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Peripheral Nervous System (PNS)Dr. Othman Al-ShboulDepartment of Physiology

الرحيم الرحمن الله بسم

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Peripheral Nervous System (PNS)

Composed of:

1. Autonomic nervous system2. Somatic Nervous System

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Peripheral Nervous System

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Composed of:

1. Sympathetic Nervous System2. Parasympathetic Nervous System

Autonomic Nervous System (ANS)

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Autonomic Nerve Pathway

Two-neuron chain

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Sympathetic Nerve Pathway

Sympathetic nerve fibers originate in the thoracic and lumbar regions of the spinal cord

Short preganglionic fibers

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

Long preganglionic fibers

Parasympathetic Nerve Pathway

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Neurotransmitters of the ANS

All preganglionic fibers release Ach

Sympathetic postganglionic fibers release NE/E mainly

Parasympathetic postganglionic fibers release Ach

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Postganglionic nerve terminals

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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

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Dual innervation of sympathetic and parasympathetic nervous systems

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Advantage of dual autonomic innervation

Enables precise control over an organ’s activity

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Sympathetic dominance

Fight-or-flight responsein emergency or stressful situations, such as a physical threat from the outside

o Heart: beats more rapidly and more forcefully, o Blood pressure is elevated by generalized constriction of the blood

vessels, o Respiratory airways open wide to permit maximal air flow, o Glycogen (stored sugar) and fat stores are broken down to release extra

fuel into the blood, o 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.

o The pupils dilate and the eyes adjust for far vision, letting the person visually assess the entire threatening scene.

o 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

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Parasympathetic dominance

Rest-and-digestin quiet, relaxed situations

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The Adrenal Medulla

Cortex

Medulla

Medulla secretes:Epinephrine (mainly) & Norepinephrine

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The Adrenal Medulla

o Part of the sympathetic nervous system

o Receives preganglionic fibers

o Does not have postganglionic fibers

o Instead, releases E & NE into the blood

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Receptor types for autonomic neurotransmitters

Specific neurotransmitter action results from specificity of receptors

2 types of receptors in ANS:o Cholinergico Adrenergic

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Cholinergic Receptors

Nicotinic receptors: found on the postganglionic cell bodies in all autonomic gangliaBind ACh

Muscarinic receptors: found on effector cell membranes (smooth muscle, cardiac muscle, and glands). Bind ACh

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Adrenergic Receptors

Found on effector cell membranes

1. alpha (α):o α1 NE > E o α2

2. beta (β)o β1 NE = Eo Β2 E > NE

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Adrenergic Receptors

o α1 Excitatory; e.g., arteriolar constrictiono α2 Inhibitory; e.g., GI smooth muscleo β1 Excitatory; e.g., increased rate and force of

cardiac contractiono β2 Inhibitory; e.g., arteriolar & bronchiolar (dilation)

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Autonomic agonists and antagonists AgonistBinds with a neurotransmitter’s receptor and elicits an effect that mimics that of the neurotransmitter.

AntagonistBinds with a neurotransmitter’s receptor and blocks the neurotransmitter’s response.

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Atropine

o Blocks the effect of acetylcholine at muscarinic receptors but does not affect nicotinic receptors

o Suppresses salivary and bronchial secretions before surgery, to reduce the risk of a patient inhaling these secretions into the lungs

Autonomic agonists and antagonists

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Salbutamol

o Selectively activates β 2 adrenergic receptors at low doses

o Dilate the bronchioles in the treatment of asthma without undesirably stimulating the heart

Autonomic agonists and antagonists

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Sympathetic vs. Parasympathetic

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Somatic Nervous System

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Somatic Nervous System

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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)

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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.

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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

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Neuromuscular junction (NMJ)

Axon of motor neuron

Synaptic vesicles

Terminal button

motor end plate

Axon of motor neuron

Neuromuscular junction

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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.)

Neuromuscular junction (NMJ)

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Somatic vs. Autonomic

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Somatic vs. Autonomic

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THE END