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motor efferent system n.
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Motor (efferent) system
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Motor (efferent) system

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  1. Motor (efferent) system • Divided into: • Somatic motor division that controls skeletal muscle • Visceral/autonomic – controls smooth and cardiac muscles and the glands (exocrine and endocrine) • These 2 system differ not only in the effector but also in their descending pathway and the response

  2. ANS Versus Somatic Nervous System (SNS) The ANS differs from the SNS in the following three areas 1. Effectors The effectors of the SNS are skeletal muscles The effectors of the ANS are cardiac muscle, smooth muscle, and glands

  3. ANS Versus Somatic Nervous System (SNS) 2. Efferent pathways In the SNS heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector (one neuron) Efferent pathways in the ANS are a two-neuron chain The preganglionic (first) neuron has a lightly myelinated axon The post-ganglionic (second) neuron extends to an effector organ Pre-ganglionic Post-ganglionic Ganglion

  4. 3. Target organ responses All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect In the ANS: Preganglionic fibers release ACh Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector ANS Versus Somatic Nervous System (SNS)

  5. On NMJ - nicotinic receptors – ACh always excitatory

  6. Receptors for neurotransmitters in the motor division Receptors are divided into 2 groups: Cholinergic – receive and respond to acetylcholine (ACh): Two subgroups that are names of chemicals that mimic some of the actions of Ach: Muscarinic (chemical found in the mushroom Amanita muscarina) Nicotinic (chemical found in the tobacco plant – nicotina tabacum) Adrenergic – receive and respond to norepinephrin(NE) / epinephrine (E) Divided into alpha and beta

  7. Cholinergic Nicotinic Receptors • Found on • Motor end plates of skeletal muscle cells • All ganglionic neurons (sympathetic and parasympathetic) • Hormone-producing cells of the adrenal medulla • Effect of ACh at nicotinic receptors is always direct and stimulatory

  8. Direct effect – receptors are part of the ion channel Nicotinic receptors

  9. Cholinergic Muscarinic Receptors • Found on • All effector cells stimulated by postganglionic cholinergic fibers • The effect of ACh at muscarinic receptors • Can be either inhibitory or excitatory • Depends on the receptor type of the target organ

  10. Indirect effect – through G-protein and 2nd messenger Muscarinic (cholinergic) and all adrenergic receptors

  11. ACh (cholinergic) receptors Muscarinic receptors (mACh) Post-ganglionic Nicotinic receptors (nACh) Pre-ganglionic Direct mechanism – open Na+ channels (depolarization) Indirect mechanism – use of G-protein and 2nd messenger system Increase intracellular Ca2+ release Inhibition of adenylate cyclase Fast excitatory effect Slow excitatory M1, M3, M5 Slow inhibitory M2, M4

  12. Adrenergic Receptors • Two types • Alpha () (subtypes 1, 2) – excited • Beta () (subtypes 1, 2 , 3) • Effects of NT depend on which subclass of receptor predominates on the target organ • Norepinephrine excites mainly Alpha () receptors • Epinephrine excites both alpha and beta equaly

  13. NE (adrenergic) receptors - all indirect through G-protein b3 b1 b2 a1 a2 Increase intracellular Ca2+ release Inhibition of adenylate cyclase 2nd messenger – cAMP Inhibition or activation of adenylate cyclase Slow inhibition slow excitation Lypolysis (excitation) Slow excitation Slow inhibition


  15. Autonomic Nervous System (ANS) function The ANS major function is to maintain homeostasis The ANS : functions via visceral (subconscious) reflexes The autonomic system is coordinated primarily by the hypothalamus and the medulla (higher centers) The brain stem and the spinal cord seem to have direct influence over autonomic functions

  16. Autonomic reflexes • Autonomic reflexes are mediated by neural circuits in the spinal cord and brain stem • The afferent pathways include both visceral and somatic fibers • The reflex includes interneurons that receive convergent input from visceral and somatic fibers • Efferent pathway can be sympathetic or parasympathetic • Main difference: visceral reflex arc has two neurons in the motor pathway

  17. Hypothalamic Function • Function as the main visceral control in the body • Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, eye pupil size etc. • Perception of pleasure, fear, and rage • Maintains normal body temperature (contains body’s “thermostat”) by initiating cooling or heat-generating processes • Regulates food intake (hunger) • Regulation of water intake and thirst – produce ADH, contain thirst center • Regulates sleep and the sleep cycle

  18. The Hypothalamus other functions • Controls the the pituitary gland via the infundibulum (will be discussed in APII) • Contains the Mammillary bodies • Relay station for olfactorypathways • The supraoptic and paraventricular nuclei produce ADH and oxytocin

  19. Autonomic control – brain stem and spinal cord • The brain stem and the spinal cord have direct influence over autonomic functions

  20. Brain Stem • Consists of three regions – • midbrain, • Pons • medulla oblongata • Similar to spinal cord – deep gray matter surrounded by white tracts but contains embedded nuclei in the white matter • Controls automatic behaviors necessary for survival (breathing, digestion, heart rate, blood pressure) • Provides the pathway for tracts between higher and lower brain centers • Associated with 10 of the 12 pairs of cranial nerves

  21. Midbrain • Located between the diencephalon and the pons • Midbrain structures include: • Cerebral pedunclesof the midbrain – two structures that contain descending pyramidal motor tracts • Cerebral aqueduct – hollow tube that connects the third and fourth ventricles • Various nuclei (what are nuclei)

  22. Midbrain Nuclei • Nuclei that control cranial nerves III (oculomotor) and IV (trochlear) • Corpora quadrigemina • 2 Superior colliculi – visual reflex centers; coordinate head and eye movement when we follow a moving object (does not have to be a conscious decision) • 2 Inferior colliculi – auditory relay centers between hearing receptors to sensory cortex. • Reflexive responses to sound – turn head toward sound

  23. Midbrain Nuclei • Substantia nigra – functionally linked to basal nuclei, contains melanin pigment (precursor of dopamine - NT) • Red nucleus – largest nucleus (rich blood supply) of the reticular formation; relay nuclei for some descending motor pathways

  24. Pons • between the midbrain and the medulla oblongata • Forms part of the anterior wall of the fourth ventricle • Fibers of the pons: • Connect higher brain centers and the spinal cord • Relay impulses between the motor cortex and the cerebellum • Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial) • Contains nuclei of the reticular formation

  25. Medulla Nuclei related to autonomic system • The medulla has an important role in the autonomic reflex center that maintain homeostasis: • Cardiovascular control center – adjusts force and rate of heart contraction • Respiratory centers – control rate and depth of breathing • Additional centers – regulate vomiting, hiccuping, swallowing, coughing, and sneezing • There is an overlap between medulla and hypothalamus that uses medullary centers to carry out instructions

  26. Other Medulla nuclei and functions • Pyramids – two longitudinal ridges formed by corticospinal tracts descending from the motor cortex (will be discussed with the somatomotor pathways) • Inferior olivary nuclei – gray matter that relays sensory information regarding the stretch of muscle and joints • Cranial nervesIX (glosopharyngeal,X (vagus), XI (accessory; neck muscle), and XII (hypoglossal; tongue) are associated with the medulla • Vestibular nuclear complex – synapses that mediate and maintain equilibrium • Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis • Serve as relay station for general somatic sensation