Pharmacology Lecture Two - PDF

Pharmacology Lecture Two - Theoratical The Autonomic Nervous System By Dr. Ahmed Khalid M. Sc Pharmacology  The Autonomic Nervous System the nervous system exerts its influence by the rapid transmission of electrical impulses over nerve fibers that...

Pharmacology Lecture Two - Theoratical The Autonomic Nervous System By Dr. Ahmed Khalid M. Sc Pharmacology  The Autonomic Nervous System the nervous system exerts its influence by the rapid transmission of electrical impulses over nerve fibers that terminate at effector cells, which respond to the release of neuromediators. Drugs that produce their primary therapeutic effect by mimicking or altering the functions of the ANS are called autonomic drugs  The nervous system is divided into two anatomical divisions: 1. central nervous system (CNS), which is composed of the brain and spinal cord, 2. peripheral nervous system, which includes neurons located outside the brain and spinal cord—that is, any nerves that enter or leave the CNS.  The peripheral nervous system is subdivided into the efferent and afferent divisions. The efferent neurons carry signals away from the brain and spinal cord to the peripheral tissues, and the afferent neurons bring information from the periphery to the CNS. Functional divisions within the nervous system The efferent portion of the peripheral nervous system is further divided into : the somatic and the ANS. The somatic efferent neurons are involved in the voluntary control of functions such as contraction of the skeletal muscles essential for locomotion.  The ANS, regulates the everyday requirements of vital bodily functions without the conscious participation of the mind. Because of the involuntary nature of the ANS as well as its functions, it is also known as the visceral or involuntary nervous system  ANS is composed of efferent neurons that innervate : 1. smooth muscle of the viscera, 2. cardiac muscle, vasculature, and the exocrine glands, thereby controlling digestion, 3. cardiac output, blood flow, and glandular secretions  Anatomy of the ANS 1. Efferent neurons: The ANS carries nerve impulses from the CNS to the effector organs through two types of efferent neurons , the preganglionic neurons and the postganglionic neuron. The cell body of the preganglionic neuron, is located within the CNS. The preganglionic neurons emerge from the brainstem or spinal cord and make a synaptic connection in ganglia (an aggregation of nerve cell bodies located in the peripheral nervous system).  The ganglia function as relay stations between the preganglionic neuron and the postganglionic neuron. The cell body of the postganglionic neuron originates in the ganglion. It is generally nonmyelinated and terminates on effector organs, such as visceral smooth muscle, cardiac muscle, and the exocrine glands.  2. Afferent neurons The afferent neurons (fibers) of the ANS are important in the reflex regulation of this system (for example, by sensing pressure in the carotid sinus and aortic arch  3. Sympathetic neurons: The efferent ANS is divided into the sympathetic and the parasympathetic nervous systems, as well as the enteric nervous system. the sympathetic and the parasympathetic neurons originate in the CNS and emerge from two different spinal cord regions. The preganglionic neurons of the sympathetic system come from the thoracic and lumbar regions (T1 to L2) of the spinal cord,  The preganglionic neurons are short in comparison to the postganglionic ones. Axons of the postganglionic neuron extend from these ganglia to the tissues that they innervate and regulate. In most cases, the preganglionic nerve endings of the sympathetic nervous system are highly branched, enabling one preganglionic neuron to interact with many postganglionic neurons. This arrangement enables this division to activate numerous effector organs at the same time.  [Note: The adrenal medulla, like the sympathetic ganglia, receives preganglionic fibers from the sympathetic system. The adrenal medulla, in response to stimulation by the ganglionic neurotransmitter acetylcholine, secretes epinephrine (adrenaline), and lesser amounts of norepinephrine, directly into the blood].  4. Parasympathetic neurons: The parasympathetic preganglionic fibers arise from cranial nerves (oculomotor), (facial), (glossopharyngeal), and (vagus), as well as from the sacral region (S2 to S4) of the spinal cord and synapse in ganglia near or on the effector organs. Thus, in contrast to the sympathetic system, the preganglionic fibers are long, and the postganglionic ones are short, with the ganglia close to or within the organ innervated. In most instances, there is a one-to-one connection between the preganglionic and postganglionic neurons, enabling discrete response of this system.  5. Enteric neurons: The enteric nervous system is the third division of the ANS. It is a collection of nerve fibers that innervate the gastrointestinal (GI) tract, pancreas, and gallbladder, and it constitutes the “brain of the gut.” This system functions independently of the CNS and controls the motility, exocrine and endocrine secretions, and microcirculation of the GI tract. It is modulated by both the sympathetic and parasympathetic nervous systems.