Chapter 16: Autonomic Nervous System (ANS)

Chapter 16 Autonomic Nervous System (ANS) · makes routine homeostatic adjustments in body systems outside of our conscious awareness · involuntary control of major body systems · directed by hypothalamus · neurons of ANS release neurotransmitters to cause changes in visceral activity · continuously works on maintaining autonomic tone · ANS directs visceral reflex responses (reflexive glandular & non-skeletal reflexes) · divided into sympathetic and parasympathetic NS · At rest → Parasympathetic NS prevails · In crisis → Sympathetic NS prevails Somatic Nervous System · consciously perceived sensations · motor neurons carry information from CNS to skeletal muscles · one motor neuron connects CNS to organ Autonomic Nervous System · involuntary inhibition or excitation of smooth muscle, cardiac muscle, or glandular secretion · motor neurons carry information to visceral motor neurons in autonomic ganglia then from autonomic ganglia to visceral effectors · two motor neurons needed to connect CNS to organ · preganglionic and postganglionic neurons Anatomy of ANS · preganglionic neuron ↳ cell body in brain or spinal cord ↳ axon is myelinated fiber (Type B Fiber)that extends to the autonomic ganglion · postganglionic neuron ↳ cell body lies outside CNS in an autonomic ganglion ↳ axon is unmyelinated fiber (Type C Fiber) that terminates in a visceral effector Divisions of ANS · sympathetic NS ↳ aka thoracolumbar division ↳ preganglionic cell bodies in T1-L2 segments of spinal cord ↳ short preganglionic fibers · Parasympathetic NS ↳ aka craniosacral division ↳ preganglionic cell bodies in 4 cranial nerves and the S2-S4 segments of spinal cord ↳ long preganglionic fibers Most organs are innervated by both branches to maintain homeostasis (with exceptions of: arteries sweat glands, arrector pili muscles) Sympathetic NS Response · Fight or Flight · Activation of SNS: ↳ increased mental alertness ↳ increased metabolic rate ↳ decreased urinary and digestive functions ↳ activation of energy reserves ↳ increased respiratory rate and bronchodilation ↳ increased heart rate and blood pressure ↳ activation of sweat glands Sympathetic NS · preganglionic neurons will synapse in one of three locations: ① Sympathetic Chain Ganglia (paravertebral ganglia) ↳ effectors in body wall, thoracic cavity, head, neck, limbs ② Collateral Ganglia ↳ innervate abdominopelvic tissues and viscera ③ Adrenal Medulla ↳ secretes neurotransmitters directly into bloodstream (neurotransmitters act as hormones) Collateral Ganglia · There are three collateral ganglia ↳ celiac ganglion: innervates stomach, duodenum, liver, gallbladder, pancreas, spleen, and kidney ↳ superior mesenteric ganglion: innervates small intestine and initial segments of large intestine ↳ inferior mesenteric ganglion: innervates terminal portions of large intestine, the kidney, urinary bladder, and sex organs Adrenal Medulla Pathway · secretes epinephrine and norepinephrine into bloodstream (80% E) ↳ impacts metabolic activity throughout body · longer lasting effects compared to direct sympathetic innervation (and neurotransmitters at synapse) Neurotransmitters · Acetylcholine (Ach) All preganglionic neurons ↳ released from sympathetic & parasympathetic preganglion neurons ↳ released from parasympathetic postganglionic neurons ↳ synapses that use Ach are cholinergic ↳ excitatory Only 1 type of postganglionic neuron · Norepinephrine ↳ released from most ganglionic neurons ↳ neurons that release NE are adrenergic ↳ a catecholamine (as are epinephrine and dopamine) ↳ effects last longer than Ach ↳ binds to adrenergic receptors (alpha and beta receptors) Sympathetic NS & Blood Pressure · epinephrine is non-selective · norepinephrine has higher affinity for alpha receptors Note that Ach causes vasodilation and NE causes vasoconstriction Parasympathetic NS Response · Rest and Digest · Activation of PNS: ↳ decreased metabolic rate ↳ decreased HR and BP ↳ increased saliva and digestive secretions ↳ increased digestive mobility ↳ stimulation of urination & defecation Parasympathetic NS · Ach released from parasympathetic preganglionic and postganglionic neurons · 2 kinds of cholinergic receptors: ① Nicotinic Receptors ↳ are cell bodies of sympathetic and parasympathetic postganglionic neurons ↳ Ach activates receptor → gated Na+ channels open → excitation ② Muscarinic Receptors ↳ are in effectors (at neuromuscular and neuroglandular junctions of PNS) ↳ innervated by parasympathetic postganglionic neurons (in smooth muscle, cardiac muscle, glands) =X Dual Innervation · organs with dual innervation receive input from both sympathetic and parasympathetic · effects are typically antagonistic · parasympathetic and sympathetic nerves intermingle in thoracic and abdominopelvic cavities · Exceptions: ↳ most arterioles and veins → sympathetic only ↳ most sweat glands → sympathetic only ↳ salivary glands → innervated by both, but not antagonistic (both stimulate secretion) Visceral Reflexes · provide autonomic motor responses to modify or facilitate higher centers · all visceral reflexes are polysynaptic · reflexes can be: ↳ long reflexes (use CNS) ↳ short reflexes (no CNS) Higher Order Functions · higher order functions share 3 characteristics: ↳ require the cerebral cortex ↳ involve conscious and unconscious information processing ↳ subject to adjustment over time (not innate or fixed) eg. memory, consciousness, cognition, language Memory · storage of acquired knowledge for later recall · Short-term memory → limited capacity · long-term memory → huge capacity · memory consolidation: the conversion of short term memory to long term · fact memories vs. Skill memories · memory emgram: a single circuit that responds to a single memory Consciousness · States of consciousness: Sleep and arousal · Sleep: ↳ needed for regeneration of tissues ↳ deep sleep: minimal cerebral cortex activity, whole body relaxation, decreased HR, BP, and respiration ↳ REM sleep: dream state, BP and respiration fluctuate ↳ coma: not rousable with normal stimulation Arousal: · involve complex interactions between reticular formation and cerebral cortex · Reticular Activating System (RAS) ↳ receives and integrates all synaptic output ↳ stimulation activates cerebral cortex (ends sleep and promotes cortical awareness) ↳ collaborates with thalamic nuclei → enhanced alertness & focus · consciousness maintained by positive feedback activity · if NOT active → decreased function of cerebral cortex → unconsciousness