smooth muscle, cardiac muscle, and glands. They make up the autonomic nervous system. Some begin in the brain and some in the spinal cord. It always takes two visceral efferent neurons to conduct an impulse from the spinal cord or brian in some cases to a muscle or gland.

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