Chapter 11 - Autonomic and Motor Nervous System

Autonomic Nervous System
     Innervates all effector organs and tissues except for skeletal muscles. It is autonomic because it functions subconsciously and involuntarily
Dual Innervation
     There are two branches or divisions of the autonomic nervous system (ANS): the sympathetic and parasympathetic. Both branches innervate most organs in an arrangement called dual innervation. The parasympathetic division is most active during rest and stimulates digestive activities. The sympathetic division is most active during times of excitement and physical activity. Animation Comparing Sympathetic and Parasympathetic Nervous Systems
     The two divisions of the ANS work together to finely control the functions of the various organs so that they operate appropriately in different situations. The two systems often perform this control by working at cross purposes. So, for example, to precisely control the heart rate the sympathetic division will increase it while the parasympathetic decreases it.
Anatomy of the ANS
     The commands of the ANS leave the central nervous system and go to effector organs by means of two efferent neurons arranged in series. The first neuron (preganglionic neuron) synapses with the second neuron (postganglionic neuron) at an autonomic ganglion.
  Anatomy of the Sympathetic Nervous System
          Preganglionic neurons of the sympathetic nervous system originate in the region of gray matter in the thoracic and upper lumbar region called the lateral horn. Pre- and postganglionic neurons are arranged in three patterns:
1. The preganglionic neuron leaves through the ventral root. After the ventral and dorsal roots fuse to form the spinal nerve the preganglionic neuron goes to sympathetic ganglia that are connected to one another and run parallel to the spinal column on either side. The chain of ganglia is the sympathetic chain or sympathetic trunk.
   The preganglionic neuron is myelinated and the axons of these form the white ramus (pl. rami) or "white branch" as they connect to the ganglia. The postganglionic neurons are unmyelinated and leave the ganglion as the gray ramus which rejoins the spinal nerves.
   Because the axons of the preganglionic neurons may branch and travel up and down the sympathetic chain, a single preganglionic neuron can synapse with many postganglionic neurons up and down the sympathetic chain. Hence, the effects of stimulation are widespread. 
2. A group of long preganglionic neurons innervate modified postganglionic cells in the adrenal medulla called chromaffin cells. These cells release epinephrine (80%), norepinephrine (20%) and a small amount of dopamine into the bloodstream in response to stimulation. The effects of these hormones spread by the bloodstream are widespread. 
3. The preganglionic neurons synapse with postganglionic neurons in collateral ganglia. The preganglionic neurons leave the spinal nerve through the white rami but do not synapse with neurons in the sympathetic chain but continue to the collateral ganglia.
   The postganglionic neurons originating from the collateral ganglia travel to specific effector organs. For example, postganglionic neurons originating from the celiac ganglion innervate some of the digestive organs. Hence, the effects of stimulation by this route are more localized and discrete. 
  Anatomy of the Parasympathetic Nervous System
     The preganglionic neurons originate in the brain stem or sacral spinal cord and are relatively long. The preganglionic neurons synapse with postganglionic neurons in ganglia near the effector organ or in the wall of the effector organ.
     Cranial nerve nuclei are in the brainstem and travel with  cranial nerves III, VII, IX and X. The X is called the vagus and innervates much of the viscera including lung, heart, stomach, small intestines and liver.
     The parasympathetic neurons that originate from the sacral spinal cord join to form distinct pelvic nerves.
Autonomic Neurotransmitters
     The peripheral nervous system uses two neurotransmitters:
1. Acetylcholine is the most common. Neurons that release it are called cholinergic. Cholinergic neurons include all preganglionic neurons of the autonomic nervous system, postganglionic neurons of the parasympathetic nervous system and some postganglionic neurons (sweat glands) of the sympathetic nervous system.
2. Norepinephrine is the other neurotransmitter and is released by neurons called adrenergic. Almost all sympathetic postganglionic neurons are adrenergic.
  Cholinergic Receptors
   There are two classes:
1. Nicotinic receptors are found on cell bodies and dendrites of sympathetic and parasympathetic neurons, on chromaffin cells of the adrenal medulla and on skeletal muscle cells. These receptors are associated with cation channels that allow both potassium and sodium ions to pass through. These receptors are associated with depolarization of the postsynaptic cells.
2. Muscarinic receptors are found on effector organs of the parasympathetic nervous system. These receptors are coupled to G proteins and may either be inhibitory or excitatory. Effector organs acted upon include heart and smooth muscles of the pupil and digestive tract.
  Adrenergic Receptors (Table 11.1)
    There are two classes of receptors a and b and each of these is divided into subclasses (a1 and  a2) and (b1, b2 and b3 ). These receptors are coupled to G proteins that activate or inhibit second messenger systems.
     Epinephrine or norepinephrine binds to a1  receptors activating a G protein that activates phospholipase C which catalyzes conversion of phosphatidyl inositol biphosphate (PIP2) to: inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 triggers release of Ca++ and DAG activates protein kinase C
     Norepinephrine or epinephrine binds to a2 receptors and activates an inhibitory G protein (Gi) that decreases activity of adenylate cyclase, decreasing synthesis of cAMP.
     The binding of norepinephrine or epinephrine to b receptors activates stimulatory G  proteins (Gs) that increase the activity of adenylate cyclase, increasing synthesis of cAMP. YouTube Video
     a receptors tend to be excitatory. b1 and b3 are generally excitatory. b2 have much more affinity for epinephrine and produce an inhibitory response. (N. B. Whether a G protein is stimulatory or inhibitory does not predict the cellular response. The cellular response, such as muscle contraction or glandular secretion, may be either promoted or inhibited.) 
Autonomic Neuroeffector Junctions
     Synapses between an efferent neuron and its effector organ is a neuroeffector junction. In contrast to typical axon terminals, neurotransmitters are released at swellings along the axon called varicosities. When action potentials reach these varicosities they are propagated by voltage-gated Na+ and K+ channels but in addition have voltage-gated Ca++ channels. 
     Action potentials that reach the varicosities open up Ca++ channels and Ca++ rushes into the cell triggering the release of neurotransmitter. Transmitter is released at all the varicosities so its release is more widespread than at typical axon terminals. 
Regulation of Autonomic Functions
     The sympathetic and parasympathetic divisions of the autonomic nervous system often work in opposition in order to maintain homeostasis. Hence the competing influences of the sympathetic and parasympathetic divisions need to be balanced or regulated to achieve homeostasis. 
     Most changes in the organ activity are controlled by visceral reflexes. These reflexes include the pupillary light reflex, accommodation, vomiting reflex, swallowing reflex, urination, defecation, erection and ejaculation. Higher centers that control autonomic function include the hypothalamus, pons and medulla oblongata
     The hypothalamus exerts an overriding influence on autonomic functions. It initiates the flight-or-fight response by activating the sympathetic branch which has immediate widespread effects. The hypothalamus contains centers that regulate body temperature, food intake and water balance. 
     The hypothalamus, in turn, receives input from the cerebral cortex and the limbic system which is concerned with the experience of emotions. 
Anatomy of Somatic Nervous System
      Somatic motor neurons originate in the ventral horn of the spinal cord. A single motor neuron innervates many skeletal muscle fibers. A motor neuron plus all the fibers innervated is a motor unit.
Neuromuscular Junction 
     The neuromuscular junction is the region where the motor neuron synapses with a skeletal motor fiber. The axon terminals of the motor neuron are called terminal boutons. Acetylcholine is stored and released here. The plasma membrane opposite the terminal bouton is called the motor end plate. The motor end plate is invaginated and contains nicotinic cholinergic receptors. Acetylcholinesterase is found in the synaptic cleft and terminates the excitatory signal.
     When an action potential arrives at the terminal bouton the depolarization causes Ca++ channels to open and Ca++ enters the cell. This triggers the release of acetylcholine by exocytosis. Acetylcholine binds to nicotinic cholinergic receptors at the motor end-plate opening cation channels. Na+ flows into the muscle cell and causes an end plate potential that is sufficient to trigger an action potential. The action potential leads to contraction of the fiber.