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Lecture 18

ANP1106 Lecture 18: Physiology notes Part 4

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Anatomy and Physiology
Jacqueline Carnegie

NEURAL INTEGRATION S ENSORIMOTOR A RC Between the input of sensory information and the output of motor • commands, there is neural integration. This integration occurs for the actions of reflexes and for the voluntary actions. We will start with the reflexes since their mechanisms are simple and very well-known. • Then we will speak in general terms about neural integration for voluntary actions because their mechanisms are far more complex and much less understood. REFLEXES B ASIC R EFLEX ARC A reflex is an automatic action triggered by a • stimulus • The action can be a movement, salivation, an increase in heart rate or others. The stimulus can be mechanical, visual, auditory or others. • The reflexes are classified 2 ways According to the control of the reflex involuntary reflex learned or conditioned reflex According to the reflex target autonomic reflexes (the effectors are the glands or the smooth muscles or cardiac muscles) somatic reflexes (the effectors are the skeletal muscles) • Elements which form part of a reflex: receptor to transduce the stimuli into an electric signal sensory neuron to forward information to the CNS (n.b. simple receptors belong to the sensory neuron) integration center which includes the neurons of the CNS that will transform the information of the sensory neurons into commands for the motoneurons (n.b. for polysynaptic reflexes, integration is done by interneurons, but for monosynaptic reflexes, integration is done by the motoneuron itself) motoneuron to activate the effector effector (e.g. muscle or gland). SOMATIC REFLEXES W ITHDRAWL REFLEX • For the arms, placing the hand in fire will cause a flexion of the arm For the legs, walking on glass will cause flexion to remove the painful stimulus • (flexor reflex) and extension of the opposite leg to maintain an upright position (crossed extensor reflex) • The expression of reflexes can be partially controlled by the voluntary actions of the cerebral cortex. Normally, the reflex for protection and balance have priority over the control of voluntary movements. But in exceptional cases, it is possible to voluntarily modify a reflex TENDON R EFLEX • Tension in the agonist causes inhibition of the agonist and excitation of the antagonist to prevent the development of too much tension in the agonist • This is a protective reflex which prevents riping the tendon out of its insertion into the bone. S TRETCH R EFLEX • Stretching of the agonist causes excitation of the agonist and inhibition of the antagonist to return agonist muscle length towards its original length • The gamma-motoneurons of the muscle spindles define the muscle length for the desired posture and the reflex corrects and perturbations in this desired posture. This reflex becomes exaggerated after spinal cord transection and may lead to muscle spasms P LANTAR R EFLEX • Scratching the sole of the foot in patients with a lesion of the cortciospinal pathway produces the sign of Babinski where the plantar reflex produces an extension of the big toe and abduction of the other toes. • Stimulation of the sole of the foot activates sensory neuronsprojecting to spinal cord interneurons that will produce the sign of Babinski unless the corticospinal projections are available to modify the behavior of the spinal cord interneurons forming this reflex AUTONOMIC REFLEXES O RGANIZATION The control of the internal milieu is done by the • hypothalamus • It receives information of the state of all the internal organs and systems. • To maintain homeostasis, the hypothalamus acts on two systems. Endocrine system secretes hormones in the blood supply that travel to target tissues and stimulate homeostatic responses. We will not be covering the endocrine system Autonomic nervous system controlled by the hyothalamus, uses neurones to control the target organs H OMEOSTASIS Homeostasis involves mechanisms that take corrective actions to prevent changes in the value of a variable S YMPATHETIC AND PARASYMPATHETIC • Almost all the targets of the autonomic nervous system receive inputs from both the sympathetic (T1-L2) and parasympathetic (cranial nerves 3, 7, 9 and 10 and S2-S4) divisions • The sympathetic ganglia form the sympathetic trunk or paravertebral ganglia on both sides and the prevertebral ganglia at the front of the spine. E FFECTS • The sympathetic and parasympathetic divisions us
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