The Autonomic Nervous System BIO 137 Anatomy & Physiology

Divisions of the Nervous System CENTRAL NERVOUS SYSTEM – Brain and Spinal Cord – Interneurons PERIPHERAL NERVOUS SYSTEM Cranial and Spinal Nerves – SENSORY Afferent neurons (input) – MOTOR Efferent neurons (output) SOMATIC – Effector: skeletal muscle AUTONOMIC – Effectors: smooth muscle, cardiac muscle, glands – PARASYMPATHETIC » maintains homeostasis – SYMPATHETIC » Fight or Flight

Autonomic Nervous System Functions independently and continuously without conscious effort Controls visceral activities Effectors are cardiac muscle, smooth muscle and glands Oversees heart rate, breathing rate, body temperature and other visceral activities that aid in maintaining homeostasis

Divisions of the PNS: Motor Divisions SOMATIC – Conscious, voluntary control – Effectors: skeletal muscle AUTONOMIC – Involuntary control, without conscious effort – Effectors: smooth muscle, cardiac muscle, glands PARASYMPATHETIC (maintains homeostasis) Active under normal, restful conditions Prepares body for normal activities Maintains homeostasis SYMPATHETIC (Fight or Flight) Prepares body for fight or flight situations Active in Stressful situations

Introduction to the ANS The ANS usually operates without conscious control, though centers in the hypothalamus and brain stem do provide regulation for ANS reflexes. – Sensory receptors called interoceptors located in blood vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment. Examples of interoceptors are chemoreceptors that monitor blood CO 2 level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels.

ANS Innervation Most visceral organs innervated by both sympathetic and parasympathetic fibers Generally exert antagonistic effects – Sympathetic = ↑ heart rate – Parasympathetic = ↓ heart rate Both systems are usually partially active

Autonomic Nervous System Sensory signals originate in skin and viscera and travel to CNS Motor impulses exit the CNS and lead to ganglia outside of CNS Information is integrated in ganglia and then travels to an effector 2 Neuron Chain

Autonomic Nerve Pathway Motor nerve pathway is a 2 neuron chain – Preganglionic neuron Cell body in CNS – Postganglionic neuron Cell body in ganglia Preganglionic axon synapses with postganglionic cell body at a ganglia Neurotransmitters are released at the end of each neuron

2 Neuron Chain in the ANS Pathway

Parasympathetic Division of the ANS Long Preganglionic neurons arise from 4 cranial nerves in the brainstem and the sacral region of cord and extend to ganglia near effectors Short postganglionic neurons lead from ganglia to effectors – The vagus nerve (CN X) carries nearly 80% of the total parasympathetic flow to the organs of the thorax and upper abdomen. – Pre & post-ganglionic fibers release acetylcholine (cholinergic) Operates under normal conditions to maintain homeostasis

Sympathetic Division of the ANS Axons of motor nerves (from T1-L2) exit through ventral root of spinal nerves, branch and enter sympathetic ganglia (trunks) located in chains along vertebral column – Sympathetic preganglionic neurons exit the spinal cord only between levels T 1 -L 2 Short pre-ganglionic fiber releases acetylcholine (cholinergic) Long post-ganglionic fiber releases norepinephrine (adrenergic) Operates under stressful conditions Fight or flight response

ANS Neurotransmitters The total number of neurotransmitters used in the entire nervous system is not known, but is well over 100. – Despite the variety of possible chemicals that could be used to transmit chemical messages in the ANS, only 2, acetylcholine and norepinephrine, are used to any great degree. Synapses at which ACh is used are termed cholinergic. Synapses at which norepinephrine or epinephrine are used are termed adrenergic.

Autonomic Neurotransmitters Cholinergic Neurons release acetylcholine All preganglionic parasympathetic and sympathetic neurons All postganglionic parasympathetic neurons Adrenergic Neurons release norepinephrine All postganglionic sympathetic neurons

ANS Receptors & Actions Cholinergic Receptors bind acetylcholine, ACh Nicotinic – usually excitatory & quick All post-ganglionic neurons Muscarinic – usually excitatory & slow Parasympathetic effector membranes

ANS Receptors & Actions Adrenergic Receptors bind norepinephine Alpha – different responses on different effectors; usually excitatory Beta - different responses on different effectors; β1-excites; β2- inhibits Sympathetic effector membranes

ANS Neurotransmitters Neurons and Neurotransmitters of the Parasympathetic Nervous System PreganglionicPostganglionic Cell body in brain or spinal cord Cell body in intramural ganglion Acetylcholine (ACh)

ANS Neurotransmitters Neurons and Neurotransmitters of the Sympathetic Nervous System PreganglionicPostganglionic oCell body in lateral horn of ospinal cord Cell body in sympathetico chain gangliono oAcetylcholine (ACh) (norepinephrine, NE) ol except sweat glands (Ach) o

Summary of ANS

Control of Autonomic Activity Controlled largely by CNS Medulla oblongata regulates cardiac, vasomotor and respiratory activities Hypothalamus regulates visceral functions Limbic system and cerebral cortex control emotional responses

Goals for Autonomic Nervous System Name each division and answer the following questions: – Where are ganglia located? – What neurotransmitter is secreted by the pre & post-ganglionic neurons? – What type of receptor is found on post-ganglionic neurons and effectors? – What is the general function of the division?