Chapter 14 Autonomic Nervous System Nerve Cells of the Enteric Plexus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

SNS vs. ANS Somatic Nervous System (SNS) Cell bodies of somatic neurons are located in CNS axons extend to skeletal muscles --have an excitatory effect Usually controlled consciously Autonomic Nervous System (ANS) Cell bodies of preganglionic neurons are located in CNS axons extend to ganglia where they synapse with postganglionic neurons The postganglionic axons can have an excitatory or inhibitory effect on Smooth muscle Cardiac muscle Glands Usually controlled unconsciously

Fig. 14.1

Tab. 14.1

Anatomy of the Autonomic Nervous System Subdivided into Sympathetic division Parasympathetic division Enteric nervous system Sympathetic and Parasympathetic divisions differ structurally in Location of preganglionic neuron cell bodies within CNS Location of autonomic ganglia

The Sympathetic Division Preganglionic cell bodies are in lateral horns of the spinal cord gray matter from T1 to L2 Preganglionic axons pass through ventral roots to white rami communicates to sympathetic chain ganglia. From there, four courses are possible Fig. 14.2

The Sympathetic Division 1) Preganglionic axons synapse (at same or a different level) with postganglionic neurons, which exit ganglia through gray rami communicates and enter spinal nerves 2) Preganglionic axons synapse (at same or a different level) with postganglionic neurons, which exit ganglia through sympathetic nerves Fig. 14.3

The Sympathetic Division 3) Preganglionic axons pass through chain ganglia without synapsing to form splanchnic nerves. Preganglionic axons then synapse with postganglionic neurons in collateral ganglia 4) Preganglionic axons synapse with cells of adrenal medulla Fig. 14.3

The Parasympathetic Division Preganglionic cell bodies are in nuclei in brainstem or lateral parts of spinal cord gray matter from S2 to S4 Preganglionic axons from brain pass to ganglia through cranial nerves Preganglionic axons from sacral region pass through pelvic splanchnic nerves to ganglia Preganglionic axons pass to terminal ganglia within wall of, or near, organ that is innervated Fig. 14.4

Tab. 14.2

Enteric Nervous System Consists of nerve plexuses within wall of the digestive tract Has contributions from 3 sources: Sensory neurons that connect digestive tract to the CNS ANS motor neurons that connect CNS to digestive tract Enteric neurons, which are confined to enteric plexuses

Distribution of Autonomic Nerve Fibers Sympathetic, parasympathetic and sensory neurons intermingle in autonomic nerve plexuses Sympathetic axons reach organs through spinal, sympathetic, and splanchnic nerves Parasympathetic axons reach organs through cranial and pelvic splanchnic nerves Sensory neurons run alongside sympathetic and parasympathetic neurons within nerves and nerve plexuses

Fig. 14.5

Neurotransmitters of the ANS Acetylcholine is released by cholinergic neurons All preganglionic neurons All parasympathetic postganglionic neurons Some sympathetic postganglionic neurons Norepinephrine (or epinephrine) is released by adrenergic neurons Most sympathetic postganglionic neurons

Receptors of the ANS Acetylcholine binds to Nicotinic receptors Found in all postganglionic neurons Excitatory Muscarinic receptors Found in all parasympathetic effector organs Some sympathetic effector organs Excitatory or inhibitory Norepinephrine and epinephrine bind to Alpha and beta receptors Found in most sympathetic effector organs

Fig. 14.6

Tab. 14.3

Regulation of the ANS Autonomic reflexes control most of activity of visceral organs, glands, and blood vessels Autonomic reflex activity can be influenced by hypothalamus and higher brain centers The sympathetic and parasympathetic divisions can influence activities of enteric nervous system through autonomic reflexes The enteric nervous system can function independently of CNS through local reflexes

Fig. 14.7

Functional Generalizations About the ANS Both divisions of the ANS produce stimulatory and inhibitory effects Most organs are innervated by both divisions Usually, each division produces an opposite effect on a given organ Either division alone or both working together can coordinate the activities of different structures The sympathetic division produces more generalized effects than the parasympathetic division Sympathetic activity generally prepares the body for physical activity Parasympathetic activity is more important for resting conditions