Peripheral Nervous System (PNS) بسم الله الرحمن الرحيم Peripheral Nervous System (PNS) Dr. Othman Al-Shboul Department of Physiology

Peripheral Nervous System (PNS) Composed of: Autonomic nervous system Somatic Nervous System

Peripheral Nervous System

Autonomic Nervous System (ANS) Composed of: Sympathetic Nervous System Parasympathetic Nervous System

Autonomic Nerve Pathway Two-neuron chain

Sympathetic Nerve Pathway Sympathetic nerve fibers originate in the thoracic and lumbar regions of the spinal cord Short preganglionic fibers sympathetic preganglionic fibers are very short, synapsing with cell bodies of postganglionic neurons within ganglia that lie in a sympathetic ganglion chain (also called the sympathetic trunk) located along either side of the spinal cord. Long postganglionic fibers originating in the ganglion chain end on the effector organs. Some preganglionic fibers pass through the ganglion chain without synapsing. Instead, they end later in sympathetic collateral ganglia about halfway between the CNS and the innervated organs, with postganglionic fibers traveling the rest of the distance

Parasympathetic Nerve Pathway Parasympathetic preganglionic fibers arise from the cranial (brain) and sacral (lower spinal cord) areas of the CNS Long preganglionic fibers

Neurotransmitters of the ANS All preganglionic fibers release Ach Sympathetic postganglionic fibers release NE/E mainly Parasympathetic postganglionic fibers release Ach

Postganglionic nerve terminals

Dual innervation of sympathetic and parasympathetic nervous systems Sympathetic and parasympathetic nervous systems generally exert opposite effects in a particular organ Both systems increase the activity of some organs and reduce the activity of others. Usually both systems are partially active; sympathetic or parasympathetic tone or tonic activity, but activity of one division can dominate the other

Dual innervation of sympathetic and parasympathetic nervous systems

Advantage of dual autonomic innervation Enables precise control over an organ’s activity like having both an accelerator and a brake to control the speed of a car. If an animal suddenly darts across the road as you are driving, you could eventually stop if you just took your foot off the accelerator, but you might stop too slowly to avoid hitting the animal. If you simultaneously apply the brake as you lift up on the accelerator, however, you can come to a more rapid, controlled stop.

Sympathetic dominance Fight-or-flight response in emergency or stressful situations, such as a physical threat from the outside Heart: beats more rapidly and more forcefully, Blood pressure is elevated by generalized constriction of the blood vessels, Respiratory airways open wide to permit maximal air flow, Glycogen (stored sugar) and fat stores are broken down to release extra fuel into the blood, Blood vessels supplying skeletal muscles dilate (open more widely). Providing increased flow of oxygenated, nutrient-rich blood to the skeletal muscles in anticipation of strenuous physical activity. The pupils dilate and the eyes adjust for far vision, letting the person visually assess the entire threatening scene. Sweating is promoted in anticipation of excess heat production by the physical exertion. Inhibition of digestive and urinary activities; not essential in meeting the threat

Parasympathetic dominance Rest-and-digest in quiet, relaxed situations

The Adrenal Medulla Cortex Medulla Medulla secretes: Epinephrine (mainly) & Norepinephrine

The Adrenal Medulla Part of the sympathetic nervous system Receives preganglionic fibers Does not have postganglionic fibers Instead, releases E & NE into the blood

Receptor types for autonomic neurotransmitters Specific neurotransmitter action results from specificity of receptors 2 types of receptors in ANS: Cholinergic Adrenergic

Cholinergic Receptors Nicotinic receptors: found on the postganglionic cell bodies in all autonomic ganglia Bind ACh Muscarinic receptors: found on effector cell membranes (smooth muscle, cardiac muscle, and glands).

Adrenergic Receptors Found on effector cell membranes alpha (α): α1 NE > E α2 beta (β) β1 NE = E Β2  E > NE

Adrenergic Receptors α1  Excitatory; e.g., arteriolar constriction α2  Inhibitory; e.g., GI smooth muscle β1  Excitatory; e.g., increased rate and force of cardiac contraction β2  Inhibitory; e.g., arteriolar & bronchiolar (dilation)

Autonomic agonists and antagonists Binds with a neurotransmitter’s receptor and elicits an effect that mimics that of the neurotransmitter. Antagonist Binds with a neurotransmitter’s receptor and blocks the neurotransmitter’s response.

Autonomic agonists and antagonists Atropine Blocks the effect of acetylcholine at muscarinic receptors but does not affect nicotinic receptors Suppresses salivary and bronchial secretions before surgery, to reduce the risk of a patient inhaling these secretions into the lungs

Autonomic agonists and antagonists Salbutamol Selectively activates β 2 adrenergic receptors at low doses Dilate the bronchioles in the treatment of asthma without undesirably stimulating the heart

Sympathetic vs. Parasympathetic

Somatic Nervous System

Somatic Nervous System

Somatic Nervous System Composed of the axons of the motor neurons which innervate skeletal muscles Motor neurons: The cell bodies are within the spinal cord. (except motor neurons supplying muscles in the head, they are in the brain stem) Release acetylcholine (ACh) Can only stimulate skeletal muscles (no inhibition) Inhibition of skeletal muscle activity can be accomplished only within the CNS through inhibitory synaptic input to the dendrites and cell bodies of the motor neurons supplying that particular muscle.

Motor neurons The dendrites and cell bodies are influenced by many converging presynaptic inputs, both excitatory (EPSP) and inhibitory (IPSP). The level of activity in a motor neuron and its subsequent output to the skeletal muscle fibers it innervates depend on the relative balance of EPSPs and IPSPs Motor neurons are considered the final common pathway, because the only way any other parts of the nervous system can influence skeletal muscle activity is by acting on these motor neurons.

Somatic Nervous System The somatic system is under voluntary control However, much of the skeletal muscle activity involving posture, balance, and stereotypical movements is subconsciously controlled

Neuromuscular junction (NMJ) Axon of motor neuron Synaptic vesicles Axon of motor neuron Terminal button Neuromuscular junction motor end plate

Neuromuscular junction (NMJ) Nerve and muscle cells do not actually come into direct contact at a neuromuscular junction The space, or cleft, between these two structures is too large to permit electrical transmission of an impulse between them The chemical messenger is acetylcholine (ACh.)

Somatic vs. Autonomic

Somatic vs. Autonomic

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