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Spinal Cord. Spinal Cord. The spinal cord is a long, thin, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. Ascending tracts (Sensory ). Descending tract ( Motor). Function.
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Spinal Cord • The spinal cord is a long, thin, tubular structure made up of nervous tissue, that extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column
Function • Transmission of sensory signals from periphery to the brain using ascending tracts • Transmission of motor impulses from brain to the periphery using descending tracts • It is the Centre for deep tendon reflexes like knee jerk • Interneurons like renshaw cells help in modulation of motor impulses • Regulation of pain with gate control mechanism • Internal circuits can assist in locomotion
Spinal cord abnormalities • Injury • Complete transection • Hemisection • Syringomylia • Tumours • Infections : tabesdorsalis
Definition • Reflex activity is the response to a peripheral nervous stimulation • It occurs without our consciousness. • It is a type of protective mechanism
Reflex Activity • Example: • When hand is placed on a hot object, it is withdrawn immediately. • When a bright light is thrown into the eyes, eyelids are closed and pupil is constricted to prevent the damage of retina by entrance of excessive light into the eyes.
REFLEX ARC • Reflex arc is the anatomical nervous pathway for a reflex action. • It is the basic unit of integrated reflex activity
REFLEX ARC • A simple reflex arc includes five components • Sense organ (receptor) • An afferent neuron (sensory neuron) • One or more synapses within a central integrating station (centre) • An efferent neuron ( motor neuron) • An effectororgan (muscle).
REFLEX ARC • Receptor • Receptor is the end organ, which receives the stimulus and produces receptor potential. • Afferent Nerve • Afferent or sensory nerve transmits sensory impulses from the receptor to center. • Center • Center receives the sensory impulses via afferent nerve fibers and in turn, it generates appropriate motor impulses. • Center is located in the brain or spinal cord.
REFLEX ARC • Efferent Nerve • Efferent or motor nerve transmits motor impulses from the center to the effector organ. • EffectorOrgan • Effector organ is the structure such as muscle or gland where the activity occurs in response to stimulus.
Clinical classification of reflex • Superficial reflex • Eg: cunjunctival reflex • Deep reflex • Knee jerk • Visceral reflex • Light reflex
Properties of Reflexes Adequate stimulus • Each reflex has a precise stimulus called the adequate stimulus. Irradiation and recruitment ( spatial ): • The extent of the response in a reflex depends on the intensity of the stimulus. Summation • Spatial summation • Temporal summation
Properties of Reflexes Recruitment • If a repetitive stimulus is maintained, the strength of the reflex contraction slowly increases to a final level After-discharge • The reflex response may continue some time after cessation of the stimulus Reciprocal innervation or reciprocal inhibition • Reflex contraction of an agonist muscle is accompanied by inhibition of the antagonist.
Properties of Reflexes Response time • Response time is an indicator of the number of synapses in the reflex arc. Rebound phenomenon • This is the exaggeration of a reflex after a temporary period of inhibition. Fatigue • On repeated stimulation of a reflex for a long time, the response begins to diminish slowly.
Abnormal reflexes • Increased reflexes : injury to brain or spinal cord • Decreased reflexes: injury to spinal nerves • Babinsski sign : injury to pyramidal tract • Pendular jerk: injury to cerebellum