ANS

Brain organisation

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Somatic or visceral sensory nerves
Cranial and spinal nerves



Sensory (afferent) divisionSomatic afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brainVisceral afferent fibers – transmit impulses from visceral organs (in the ventral body cavity) to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs Peripheral Nervous System (PNS): Two Functional Divisions


Somatic nervous system (voluntary)Conscious control of skeletal musclesAutonomic nervous system (ANS) (involuntary)Regulates smooth muscle, cardiac muscle, and glandsDivisions – sympathetic and parasympathetic Motor (efferent) Division: Two Main Parts

Divisions of the ANS

The two divisions of the ANS are the sympathetic and parasympatheticThe sympathetic mobilizes the body during extreme situationsThe parasympathetic performs maintenance activities and conserves body energy The two divisions counterbalance each other’s activity

Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS)
The stability of our internal environment depends on the activity of the ANS (auto=self, nom=govern). The ANS consists of motor neurons that: Innervate smooth and cardiac muscle and glands Make adjustments to ensure optimal support for body activities Operate via subconscious control (involuntary) Have viscera as most of their effectors

Comparison of Somatic and Autonomic Systems

Figure 14.2



ANS Versus Somatic Nervous System (SNS)
The ANS differs from the SNS in the following three areas Effectors, Efferent pathways, Target organ responses Effectors The effectors of the SNS are skeletal muscles The effectors of the ANS are cardiac muscle, smooth muscle, and glands Efferent pathways : Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector Axons of the ANS are a two-neuron chain The preganglionic (first) neuron has a lightly myelinated axon The postganglionic (second) neuron extends to an effector organ(unmyelinated) Neurotransmitter Effects : All somatic motor neurons release Acetylcholine (ACh), which has an excitatory effect In the ANS: Preganglionic fibers release ACh Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory or inhibitory ANS effect on the target organ is dependent upon the neurotransmitter released and the receptor type of the effector

Divisions of the ANS

The two divisions of the ANS are the sympathetic and parasympatheticThe sympathetic mobilizes the body during extreme situationsThe parasympathetic performs maintenance activities and conserves body energy The two divisions counterbalance each other’s activity

Role of the Parasympathetic Division

Concerned with keeping body energy use lowInvolves the D activities – digestion, defecation, and diuresisIts activity is illustrated in a person who relaxes after a mealBlood pressure, heart rate, and respiratory rates are lowGastrointestinal tract activity is highThe skin is warm and the pupils are constricted

Role of the Sympathetic Division

The sympathetic division is the “fight-or-flight” systemInvolves E activities – exercise, excitement, emergency, and embarrassmentPromotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increasedIts activity is illustrated by a person who is threatenedHeart rate increases, and breathing is rapid and deepThe skin is cold and sweaty, and the pupils dilate

Anatomy of ANS

Figure 14.3
Division
Origin of Fibers
Length of Fibers
Location of Ganglia
Sympathetic
Thoracolumbar region of the spinal cord
Short preganglionic and long postganglionic
Close to the spinal cord
Parasympathetic
Brain and sacral spinal cord
Long preganglionic and short postganglionic
In the visceral effector organs

Parasympathetic Division Outflow

Cranial Outflow
Cranial Nerve
Ganglion
Effector Organ(s)
Occulomotor (III)
Ciliary
Eye
Facial (VII)
PterygopalatinSubmandibular
Salivary, nasal, and lacrimal glands
Glossopharyngeal (IX)
Otic
Parotid salivary glands
Vagus (X)
Located within the walls of target organs
Heart, lungs, and most visceral organs
Sacral Outflow
S2-S4
Located within the walls of the target organs
Large intestine, urinary bladder, ureters, and reproductive organs
Midbrain
Medulla
Pons

Sympathetic Outflow

Arises from lateral horn of spinal cord segments T1 through L2 Sympathetic neurons produce the lateral horns of the spinal cord Preganglionic fibers pass through the white rami communicantes and synapse in the chain (paravertebral) ganglia Fibers from T5-L2 form splanchnic nerves and synapse with collateral (prevertebral) ganglia Postganglionic fibers innervate the numerous organs of the body

Sympathetic Trunks and Pathways

Pathways of sympathetic axons with synapses A preganglionic fiber follows one of three pathways upon entering the paravertebral ganglia Synapse with the postganglionic neuron within the same ganglion Ascend or descend the sympathetic chain to synapse in another chain ganglion Pass through the chain ganglion and emerge without synapsing but synapse in collateral ganglia located anterior to spinal column and close to large abdominal arteries and hence their names are celiac (below diaghram), sup mesenteric (upper abdomen) and inferior mesenteric (middle abdomen)

Sympathetic Trunks and Pathways

Figure 14.6
1. Synapse in a chain ganglion at same level. 2. Synapse at a chain ganglion at a different level. 3. Synapse in a collateral ganglion

Pathways with Synapses in Sympathetic Chain Ganglia (paravertebral gangioa)

The white rami carry preganglionic (myelinated) axons to the sympathetic chains. Gray rami carry postganglionic (non myelinated) axons. Postganglionic axons enter the ventral ramus of the spinal nerve via the gray rami communicantes

White ramus

Grey ramus

Pathways with Synapses in Collateral (prevertebral) Ganglia

The preganglionic fibers (T5-L2) leave the sympathetic chain without synapsing They form thoracic, lumbar, and sacral splanchnic nerves They synapse in prevertebral ganglia (celiac, superior mesenteric, inferior mesenteric and hypogastric ganglion)

Pathways with Synapses in the Adrenal Medulla

Fibers of the thoracic splanchnic nerve pass directly to the adrenal medulla Upon stimulation, medullary cells secrete norepinephrine and epinephrine into the blood

Neurotransmitters and Receptors

Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANSACh is released by all preganglionic axons and all parasympathetic postganglionic axonsCholinergic fibers – ACh-releasing fibers Adrenergic fibers – sympathetic postganglionic axons that release NE Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type

Cholinergic Receptors

The two types of receptors that bind ACh are nicotinic and muscarinic These are named after drugs that bind to them and mimic ACh effects Nicotinic receptors are found on: Motor end plates (somatic targets) All preganglionic axons of both sympathetic and parasympathetic divisions The hormone-producing cells of the adrenal medulla The effect of ACh binding to nicotinic receptors is always stimulatory Muscarinic Receptors Muscarinic receptors occur on all effector cells stimulated by postganglionic parasympathetic cholinergic fibers The effect of ACh binding: Can be either inhibitory or excitatory Depends on the receptor type of the target organ

Adrenergic Receptors

The two types of adrenergic receptors are alpha and betaEach type has two or three subclasses (1, 2, 1, 2 , 3)Effects of NE binding to:  receptors is generally stimulatory  receptors is generally inhibitoryA notable exception – NE binding to  receptors of the heart is stimulatoryBeta-blockers – attach mainly to 1 receptors and reduce heart rate and prevent arrhythmias (without interfering with other sympathetic effects)
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Interactions of the Autonomic Divisions

Most visceral organs are innervated by both sympathetic and parasympathetic fibers This results in dynamic antagonisms that precisely control visceral activity Sympathetic fibers increase heart and respiratory rates, and inhibit digestion and elimination Parasympathetic fibers decrease heart and respiratory rates, and allow for digestion and the discarding of wastes