Chapter 15- Autonomic Nervous System

Somatic Nervous System Overview

• effect is always excitation

• under voluntary control

• can receive sensory input from somatic senses

• is a 2-neuron pathway

ANS Contributes to Homeostasis by Responding to

• subconscious visceral sensations

• exciting or inhibiting smooth muscle, cardiac muscle, or glands

ANS Structurally

• autonomic sensory neurons

• integrating centers of the CNS

• autonomic motor neurons

ANS Characteristics

• involuntary control

• sensory receptors called interceptors located in blood vessels, visceral organs, muscles, and the nervous system monitor conditions in the internal environment

Chemoreceptors

monitor blood CO2 level

Mechanoreceptors

detect the degree of stretch in the walls of organs or blood vessels

ANS Motor Pathways Overview

• regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities in their effector tissues

ANS Motor Pathways Consist of Two Motor Neurons in a Series

• preganglionic neuron

• postganglionic neuron

Preganglionic Neuron

has its cell body in the central nervous system and the axon extends to an autonomic ganglion

Postganglionic Neuron

has its cell bodies in the intermediate ganglion and its unmyelinated axon extending from the ganglion to the effector

Divisions of ANS (2)

• sympathetic division (thoracolumbar)

• parasympathetic division (craniosacral)

Dual ANS Innervation

body organs receive impulses from both sympathetic and parasympathetic neurons

usually the nerve impulses from one division stimulate an organ, while impulses from the other division decrease activity

Sympathetic Division (Thoracolumbar)

• preganglionic neurons have cell bodies in the lateral horns of grey matter in the 12 thoracic segments

• synapse to postganglionic neuron

• fight or flight response

Parasympathetic Division (Crainosacral)

• preganglionic neurons have cell bodies in the nuclei of cranial nerves and lateral grey matter of the 2-4 sacral segments

• synpase of postganglionic neurons at terminal ganglia near or within an effector

• rest and relax

Cholinergic

synpases at which acetylcholine is released

Adrenergic

synapses at which norepinephrine is released

Cholinergic Receptors Subtypes

• nicotinic receptors

• muscarinic receptors

Nicotinic Receptors

• in plasma membrane of dendrites and cell bodies of sympathetic and parasympathetic postganglionic neurons, plasma membrane of chromaffin cells of the adrenal medulla and the motor end plate of the NMJ

• activation of nicotinic receptors by ACh causes depolarization (excitation) of a postganglionic neuron

• autonomic effector of skeletal muscles

Muscarinic Receptors

• in plasma membrane of all effectors innervated by parasympathetic postganglionic axons

• activation by ACh sometimes causes depolarization or hyperpolarization

• ACh inhibits (relaxes) smooth muscles sphincters in the GI tract but excites (contracts) smooth muscles of the iris

Adrenic Receptors

receptors that respond to norepinepherine (NE) and epinephrine (E) released by a sympathetic postganglionic neuron or as a hormone by chromaffin cells into blood

Adrenic Receptors Subtypes

• alpha receptors

• beta receptors

Alpha Receptors

• alpha 1 (excitation)

• alpha 2 (inhibition)

Alpha 1 Receptors

• in smooth muscles of blood vessels of salivary glands, skin, kidneys

• causes contractions causing vasoconstriction and closing of sphincters

• in salivary glands: cause secretion of potassium ions and water

• in sweat glands: cause increased sweating

Alpha 2 Receptors

• in smooth muscle of some blood vessels

• cause relaxation and vasodilation

• cells of pancreas: cause decreased insulin secretion

• platelets of blood: cause aggregation to form platelet plug

Beta Receptors

• beta 1 (excitation)

• beta 2 (inhibition)

Beta 1 Receptors

• in cardiac fibres: cause excitation (increased force and rate of contraction)

• in posterior pituitary: cause ADH secretion

• in adipose cells: cause breakdown of triglycerides

Beta 2 Receptors

• in smooth muscle of walls of airways, blood vessels that serve the heart, skeletal muscle adipose tissue, liver and walls of visceral organs

• cause relaxation and vasodilation

• cause glycogenolysis in the liver

Autonomic Tone

• defined as the balance between sympathetic and parasympathetic activity

• regulated by the hypothalamus

• divisions can affect body organs differently because postganglionic neurons release different neurotransmitters and because effector organs posses different cholinergic and adrenergic receptors

Sympathetic Responses

• dominates during times of physical or emotional stress

• reduces body functions that favour the storage of energy

Sympathetic Responses - E Situations

• exercise

• emergency

• excitment

• embarrassment

Sympathetic Responses - Steps

1. sympathetic postganglionic neurons diverge

2. acetylcholinesterase rapidly inactivates ACh

3. epinephrine and norepinephrine secretes into blood from adrenal medulla prolong an intensify responses of NE

Sympathetic Stimulation Leads to…

• secretion of NE by the adrenal glands

• an increase in the rate and strength of heart beat

• constriction of blood vessels of non-essential organs

• dilation of vessels of essential organs

Parasympathetic Receptors…..

• enhances rest and digest activities

• high parasympathetic responses favours body functions that conserve and restore energy

SLUDD

• salivation (increased)

• lacrimation (increased)

• urination (increased)

• digestion (increased)

• defecation (increased)

Visceral Effector Glands - Adrenal Medulla

• sympathetic: secretion of E and NE

Visceral Effector Glands - Lacrimal

• sympathetic: slight tears

• parasympathetic: secretion of tears

Visceral Effector Glands - Pancreas

• sympathetic: inhibits secretion of digestive enzymes and insulin while promoting secretion of glucagon

• parasympathetic: secretion of digestive enzymes and insulin

Visceral Effector Glands - Posterior Pituitary

• sympathetic: secretion of ADH

Visceral Effector Glands - Pineal

• sympathetic: increase synthesis and release melatonin

Visceral Effector Glands - Sweat

• sympathetic: increased sweating in most body regions

Visceral Effector Glands - Adipose Tissue

• sympathetic: lipolyis and release fatty acids

Visceral Effector Glands - Liver

• sympathetic: decreased bile secretion

• parasympathetic: glycogen synthesis, increased bile secretion

Visceral Effector Glands - Kidneys

• sympathetic: secretion of renin

Visceral Effector Glands - Cardiac Muscle

• sympathetic: increased heart rate and force of atrial and ventricular contractions

• parasympathetic: decreased heart rate and force of atrial contraction

Visceral Effector Glands - Lungs

• sympathetic: airway dilation

• para: airway constriction

Visceral Effector Glands - Stomach

• sympathetic: decreased motility and tone, contraction of sphincters

• para: increased motility and tone, relaxation of sphincters

Visceral Effector Glands - Urinary Bladder

• sympathetic: relaxation of wall, contraction of internal urethral sphincter

• para: contraction of wall, relaxation of internal urethral sphincter

Autonomic (visceral reflexes)

are responses that occur when nerve impulses pass through autonomic reflex arc

Reflex Arc

a neural pathway that controls a reflex

play a key role in blood pressure, digestion, and defecation

Components of Autonomic Reflex Arc

• sensory receptor

• sensory neuron

• integrating centre

• motor neurons

• effector

Components of Autonomic Reflex Arc - Sensory Receptor

distal end of the sensory neuron which responds to a stimulus and produces a change that will trigger a nerve impulse

usually associated with interoceptors which respond to internal stimuli

Components of Autonomic Reflex Arc - Sensory Neuron

conducts nerve impulses from receptors to CNS

Components of Autonomic Reflex Arc - Integrating Centre

interneurons in CNS relay signals from sensory to motor neurons and main integrating centres for autonomic reflexes are in hypothalamus

Components of Autonomic Reflex Arc - Motor Neurons

two motor neuron pathway conducts nerve impulse from integrating centre out of CNS to effectors