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Spinal Cord Lec:2 Assis.Professor Dr. Farah Nabil Abbas MBChB, MSc, PhD Spinal Cord • Within bony vertebral column. • Cylindrical of soft tissue (little finger). • Cross-section central butterfly (gray matter): i. ii. iii. iv. v. Interneuron Cell bodies Dendrites of efferent neurons Entering fibers of afferent neurons Glial cells. Spinal Cord • Gray matter (Lacks myelin). • White matter • Myelinated axons of interneuron “fiber tracts or pathways”. 1) Descending: Relay information from brain to spinal cord. 2) Ascending: Transmit information to brain. 3) Others: Transmit information between different levels of brain and spinal cord. Spinal Cord • Afferent fibers enter spinal cord via dorsal root. • Cell bodies of the afferent neurons “dorsal root ganglia. • Efferent neurons leaves spinal cord via ventral roots. • Dorsal and ventral roots from same level combine spinal nerve, one on each side of spinal cord. Motor Functions of the Spinal Cord (Cord Reflexes) Reflex • An involuntary motor response to an adequate sensory stimulus that occurs subconsciously, without the mediation of the cerebral cortex • i.e., withdrawal reflex. Characters of Reflex Action 1. Important role in protection of the body. 2. Responsible for maintenance of: a. muscle tone. b. body posture. 3. Center can be anywhere except cerebral cortex. 4. Center can be in spinal cord or in brain stem. Classification of the Reflexes 1. Anatomical – classified involved into:• Segmental reflex according to the spinal cord segment • reflex arc involves one side of spinal segment. • Receptors organ, afferent neuron, efferent neuron and effector organ are located on same side of spinal segment, i.e., stretch reflex. Classification of the Reflexes • Intersegmental reflex • Reflex arc involves both sides of spinal segment • Efferent neuron and effector organ are on opposite sides of receptor organ, i.e., crossed extensor reflex. • Suprasegmental • Segments above the level of reflex arc are involved, i.e., postural reflex. Classification of the Reflexes 2. Physiological: • Classified into • flexor/ withdrawal reflex • extensor / stretch reflex. 3. Clinical Reflexes: classified according to whether they were present at birth or developed later into: • Unconditioned & Conditioned reflexes Classification of the Reflexes A. Conditioned reflexes: • Not present at birth but acquired later on by training • i.e., cycling and swimming. B. Unconditioned reflexes: • Inborn - present since birth • i.e., suckling reflex. • They are of three types: Unconditioned Reflexes A. Superficial reflexes • Elicited by stimulation of receptor organs that are present superficially in the skin or mucous membrane i. Conjunctival reflex ii. Abdominal reflex iii. Planter reflex Unconditioned Reflexes B. Deep reflexes • Elicited by tapping the tendon of a slightly stretched muscle i. Knee jerk ii. Ankle jerk. Unconditioned Reflexes C. Visceral reflexes • Concerned with the reflex activity of internal organs. • Part of the reflex is made by autonomic NS. i. ii. iii. Cardiovascular reflex Gastrointestinal reflex Micturition reflex. Reflex Arc • Basic unit of integrated neural activity. • Made up of sense organ, an afferent neuron, one or more synapses, and an efferent neuron. • Monosynaptic “single synapse” – simple arc • Polysynaptic “multiple interneuron". • Number of synapses in the arcs - 2 to 1000s Organization of Spinal Cord for Motor Functions • Each spinal segment - several million neurons in the gray matter. •Gray matter - integrative area of cord reflexes. • Neurons (2 types) 1. Anterior motor neurons 2. Internurons Anterior Motor Neurons • Anterior horn - Several 1000s neurons / each segment . • 50-100% larger than most of others. • Give rise to nerve fibers leaves by way of ventral roots and innervates skeletal muscle fibers. • 2 types Anterior Motor Neurons • Alpha Motor Neurons • Transmit impulse through Aα fibers (14μm) innervate large skeletal muscle fibers. • Stimulation excites 3-100s skeletal muscle fibers called the "motor units". • Gamma Motor Neurons • Transmit impulse through Aδ fibers (5μm) innervate intrafusal fibers - part of the muscle spindle help to control basic muscle tone. Interneuron • All areas of gray matter. • 30 times > anterior motor neurons. • Small and highly excitable. • Interconnections with each other • Directly innervate anterior motor neurons.