Spinal Cord Lec:2
Assis.Professor Dr. Farah Nabil Abbas MBChB, MSc, PhDSpinal Cord
Within bony vertebral column.Cylindrical of soft tissue (little finger).Cross-section central butterfly (gray matter):InterneuronCell bodiesDendrites of efferent neuronsEntering fibers of afferent neuronsGlial cells.Spinal Cord
Gray matter (Lacks myelin).White matterMyelinated axons of interneuron “fiber tracts or pathways”.Descending: Relay information from brain to spinal cord.Ascending: Transmit information to brain.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)
ReflexAn 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
Important role in protection of the body. Responsible for maintenance of: a. muscle tone. b. body posture. Center can be anywhere except cerebral cortex. Center can be in spinal cord or in brain stem.Classification of the Reflexes
1. Anatomical – classified according to the spinal cord segment involved into:- Segmental reflexreflex 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 reflexesClassification of the Reflexes
Conditioned reflexes: Not present at birth but acquired later on by training i.e., cycling and swimming. 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 Conjunctival reflex Abdominal reflex Planter reflexUnconditioned Reflexes
B. Deep reflexes Elicited by tapping the tendon of a slightly stretched muscle Knee jerk Ankle jerk.
Unconditioned Reflexes
C. Visceral reflexes Concerned with the reflex activity of internal organs. Part of the reflex is made by autonomic NS. 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 1000sOrganization 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) Anterior motor neurons InternuronsAnterior 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 typesAnterior Motor Neurons
Alpha Motor NeuronsTransmit impulse through Aα fibers (14μm) innervate large skeletal muscle fibers.Stimulation excites 3-100s skeletal muscle fibers called the "motor units".Gamma Motor NeuronsTransmit 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.