autonomic nervous system

roles of the ANS

involuntary branch of the efferent division of the peripheral nervous system (PNS)

controls the body’s internal environment in a coordinated manner

• regulates contraction/relaxation of cardiac muscle, smooth muscle, glands

• heart rate, circulation, respiration, digestion

internal organs controlled by ANS: heart, lungs, stomach etc.

CNS control of ANS

spinal cord

• integrates autonomic reflexes not subject to higher control e.g. urination, defecation

medulla and pons

• regulate cardiovascular, respiratory and digestive tracts

hypothalamus

• heart rate, blood pressure, respiration (via medulla)

• hypothalamus-pituitary-adrenal axis (HPA) (hormone secretion)

ANS control of sympathetic nervous system

dilates pupils (distance vision)

increases heart rate and contractility, blood pressure rises

dilates airways (bronchi, bronchioles)

inhibits digestive activity

closes sphincters

ANS control of parasympathetic nervous system

constricts pupils

slows heart rate and contractility

constricts airways (bronchi, bronchioles)

stimulates digestive activity

opens sphincters

nerve pathways in the ANS

every ANS pathways from the CNS to the effector organ/tissue is a chain of 2 neurons

CNS → preganglionic neuron → ganglion → postganglionic neuron → effector organ

parasympathetic nerve path

pre ganglionic nerves originate from cranial (X-vagus nerve) and sacral (lower spinal cord) regions of spinal cord

pre ganglionic nerves are long

pre ganglionic nerves release ACh and nicotinic receptors

post ganglionic nerves are short

post ganglionic nerves release ACh and muscarinic receptors

sympathetic nerve paths

pre ganglionic nerves originate from thoracic and lumbar regions of spinal cord (T1-L3)

pre ganglionic nerves are short

pre ganglionic nerves release ACh and nicotinic receptors

post ganglionic nerves are long

post ganglionic nerves release Noradrenaline and alpha and beta receptors

ANS NT receptors

all the effects of the ANS are carried out by ACh and noradrenaline (and adrenaline)

neurotransmitters can stimulate or inhibit in the target organs/tissues - the responses are determined by the type of receptors present

acetylcholine: cholinergic receptors

• nicotinic

• muscarinic (effector organs)

noradrenaline/adrenaline: adrenergic receptors

• Alpha

• Beta

adrenergic receptors

found at effector organs in sympathetic system

two major classes alpha and beta

• alpha 1: excitatory e.g. increases contraction of arterioles → raises blood pressure

• alpha 2: inhibitory e.g. decreases smooth muscle contraction in digestive system → reduces GIT motility

• beta 1: excitatory e.g. increases contraction of cardiac muscle → increases rate and force of contractions

• beta: inhibitory e.g. relaxes smooth muscle in respiratory tract → dilation of bronchioles

all are metabotropic

adrenal medulla

adrenal medulla is a modified part of the sympathetic nervous system

• no post ganglionic fibers

• pre ganglionic fibers release ACh which directly stimulates the adrenal medulla

• adrenal medulla secretes NT (hormones) into blood

sympathetic and parasympathetic systems

dual reciprocal innervation

• most organs innervated by both systems

• systems mostly exert opposing effects

• enable rapid and precise control

• enables rapid transitions from rest state to fight or flight

under most circumstances there is partial activation of both systems (tonic activity)

• sympathetic dominates in emergency or stressful situations

• parasympathetic dominates in quiet, relaxed situations

dual reciprocal effects of ANS on various organs

ANS control of heart

sino-atrial (SA) node regulates heart rate

myocytes regulate force of contraction

these are innervates by both sympathetic and parasympathetic NS

both systems are active at rest - your heart rate and the force of contraction are determined by the balance of both systems

exceptions

most arterioles and veins are only innervated by sympathetic nerves

most sweat glands are only innervated by sympathetic nerves

in some cases, other NTs or signaling molecules are involved in the ANS e.g. GABA, serotonin, dopamine