Autonomic nervous system
- influences smooth muscle, glands and the heart
- 2 subdivisions: SYMPATHETIC and PARASYMPATHETIC
- regulates organs to maintain homeostasis
- regulates stress response, reproduction, thermoregulation & enteric system (digestion)
Preganglionic neuron
synapses with postganglionic neuron within a ganglion
–> a cluster of neuronal bodies outside the CNS
Postganglionic neurons
innervates the effector organ
Parasympathetic N.S.
preganglionic neurons originate in the brainstem or sacral spinal cord
- long preganglionic neurons to terminal ganglia near the effector organ
- short postganglionic neurons from ganglia to effector organs
Neurotransmitters of the ANS
Preganglionic neurons: release ACh for both sympathetic and parasympathetic
***means they both have receptors for ACh
Postganglionic neurons:
- Sympathetic postganglionic neurons: norepinephrine (adrenergic fibres)
- Parasympathetic postganglionic neurons: ACh (cholinergic fibres)
Adrenal medulla 80% epinephrine and 20% norepinephrine (these move through blood so widespread effect)
Receptor types
Neurotransmitters can either:
- stimulate activity in some tissues
- inhibit activity in other tissues
Response depends upon specialization of the tissue (effector organ) cells
–> have one or more of different types of receptor proteins (and within other receptors)
Nicotinic cholinergic receptors
- activated by nicotine
- found on postganglionic cells in all autonomic ganglia
- respond to release of ACh from both sympathetic and parasympathetic preganglionic fibres
- channel opening results in depolarization
- effect depends on target
Two types: N1 & N2
Muscarinic cholinergic receptors
- activated by mushroom poison, muscarine
- found on effector cell membranes (smooth muscle, cardiac muscle and glands) –> found on effector cell after postsympathetic neuron
- bind ACh released by parasympathetic postganglionic fibres
- 5 subtypes: M1-M5
Adrenergic receptors
- bind epinephrine & norepinephrine
- two classes: Alpha (a1-2) & Beta (b1-b3)
Alpha adrenergic receptors
- located in effector organs of sympathetic nervous system
- most common
usually excitatory and stimulate smooth muscle or glands - affinity greater for norepinephrine>epinephrine
a1 receptors
- excitatory response
e. g. arteriolar constriction –> reduces blood flow to an area - when you get into fight or flight, going to inhibit certain activities such as muscle contractions for stomach/blood flow for digestion
a2 receptors
- inhibitory response in effector organ
e. g. decreased smooth muscle contraction in digestive tract
B1 adrenergic receptors
- located in cardiac muscle and kidneys
- usually excitatory
- equal affinity for norepinephrine and epinephrine
B2 adrenergic receptors
- located in some blood vessels and smooth muscle
- usually inhibitory
- greater affinity epinephrine>norepinephrine
B3 adrenergic receptors
- not very common
- located in adipose tissue (breakdown of fat molecules) –> energy/produce ATP important for flight or fight
- usually excitatory
- equal affinity for norepinephrine and epinephrine
