KINE 301 Chapter 11

Describe the process involved in the release of ACh, and the process by which the postsynaptic membrane depolarizes.

1. Release of ACh: action potential arrive at axon terminal, causes voltage-gated Ca2+ channel to open, Ca2+ entry causes synaptic vesicles to fuse with presynaptic membrane, Ach is released into synaptic cleft 2. Postsynaptic membrane depolarization: ACh binds to nicotinic receptor, opens cation channel, net entry of Na+ into muscle fiber depolarizes fiber. ACh in synapse is broken down by AChE.

What is a bouton? What is the neuromuscular junction? What is the motor end plate?

A bouton is a button-like swelling on an axon where it has a synapse with another axon (enlarged axon terminal). A neuromuscular junction is the synapse of a somatic motor neuron on a muscle fiber. On the postsynaptic side of this junction, the muscle cell membrane that lies opposite the axon terminal is modified into a motor end plate, a series of folds with nicotinic ACh receptor channels clustered together in an active zone.

Briefly describe a1, a2, b1, and b2, adrenergic receptors (in terms of sensitivity to catecholamines, location, and main target tissues.

Alpha 1 receptors are found in most sympathetic target tissues, are more sensitive to norepinephrine than epinephrine, and cause muscle contraction or secretion by exocytosis. Alpha 2 receptors are found in the GI tract and pancreas, are more sensitive to norepinephrine than epinephrine, and cause smooth muscle relation or decreased secretion. Beta 1 receptors are found in heart muscle and kidney, are equally sensitive to norepinephrine and epinephrine, and enhance cardiac muscle contraction. Beta 2 receptors are found in certain blood vessels and smooth muscle of some organs, are more sensitive to epinephrine than norepinephrine, and relax smooth muscle in many tissues.

Describe the general structure or pathway of neurons for the efferent side of the autonomic nervous system. What is a ganglion?

Autonomic pathways have two efferent neurons in a series. The first is called the preganglionic neuron, which originates in the CNS and extends to an autonomic ganglion outside the CNS. There, the preganglionic neuron synapses with the second neuron in the pathway, the postganglionic neuron, which projects its axon to the target tissue.

What structure ensures that the axon terminal remains adjacent to the motor end plate?

Between the axon and muscle, the synaptic cleft is filled with a fibrous matrix whose collagen fibers hold the terminal and motor end plate in proper alignment.

What is the difference (in terms of location) between the cholinergic nicotinic receptors and the cholinergic muscarinic receptors?

Cholinergic nicotinic receptors are on postganglionic cells, while muscarinic cholinergic receptors are on the target cells.

Adrenergic receptors for catecholamines and cholinergic muscarinic receptors are coupled to --on the inside of the cell membrane.

G proteins

What are the roles of monoamine oxidase and acetylcholinesterase?

MAO is the main enzyme responsible for deflation of catecholamines. AChE is the enzyme that rapidly deactivates ACh by degrading it into acetyl and choline.

Explain the location of origin for the sympathetic and parasympathetic neurons?

Most sympathetic pathways originate in the thoracic and lumbar regions of the spinal cord. These ganglia are found in to ganglion chains that run along either side of the bony vertebral column, with additional ganglia along the descending aorta. Parasympathetic pathways originate in the brain or sacral regions. These ganglia are located on or near their target organs.

Describe the general pathway of a somatic motor neuron. Where do they originate? What is the upper limit of their length?

Somatic motor pathways have a single neuron that originates in the CNS and projects its axons to the target tissue, which is always a skeletal muscle. These pathways are always excitatory. The cell bodies of somatic motor neurons are located either in the ventral horn of the spinal cord or in the brain. They may be a meter or longer in length.

What are the two divisions of the efferent side of the peripheral nervous system?

Somatic, autonomic

What are the divisions of the autonomic nervous system? How do these divisions differ from one another?

Sympathetic Parasympathetic The two divisions can be distinguished by observing the situations in which they are active. The parasympathetic branch is dominant during routine, quiet activities of day-to-day living. The sympathetic branch dominates in stressful situations.

How is the release of the neurotransmitter from varicosities less directed than the communication which occurs between a somatic motor neuron and a skeletal muscle?

Target cell membranes do not possess clusters of neurotransmitter receptors in specific sites. Neurotransmitter is simply released into the interstitial fluid to diffuse to wherever the receptors are located; a single postganglionic neuron can effect a large area of target tissue.

Describe the adrenal medulla, and its contribution to the sympathetic branch of the autonomic nervous system.

The adrenal medulla is specialized neuroendocrine tissue that primarily secretes epinephrine. In response to alarm signals from the CNS, the medulla releases large amounts of epinephrine for general distribution throughout the body as part of a fight-or-flight response. It also forms the core of the adrenal glands.

What is the neuroeffector junction? What is a varicosity?

The neuroeffector junction is the synapse between a postganglionic autonomic neuron and its target cell. Varicosities are swollen regions along autonomic axons that store and release neurotransmitter.

In addition to the neurohormonal activity of the autonomic nervous system, neurotransmitters can mediate a response. Name the two main neurotransmitters of the ANS, and describe which is released at each neuron (preganglionic and postganglionic) of the sympathetic and parasympathetic pathways respectively.

The primary autonomic neurotransmitters are acetylcholine and norepinephrine. Sympathetic pathways use acetylcholine and norepinephrine. Both sympathetic and parasympathetic preganglionic neurons release ACh. Postganglionic sympathetic neurons release NE; postganglionic parasympathetic neurons secrete ACh.

Give two exceptions to the general rule that sympathetic and parasympathetic divisions exhibit antagonistic control. Give one well-studied example of how receptors mediate different responses in the same type of tissue.

Two exceptions to dual antagonistic innervation include the sweat glands and the smooth muscle in most blood vessels (tonic control). An example is adrenergic receptors in blood vessels. Most blood vessels contain one receptors for smooth muscle vasoconstriction; however, some blood vessels contain two types of receptors, one for vasoconstriction and one for vasodilation.

Explain denervation hypersensitivity of the sympathetic system.

When sympathetic neurons degenerate, target tissues up-regulate by putting more receptors into the cell membrane to maximize the cell's response to available norepinephrine. The increase in receptor abundance leads to denervation hypersensitivity, a state in which the administration of exogenous adrenergic agonists causes a greater-than expected response.

What is the consequence of a lack of communication between a motor neuron and the muscle cells it innervates?

Without communication between the motor neuron and the muscle, the skeletal muscles for movement and posture weaken, as do the skeletal muscles for breathing. This loss of respiratory function could potentially be fatal unless the patient is placed on artificial ventilation. ex. Myasthenia gravis

Many cases of sympathetic dysfunction are manifested most strongly in the

cardiovascular system.

An exception to the answer in the previous question is described when -- neurons, such as those that terminate on sweat glands, secrete ACh instead of norepinephrine.

sympathetic cholinergic

The autonomic division is often called the -- due to its control over the function of internal organs.

visceral nervous system