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Figure 14.17b. Table 14.5. Figure 14.18. Concept 14.4 Reflexes and SNS Reflex Arcs. Reflexes. somatic reflexes involve contraction of skeletal muscles autonomic reflexes not consciously perceived responses of smooth muscle, cardiac muscle, and glands reflex arc
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Reflexes • somatic reflexes involve contraction of skeletal muscles • autonomic reflexes • not consciously perceived • responses of smooth muscle, cardiac muscle, and glands • reflex arc • pathway followed by nerve impulses that produce reflex • may be monosynaptic or polysynaptic
Reflexes • fast involuntary, unplanned sequence of actions that occurs in response to a particular stimulus • some present from birth • some are learned or acquired • cranial reflex • integration occurs in brain • spinal reflex • integration occurs in spinal cord • SNS reflexes are always excitatory
SNS Reflex Arc Components • sensory receptor • dendrite or associated sensory structure • sensory neuron • axon and axon terminals • integrating center • interneuron(s) that relay impulses • motor neuron • impulse triggered by integrating center • effector • body part that responds to impulse
Stretch Reflexes • Triggered by tapping on tendons attached to muscles at elbow, wrist, knee, and ankle joints • Monosynaptic • muscle spindles detect slight stretch • muscle spindle generates impulse(s) to sensory neuron to posterior root of spinal nerve • sensory neuron synapses with motor neuron in gray matter of spinal cord • strong enough impulse triggers AP of motor neuron • ACh at NMJ triggers contraction of skeletal muscle
Stretch Reflexes • Monosynaptic reflexes are called ipsilateral reflexes • propagate into and out of the same side of spinal cord • reflex helps prevent injury by preventing overstretching of muscles • reciprocal innervation • polysynaptic reflex arc to antagonistic muscles operates at the same time • three neurons and two synapses
Flexor Reflexes • Triggered by painful AKA withdrawal reflex • Polysynaptic or intersegmental reflex arc • pain-sensitive sensory neuron stimulated • impulse propagates into spinal cord • sensory neuron activated interneuron and signal sent to several segments • several motor neurons activated and motor impulse propagates toward several NMJs • ACh released into synaptic cleft causes flexor muscles to contract withdrawing body part from painful stimulus
Flexor Reflexes • reciprocal innervation occurs as in stretch reflex
ANS versus SNS SNS • produces voluntary movements • in response to consciouslyperceived sensory input ANS • produces involuntary movements in • cardiac muscle • smooth muscle • glands • in response to unconscious sensory input
ANS Reflex Arcs • regulates activity of smooth, cardiac muscle, and many glands • continual flow of nerve impulses from autonomic sensory neurons in visceral organs and blood vessels propagates into integrating centers of CNS • impulses in autonomic motor neurons propagate to various effector tissues • can excite or inhibit activities of effector tissues • ANS activity is regulated by hypothalamus and brain stem of CNS
ANS Divisions sympathetic division parasympathetic division • most organs have dual innervation by both sympathetic and parasympathetic divisions • transmit opposing nerve impulses enteric division • enteric plexuses • network of neurons that extend throughout GI tract walls • contain • sensory neurons • interneurons • motor neurons
Components of ANS • Preganglionic Neurons • Autonomic Ganglia • Postganglionic Neurons • Effectors
Preganglionic Neurons • cell body in CNS • sympathetic division • in gray matter segments T1-T12, L1 and L2 • parasympathetic division • in nuclei of four cranial nerves in brain stem • in gray matter segments S2-S4 • small-diameter myelinated fiber • extends to an autonomic ganglion • synapses with postganglionic neuron
Ganglia • Sympathetic ganglia • sympathetic trunk ganglia • vertical row on either side of vertebral column • prevertebral ganglia • celiac ganglion • superior mesenteric ganglion • inferior mesenteric ganglion • Parasympathetic ganglia • terminal ganglia • located close to or actually within wall of visceral organ • longer than most axons of sympathetic preganglionic neurons
Autonomic Plexuses • many lie along major arteries • may contain sympathetic ganglia and axons of autonomic sensory neurons • often named after associated artery • thoracic plexuses • cardiac plexus • pulmonary plexus • abdominal and pelvic plexuses • celiac plexus • superior and inferior mesenteric plexus • hypogastric plexus • renal plexus
Postganglionic Neurons • Sympathetic neurons connect with postganglionic neurons by • synapse with first ganglion it reaches • may ascent or descent to higher or lower ganglion before synapsing with postganglionic neurons • without synapsing it may continue through sympathetic trunk ganglion • end at prevertebral ganglion • synapse with postganglionic neurons there
Postganglionic Neurons Sympathetic effectors • some preganglionic sympathetic axons directly innervate adrenal medullae • each has many axon collaterals • single sympathetic preganglionic fiber may synapse with 20 or more postganglionic neurons • example of divergence • explains why responses affect almost entire body simultaneously Parasympathetic effectors • preganglionic neurons pass to terminal ganglia near or within a visceral effector • can be localized to single effector
ANS Signal Transmission Classification based on neurotransmitter released • Cholinergenic neurons • release acetylcholine • Adrenergenic neurons • release norepinephrine
Cholinergenic neurons • release acetylcholine • preganglionic • both sympathetic and parasympathetic • sympathetic postganglionic that innervate most sweat glands • all parasympathetic postganglionic neurons
Adrenergenic neurons • release norepinephrine • most sympathetic postganglionic neurons • receptors • bind both neurotransmitter norepinephrine and hormone epinephrine • four classifications • alpha 1, alpha 2, beta 1, and beta 2 • alpha 1 and beta 1 are generally excitatory receptors • alpha 2 and beta 2 are generally inhibitory receptors
Sympathetic • dominates during extreme physical or emotional stress • favors activities that can support high production of ATP and high physical activity • fight or flight response • pupil dilation • increased heart function • dilation of airways • reduced blood flow to viscera • increased blood flow to heart, skeletal muscles, liver and adipose tissue • increased glycogen and fatty acid breakdown • release of glucose from liver • inhibition of processes not essential for emergency response
Parasympathetic • enhances rest and digest activities • favors activities that can support body functions that conserve and restore energy during times of rest and recovery • SLUDD • salivation • lacrimation • urination • digestion • defecation • three decreases • heart rate • diameter of airways • diameter of pupils
Autonomic Reflexes Components: • Receptor • Sensory neuron • Integrating center • Motor Neuron • Effector
Hypothalamus • control and integration center of ANS • receives sensory input regarding • visceral function • olfaction • gustation • blood, temperature, osmolarity, and substance concentration • emotions from limbic system • output via reticular formation to • brain stem • spinal cord • posterior and lateral control sympathetic activities • anterior and medial control parasympathetic activities
