Spinal Nerves and Somatic Reflexes: Anatomy, Sensation, and Function

Anatomy of a Spinal Nerve • 31 pairs of spinal nerves – 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal Connective Tissues of a Spinal Nerve A nerve is a bundle of nerve fibers (axons) covered with 3 layers of Connective Tissue: • Epineurium covers nerves and conducts blood vessels along nerves. • Perineurium surrounds a fascicle (bundle) of axons and conducts blood vessels into the nerve. • Endoneurium is the basal lamina and loose connective tissue fibers around the Schwann cells. Blood vessels do not penetrate through the endoneurium. Spinal Nerve Roots Dorsal Root Ganglion • Ganglia in the PNS are clusters of neuron somas (cell bodies) in a nerve. • Example: Dorsal Root Ganglion is the location of the sensory neuron somas. Branches of the Spinal Nerves • Proximal Branches – dorsal root (sensory input to spinal cord) – ventral root (motor output of spinal cord) • Distal Branches – dorsal ramus serves dorsal body muscle and skin – ventral ramus serves ventral body and limb muscles and skin – meningeal branch serves meninges, vertebrae and ligaments Cutaneous Innervation and Dermatomes • Each spinal nerve receives sensory input from a specific area of skin called a dermatome. Pain • Pain is discomfort caused by tissue injury or noxious stimulation. • Pain is a valuable sense because it helps us learn how to avoid serious injury. - Neuropathy is the loss of the sense of pain. - Diseases including diabetes mellitus and leprosy can cause neuropathy. Nociceptors • Nociceptors are specialized sensory nerve fibers that sense pain. • Nociceptors are abundant in skin, mucous membranes, organs, meninges, but NOT the brain. • Fast Pain is transmitted through myelinated fibers and produces instantaneous sharp, localized, stabbing pain. • Slow Pain follows fast pain and is transmitted through unmyelinated fibers and produces longer-lasting, dull, diffuse feeling of pain. • Somatic Pain comes from the skin, muscles and joints. • Visceral Pain comes from internal organs. • Bradykinin released by injured tissues is the most potent pain stimulus known, and triggers a cascade of reactions that promote healing. • Seratonin, prostaglandins, histamine, K+, and ATP also stimulate nociceptors. Shingles • Skin eruptions along the path of a spinal nerve (dermatome) caused by the chicken pox virus (Varicella zoster) that lives in the dorsal root ganglia for life. • Periodic flair-ups can occur along the path of an infected nerve. • Flair-ups are more common after age 50 or if the person’s immune system is compromised. • Treated with aspirin and cortisone creams to relieve pain and inflammation. In 1998, at age 38, Mary Ellen Nelsen had a shingles outbreak. Antiviral drugs cleared up the painful shingles rash on her face and scalp, but a ferocious itch took its place. “It was relentless,” Nilsen says. Over a 13 month period, Nilsen scratched, despite her best efforts not to, and despite her horror at the growing lesions she saw in the mirror. At the time, Nilsen says, she had no idea that the damage she was doing to herself was more than skin deep, but she ended up in a Boston emergency room with brain tissue protruding through a hole she scratched through her skull which resulted in frontal-lobe brain damage. The virus damaged sensory nerves in a way that induced itch, but left her unable to feel pain from the scratching-induced wounds. G. Miller (2007) Grasping for Clues to the Biology of Itch. Science 12 October 2007 Vol 318 p. 189. Gray Matter in the Spinal Cord is divided into Horns • Dorsal horns lead to the dorsal roots of spinal nerves – dorsal root is totally composed of sensory fibers – dorsal root ganglion contains the somas of unipolar sensory neurons • Ventral horns lead to the ventral roots of spinal nerves – ventral root is totally composed of motor fibers • Gray Commissure forms a bridge of gray matter between the horns • Central canal filled with CSF and is continuous with the 4th ventricle of the brain White Matter in the Spinal Cord is divided into Tracts • Ascending Tracts carry signals up to brain. • Descending tracts carry signals down spinal cord. • Tracts can be Contralateral (origin and destination are on opposite sides) or Ipsilateral (origin and destination are on the same side). Somatic Reflexes • Somatic Reflexes are quick, involuntary, stereotyped reactions of glands or muscle in response to sensory stimulation • Automatic responses to sensory input that occur without our intent or often even our awareness occur through a Somatic Reflex Arc: 1) stimulation of somatic receptors 2) afferent fibers carry signal to dorsal horn of spinal cord 3) interneurons integrate the information in spinal cord 4) efferent fibers carry impulses to skeletal muscles 5) skeletal muscles respond • Examples: Flexor Withdrawal Reflex and Crossed Extensor Reflex Flexor Withdrawal Reflex • Flexor Withdrawal Reflex quickly withdraws foot from pain. • Neural circuitry in spinal cord controls sequence and duration of muscle contractions without the brain. Crossed Extensor Reflex 2 3 4 and maintains balance The Stretch (Myotatic) Reflex • When a muscle is stretched quickly, it contracts to help maintain equilibrium and posture. – example: when your head starts to tip forward as you fall asleep, the muscles are stretched and respond by quickly contracting to raise the head and correct the posture. – the reflex maintains posture by balancing tension in extensors and flexors at particular joints (neck, knees, hips, spine, etc.). • Stretch is sensed by modified muscle cells called Muscle Spindles that are innervated by special sensory neurons called primary afferent neurons. • Special motor neurons called alpha-motor neurons quickly respond to contract the stretched muscle. • Reciprocal inhibition prevents flexors and extensors from working against each other.