Chapter 16

Anatomy & Physiology

Fifth Edition

Seeley/Stephens/Tate

(c) The McGraw-Hill Companies, Inc.

The Autonomic System ---simplified

• The peripheral nervous system consist of:– Afferent neurons: from the periphery to CNS,

Reflex loop (sensory - ascending)– Efferent neurons: from the CNS to periphery,

Somatic motors – skeletal muscles

ANS – smooth muscles, cardiac muscle and glands

(motor – descending)• Innervation of these two systems differ:

– Afferent neurons connect directly to CNS through synapses.

• Efferent neurons of somatic division connect directly from the CNS to the skeletal muscles.

• Efferent neurons of ANS, which have come out from the spinal cord goes through autonomic ganglions.– Preganglionic neuron (myelinated)--- autonomic ganglion ---

postganglionic neuron (unmyelinated)

• There are two types of ANS; sympathetic division and parasympathetic division.– The sympathetic division: preganglionic fibers from the T and

L regions of the spinal cord make ganglia at the synaptic chain ganglia. They usually stimulate tissue metabolism, increases alertness, get ready for emergencies.

– The parasympathetic division: they originate from the brainstem and the S region of the spinal cord. Conserve energies and sedentary. Also digestion.

• The neuro transmitters of ANS• Recall, the neurotransmitters for the afferent neurons

and somatic efferent neurons is Ach.• In ANS both Ach and norepinephrine (NE), are used.

– Cholinergic neurons use Ach..– Adrenergic neurons use NE.

– In the sympathetic division:• Preganglionic neurons are cholinergic

• Most of the post ganglionic neurons are adrenergic

• While the postganglionic sweat glands, thermoregulaters are cholinergic.

• In the parasympathetic division

– Both preganglionic and postganglionic neurons are cholingeric (nicotinic or muscarinic), they generally excitatory (sm. muscles), but can be inhibitory (heart).

– There are other neurotransmitters of ANS, such as, fatty acids like prostaglandins and peptides such as, gastrin, somatostatin, dopamine, etc…

• Receptors of ANS

– There are two types:

1. Cholinergic receptors: ( two subtypes)• Nicotinic receptors for nicotine (alkaloid from tobacco).

– Found in postganglionic neurons in the autonomic neurons and skeletal muscles.

• Muscrininc receptors for muscarine ( from mushrooms)– Found in the effector cell responding to Ach.

• Ach on the nicotinic receptor opens Na+ channels ( excitatory)

• Ach on the muscarinic receptors modulate G-proteins and the response could be either excitatory or inhibitory. Examples,– Ach + muscarine receptor of cardiac muscle is

inhibitory.– Ach + muscarine receptor of smooth muscle is

excitatory.

2. Adrenergic receptors– NE is released from the adrenergic postganglionic neurons of the

sympathetic division ---- to adrenergic receptors of the effector cells.

– Two types of adrenergic receptors could be found in the same cell, alpha receptors and beta receptors.

– The action of NE on the adrenergic receptors;• modulates G-proteins.• The response could be either excitatory or inhibitory depending

on the tissues. For example:– NE + beta receptor of cardiac muscles is excitatory.– NE + beta receptor of the stomach smooth muscle is inhibitory.

– Both epinephrine and NE can bind to alpha and beta receptors.– Epinephrine binds to alpha or beta receptors equally.– NE binds to alpha more strongly than beta.

• Dual innervations of organs with sympathetic and parasympathetic divisions with opposing or cooperative effects.

• The sympathetic division has a more general effect than the parasympathetic division, because epinephrine and norepinephrine are released from the adrenal medulla and circulates through the circulatory system.

• Review clinical focus.

The End.