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Spinal Nerves and Reflexes

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Spinal Nerves and Reflexes Central Nervous System - Spinal Nerves and Reflexes Chapter 13B Spinal Nerves - Number There are 31 pairs of spinal nerves…a total of 62 nerves. Spinal cord is located in the vertebral canal. Spinal nerves exit vertebral column through intervertebral foramina. Intervertebral foramen Vertebral canal Spinal Nerves Interneuron Sensory neuron Sensory fiber Spinal nerve Motor neuron Motor fiber All spinal nerves are mixed nerves….contain sensory and motor fibers. Spinal Nerves - Supply N V C2–C3 C2 C 3 C3 C4 Spinal nerves go to skin, muscles and some T2 C4 C5 T3 T1 of the internal organs. T4 T2 T5 C5 T3 T T 6 4 T7 T5 T8 Dermatomes: areas of the skin that is T2 T6 T9 T T2 T7 10 connected to a specific spinal nerve. T11 T8 T12 T9 C L1 6 T10 L2 T T L3 1 11 L4 C Myotomes: specific muscles that are C6 L 7 T12 5 L1 supplied by a specific spinal nerve. S4S L 3 2 S2 C8 C8 T L3 L1 1 1 S5 C7 S1 L5 L4 S2 L2 KEY L5 L Spinal cord regions 3 = Cervical = Thoracic S = Lumbar 1 = Sacral L4 ANTERIOR POSTERIOR Spinal Nerves - Branches Spinal nerve Dorsal Dorsal root Dorsal root ganglion ramus Spinal nerve Ventral Dorsal horn ramus Ventral Ventral root horn Rami communicantes After exiting vertebral column, EACH spinal nerve splits into branches, called rami: 1. Dorsal ramus: contains nerves that serve the dorsal portions of the trunk- carry visceral motor, somatic motor, and sensory information to and from the skin and muscles of the back. 2. Ventral ramus: contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs- carry sensory information from the body wall and the limbs and motor information to smooth muscle, skeletal muscle and glands. 3. Rami communicates: Contain autonomic nerves that carry visceral motor and sensory information to and from the visceral organs. Roots: each root is either sensory or motor; Rami: each rami is sensory and motor. Spinal Nerves - Plexus Reminder: Each spinal nerve gives off a branch called ventral/anterior ramus goes to the muscles and skin on the anterior surface of the trunk, arms, legs, head and shoulders. Ventral/anterior rami branches of thoracic nerves T2-T12 (Do not form plexus) give rise to intercostal nerves- innervates intercostal muscles, muscles and skin of anterolateral thorax and most of the abdominal wall. Ventral/anterior rami branches of other spinal nerves- C1-C8, L1-L5, S1-S5 reorganize and form network on both sides of the spinal cord before going to the muscles and skin of their respective areas. This network of intersecting nerves is called plexus. Nerve plexus are found only on --------? Ventral rami Spinal Nerves - Plexus Plexus: Refers to a network formed by the ventral rami branches of the spinal nerves (exception of T1-T12). There are four major plexi formed next to the spinal cord: Cervical plexus: in the neck region. Brachial plexus: in the shoulder region. Lumbar plexus: in the lower back region. Sacral plexus: in the buttock region. Spinal Nerves – Cervical Plexus Cervical Plexus: Formed by the ventral rami branches of the spinal nerves C1 – C5. Supplies to the skin and muscles of the C1 C2 Cervical head, neck and upper part of the C3 plexus C4 shoulders. C5 Phrenic nerve- A major nerve of cervical plexus-originates from C3, C4 and C5 spinal nerves and supplies diaphragm, a key respiratory muscle. “C3, C4, C5 keeps the diaphragm alive” Spinal Nerves – Brachial Plexus Brachial Plexus: Formed by the ventral rami branches of the spinal nerves C5 – C8 and part of T1. Supplies to the skin and muscles of the shoulders and upper extremities (arms). C5 C6 C7 Brachial C8 T1 plexus 1) Axillary nerve- innervates deltoid muscle. 2) Musculocutaneous nerve- innervates flexor muscles on arm (biceps brachii, brachialis and coracobrachilais). 3) Radial nerve-Innervates the extensor muscle of the arm (triceps brachii) and forearm. 4) Ulnar nerve- innervates flexor muscles of wrist (flexor carpi ulnaris). 5) Median nerve- innervates flexor muscles of the forearm. Spinal Nerves – Lumbar Plexus T12 Lumbar Plexus: L1 Lumbar Formed by the ventral rami branches of L 2 plexus the spinal nerves part of T12, L1 – L4. L3 L4 Supplies to the skin and muscles of the lateral and anterior abdominal wall, external genitals and part the thighs. Major nerves include:- 1) Genitofemoral nerve 2) Lateral femoral cutaneous nerve 3) Femoral nerve Spinal Nerves – Sacral Plexus Sacral Plexus: Formed by the ventral rami branches of the spinal nerves L4 – L5 and S1 – S4. L4 Supplies to the skin and muscles of the L5 S1 buttocks and lower extremities (legs). Sacral S2 plexus S3 Major nerve:- S4 Sciatic nerve (originates from L4-S3)-longest and thickest nerve in the body-innervates gluteal muscle, hamstrings and calf muscle. Sciatic nerve Reflex Sensory fiber Cell body of Sensory neuron Dorsal/posterior root ganglion Denticulate ligament Spinal nerve Reflex: Motor fiber Refers to a fast response to an external or internal stimulus to maintain homeostasis. Example: Prick your finger muscles of the arm contract you pull your arm to prevent further injury and fluid loss. Brain is not usually involved in reflex action. With age, conduction rate decreases Does that increase or decrease reflex time?? Reflex Arc 1 2 Dorsal Arrival of Activation of a root Sensation stimulus and sensory relayed to the activation of neuron brain by axon receptor collaterals Spinal cord 3 Information REFLEX processing ARC in the CNS Receptor Stimulus Response by a peripheral effector Effector Ventral root KEY 4 Sensory neuron Activation of a (stimulated) motor neuron Interneuron Motor neuron (stimulated) Reflex arc: refers to the pathway followed in order to cause a reflex. It involves: 1. Receptor- receives the stimulus causes generation of an impulse. Could be dendrite of the sensory neuron or a specialized cell. 2. Sensory neuron- carries impulse to CNS (spinal cord/brain). 3. Integration center- CNS where information is analyzed. Association neuron/interneurons are involved. 4. Motor neuron- takes impulses from CNS to where the response occurs. 5. Effector- the structure that responds….muscle or a gland. Neuronal Circuits Human body has about: 10 million sensory neurons that bring impulses to CNS. Half million motor neurons that take impulses from CNS to the effectors to get a response. 20 billion interneurons that connect sensory neurons to correct motor neurons to get correct response! How interneurons coordinate all body functions….billions of interneurons are organized in much smaller number of units called neuronal pools- a group of neurons dedicated to a particular function/functional group of neurons that process & integrate information. Neuronal circuit-structural orientation of neurons in the neuronal pool. Neuronal Circuits Divergence Convergence Serial processing Parallel processing Reverberation Neuronal circuits can be of different types: 1. Divergence: one neuron triggers many other neurons-broad distribution of a specific input- causes amplification of signal, e.g.- a motor neuron innervating 1000’s of muscle fibers. 2. Convergence- many neurons converge to one neuron-causes concentration of signal, e.g.-different types of sensory stimuli can have same ultimate effect-linking different senses like sight, sound and smell to associate it with a memory. 3) Serial processing- a type of neuronal processing where one neuron stimulates the next, eventually causing a specific anticipated response. 4) Parallel processing-several neurons/neuronal pools process information simultaneously- many responses can occur at the same time-e.g. step on a nail pain, “ouch”, lift foot, balance your body…all at the same time. 5) Reverberation- a positive feedback mechanism-collateral branches of axon extends back toward the source of an impulse and further stimulates the presynaptic neuron to produce prolonged and repetitive effect, e.g. complex circuits that control consciousness, muscle coordination, breathing. Reflex Arc - Types Patellar reflex A. Innate reflexes: reflexes you are born with…blinking, suckling, chewing. Acquired reflexes: reflexes you learn…driving, jumping ropes. Withdrawal reflex B. Somatic reflex arc: results in contraction of skeletal muscle…voluntary effector. Autonomic/Visceral reflex arc: results in contraction of smooth/cardiac muscle or secretion of a gland…involuntary effectors. C. Monosynaptic reflex arc: does not involve interneuron…one synapse…sensory motor, e.g. patellar reflex. Polysynaptic reflex arc: has one or more interneurons…2 or more synapses, e.g. withdrawal reflex Intersegmental reflex arc: sensory neurons enter at certain level/segment of the spinal cord interneurons take to another level/segment motor neurons exit at different level. D. Spinal reflex arc: involves spinal cord as the integration center…simpler. Cranial reflex arc: involves brain as the integration center…more complex. Reflexes & Neurological Impairment Somatic reflexes are often used to diagnose nervous system injuries. Lack of reflex indicates pathway injuries. Patellar reflex: patellar ligament is tapped sensory impulses are sent to the spinal cord motor impulses exit on the same side thigh muscle- quadriceps femoris contracts extension of the leg at the knee. Achilles reflex: achilles tendon is tapped contraction of gastrocnemium muscle foot extends at the ankle. Reflex Arc - Types Spinal Cord Disorders Spinal cord trauma: damage to the spinal cord loss of muscle control paralysis. Paraplegia: where both the lower extremities are paralyzed. Quadriplegia: where all four limbs are paralyzed. Neuritis: inflammation of the nerves. Due to injuries, bone fracture, drugs, etc. Painful and affects muscle contraction. Sciatica: a type of neuritis. Compression of sciatic nerve against coxal bone or hip injury injury to sciatic nerve numbness of the lower extremity or severe pain radiating from the lower back to the leg. Shingles: where chicken pox virus, hiding in the peripheral nerves becomes re-activated when immune system in compromised starts multiplying spreads along the peripheral nerve form painful blisters on the area of the skin served by that nerve (dermatome).
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