Autonomic &Somatic Nervous Systems.

By Syed Abdul Naveed.

M.Pharm (Pharmacology).

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which consists of

is divided into

that make up

which is divided into

The Nervous System

Sensory nervesMotor nerves

Autonomic nervous system

Somatic nervous system

Central nervous system

Peripheral nervous system

Sympathetic nervous system

Parasympathetic nervous system

Enteric nervous system

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Autonomic nervous system

• The autonomic nervous system is the subdivision of the peripheral nervous system that regulates body activities that are generally not under conscious control (or) responsible for control of involuntary or visceral body functions.

• The A.N.S composed of efferent neurons that innervates smooth muscle of the viscera, cardiac muscle ,vasculators & the exocrine gland ,cardiac output, blood flow & glandular secretion.

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Autonomic nervous systemDivisions of the autonomic nervous system:• Parasympathetic division• Sympathetic divisionServe most of the same organs but cause opposing or antagonistic

effects.

• Parasysmpathetic: routine maintenance “rest &digest”.

• Sympathetic: mobilization & increased metabolism “fight, flight or fright”.

• Efferent (Motor) Nerve : Carry impulses from cns (brain and spinal cord) to muscle or organ (peripheral tissues).

• Afferent (Sensory)Nerve :Neuron which transmits impulses from sense organs (bring information from periphery ) to cns

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Introduction (efferent neurons)

• Efferent (motor)neuron carrier nerve impulse from CNS to the effector organ by way of two types of efferant neurons.

• First nerve is called Preganglionic neuron

and its cell body is located in the CNS.

• Preganglionic neurons emerge in the brain stem or spinal cord and makes a synaptic connection in the ganglia.

• The ganglia function as a relay station between a preganglionic neuron and second nerve cell (postganglionic neuron)

• The postganglionic neuron has a cell body originating in the ganglia and terminates on effector organ.

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Introduction to somatic motor neurons

• Somatic efferent neurons are involved in the voluntary contol of function such as contraction of the skeletal muscle.

• Input from sense organs & out put to skeletal muscle.

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Autonomic and Somatic Motor Systems

Autonomic motor systemChain of two motor neurons

•Preganglionic neuron•Postganglionic neuron

Conduction is slower due to thinly or unmyelinated axons

Somatic motor systemOne motor neuron extends from the CNS to skeletal muscleAxons are well myelinated, conduct impulses rapidly

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Sympathetic Nervous system(fight, flight or fright)

• Also called thoracolumbar system: all its neurons are in lateral horn of gray matter from T1-L2

• Preganglionic neuron of the sympathetic system comes from thoracic and lumbar region of the spinal cord and they synapse in the two cord like chain of ganglia that run parallel on each side of the spinal cord.

• Preganglion neurons are short ,. Postganglion neuros are long

• Axons of the postganglionic neuron extended from these ganglion to the tissue that they innervate and regulate.

• The adrenal medulla (large synaptic ganglion receives preganglion fiber from the sympathetic system.

• The adrenal medulla in response to stimulation secreates epinephrine also know as adrenaline, and lesser amount of norepinephrine in to the blood.

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Ach

Ach

Ach

Ach NE

AchEPI/NE

Ach Ach

Somatic

Sympathetic

Sympathetic

Sympathetic

Para-sympathetic

Postganglionic Fiber;Adrenergic

Adrenal Gland

Motor Fiber

Sweat Glands

Smooth MuscleCardiac CellsGland Cells

Smooth MuscleCardiac CellsGland Cells

SkeletalMuscle

Ganglion

Ganglion

Ganglion

Neurotransmitter released by preganglionic axons•Acetylcholine for both branches (cholinergic)

Neurotransmitter released by postganglionic axons•Sympathetic – most release norepinephrine (adrenergic)•Parasympathetic – release acetylcholine 9

Role of the Sympathetic Division

• The sympathetic division is the “fight-or-flight” system

• Involves E activities – exercise, excitement, emergency, and embarrassment

• Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is increased

• Its activity is illustrated by a person who is threatened

– Heart rate increases, and breathing is rapid and deep

– The skin is cold and sweaty, and the pupils dilate

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Sympathetic Division of the ANS

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The Role of the Adrenal Medulla in the Sympathetic Division

• Adrenalin (also called epinephrine) is produced in the medulla of the adrenal glands.

•The adrenal glands are located on the top of each kidney.

•The sympathetic nervous system stimulates the adrenal gland (an effector) to release the hormone adrenalin or epinephrine into the bloodstream.

•Adrenalin is a modified amino acid hormone. The target tissue for adrenalin is mainly cardiac and skeletal muscle.

•Adrenalin increases heart rate and blood pressure providing more oxygen to working muscles.

• It also increases blood sugar levels providing more energy to cardiac and skeletal muscles.

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Role of the Parasympathetic Division

• Concerned with keeping body energy use low

• Involves the D activities – digestion, defecation, and diuresis

• Its activity is illustrated in a person who relaxes after a meal

– Blood pressure, heart rate, and respiratory rates are low

– Gastrointestinal tract activity is high

– The skin is warm and the pupils are constricted

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Parasympathetic neurons(Rest and Digest)

Allow body to function under Rest and digest

Pre-ganglionic fiber rises from the cranium(cranium fibers)

[III , VII, IX, X]and from sacral region of the spinal cord and synapse in ganglia near or on the effector organ.

Cranial outflow

III –oculomotor nerve(- pupils constrict)

VII -facial nerve –( tears, nasal mucus

IX -gloss-pharyngeal nerve(parotid salivary gland)

X –vagus nerve(visceral organs of thorax & abdomen): Stimulates digestive glands

Increases motility of smooth muscle of digestive tract

Decreases heart rate

Causes bronchial constriction

Sacral outflow (S2-S4): form pelvic splanchnic nerves

Innervates organs of the pelvis and lower abdomen

Supply 2nd half of large intestine

Supply all the pelvic (genitourinary) organs

Preganglionic fibers are long

Postganglionic fibers are short , with a ganglia

close or with in the organ.

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Summary of parasympathetic neurons and synapses

Preganglionic neurons• Long

• Synapse with postganglionic neurons at ganglia.

• Release acetylcholine (ACH) to activate nicotinic receptors on postganglionic neurons

Postganglionic neurons• Short

• Synapse on the target organ

• Release acetylcholine (ACH) to activate muscarinic receptors on the target organ

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INNERVATION BY THE A.N.S

DUAL INNERVATION

• Most organs in the body are innervated by both divisions of the a.n.s.

• Despite this dual innervation one system usually predominates in controlling the activity of a given organ.

Ex: In the heart vagus nerve is the predominant factor for controlling rate.

ORGAN RECIVING ONLY SYMPATHETIC INNERVATION

• The adrenal medulla pilomotormuscle and sweat glands.

• The control of blood pressure is also mainly a sympathetic activity with essential no participation by the parasympathetic system

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Enteric neurons

• It is third division of ANS

• It is collection of nerve fibers that innervates the GIT, Pancreas and Gall Bladder

• This system function independently of the CNS and control the motility exocrine and endocrine secretion and micro-circulation of the GIT.

• It is modulated by both sympathetic and parasympathetic nervous system.

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Neurotransmitter in sympathetic and parasympathetic nervous system

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DIFFERENCES BETWEEN SNS&PNS

SNS PNS

ORIGIN DORSO-LUMBAR(T1 TO L2 OR L3) CRANOI SACRAL(III,VII,IX,X AND

S2-S4)

DISTRIBUTION WIDE LIMITED

GANGLIA AWAY FROM ORGANS ON/ CLOSE TO THE ORGANS

POST GANGLIONIC FIBRES LONG SHORT

PRE:POST FIBRES RATIO 1:20 TO 1:100 1:1 TO 1:2

TRANSMITTER NE (MAJOR)

Ach (MINOR)

Ach

STABILITY OF TRANSMITTER NA STABLE Ach (RAPIDLY DESTROYS)

IMP FUNCTION TACKLING STRESS &

EMERGENCY

ASSIMILATION OF FOOD

,CONSERVATION OF ENERGY22

Adrenergic Receptors α1-receptors: vasoconstriction, relaxation

of gastrointestinal smooth muscle, salivary

secretion and hepatic glycogenolysis

α2-receptors: inhibition of transmitter

release (including noradrenalin and

acetylcholine release from autonomic

nerves), platelet aggregation, contraction of

vascular smooth muscle, inhibition of insulin

release

β1-receptors: increased cardiac rate and

force

β2-receptors: bronchi dilatation,

vasodilatation, relaxation of visceral smooth

muscle, hepatic glycogenolysis and muscle

tremor

β3-receptors: lipolysis. 23

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 receptor

• Nicotinic receptors are found on:

– Motor end plates (somatic targets)

– All ganglionic neurons 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 receptor

• Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers

• The effect of ACh binding:

– Can be either inhibitory or excitatory

– Depends on the receptor type of the target organ24

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Regulation of ANS Autonomic reflexes control most of activity of visceral organs, glands,

and blood vessels. Autonomic reflex activity influenced by hypothalamus and higher

brain centers, but it is the hypothalamus that has overall control of the ANS.

Sympathetic and parasympathetic divisions influence activities of enteric (gut) nervous system through autonomic reflexes. These involve the CNS. But, the enteric nervous system can function independently of CNS through local reflexes. E.g., when wall of digestive tract is stretched, sensory neurons send information to enteric plexus and then motor responses sent to smooth muscle of gut wall and the muscle contracts.

• Centers of the hypothalamus control:

– Heart activity and blood pressure

– Body temperature, water balance, and endocrine activity

– Emotional stages (rage, pleasure) and biological drives (hunger, thirst, sex)

– Reactions to fear and the “fight-or-flight” system 26

Levels of ANS Control

• The hypothalamus is the main integration center of ANS activity

• Subconscious cerebral input via limbic lobe connections influences hypothalamic function

• Other controls come from the cerebral cortex, the reticular formation, and the spinal cord

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synapse

• The junction between two neurons is called a synapse.

• An action potential cannot cross the synaptic cleft between neurons, and instead the nerve impulse is carried by chemicals called neurotransmitters.

• These chemicals are made by the cell that is sending the impulse (the pre-synaptic neuron) and stored in synaptic vesicles at the end of the axon.

• The cell that is receiving the nerve impulse (the post-synaptic neuron) has chemical-gated ion channels in its membrane, called neuroreceptors.

• These have specific binding sites for the neurotransmitters 28

1. At the end of the pre-synaptic neuron there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell.

2. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis.

3. The neurotransmitters diffuse across the synaptic cleft.

4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open. In the example shown these are sodium channels, so sodium ions flow in.

5. This causes a depolarisation of the post-synaptic cell membrane, which may initiate an action potential.

6. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neuron by endocytosis and used to re-synthesis more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on.

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General Features of PeripheralAutonomic Neurotransmission

Membrane Depolarization ofPre- or Postganglionic Fiber

Calcium Entry intoVaricosity

Exocytosis of NT

Diffusion of NT AcrossNeuroeffector Junctionor Synapse

Activation of NT Receptors

Depolarization of Postganglionic Fiber or Response of Effector Cell

Nerve Impulse

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SYNTHESIS OF NOREPINEPHRINE

Nor epinephrine (NE) is the primaryneurotransmitter For postganglionic sympathetic adrenergic nerve. It is synthesized inside the nerve axon, stored within vesicles, then released by the nerve when an action potential travels down the nerve. Synthesis of NE: The amino acid tyrosine is transportedinto the sympathetic nerve axon.Tyrosine (Tyr) is converted to DOPAby tyrosine hydroxylase,(rate-limiting step for NE synthesis).DOPA is converted to dopamine (DA)by DOPA decarboxylase.Dopamine is transported into vesicles then converted to norepinephrine (NE) by dopamine β-hydroxylase (DBH); transport into the vesicle can by blocked by the drug reserpine.

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An action potential traveling down the axon depolarizes the membrane and causes calcium to enter the axon.

Increased intracellular calcium causes the vesicles to migrate to the axonal membrane and fuse with the membrane, which permits the NE to diffuse out of the vesicle into the extracellular (junctional) space. DBH, and depending on the nerve other secondary neurotransmitters (e.g., ATP), is released along with the NE.

The NE binds to the postjunctional receptor and stimulates the effector organ response

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Synthesis of epinephrine

Epinephrine is synthesized from norepinephrine within the adrenal medulla,which are small glands associated with the kidneys.

Preganglionic fibers sympathetic adrenergic nerves synapse within the adrenals.Activation of these fibers releases acetylcholine, which binds to postjunctionalnicotinic receptors in the tissue.

This leads to stimulation of NE synthesis within adenomedullary cells, but unlike sympathetic neurons, there is an additional enzyme (phenyl ethanolamine-N-methyltransferase ) that adds a methyl group to the NE molecule to form epinephrine.

The epinephrine is released into the blood perfusing the glands and carried throughout the body.

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Synthesis of acetylcholine

Acetylcholine Synthesis

• Acetyl-CoA is synthesized from pyruvate by mitochondria within cholinergic nerves.

• This acetyl-CoA combines with choline that is transported into the nerve axon to form acetylcholine (ACh).

• The enzyme responsible for this is choline acetyltransferase.

• The newly formed ACh is then transported into vesicles for storage and subsequent release similar to what occurs for NE.

• After ACh is released, it is rapidly degraded within the synapse by acetylcholineesterase, to form acetate and choline.

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COTRANSMISSION

Release of More Than One

Neurotransmitter from the Same Nerve Terminal

Cotransmitter B

Synergistic or Opposite Actions

Cotransmitter A

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Many Examples of NANC Neurotransmitters

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Many Examples of NANC Neurotransmitters

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MODULATION OF NEUROTRANSMISSION

Modulation of Neurotransmisson

Presynaptic/Prejunctional

Modulation

Postsynaptic/Postjunctional

Modulation

Presynaptic/Prejunctional

Inhibition

Presynaptic/Prejunctional

Facilitation

Postsynaptic/Postjunctional

InhibitionPostsynaptic/

Postjunctional

Facilitation

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MODULATION OF NEUROTRANSMISSION

From Nerve Terminal Being

Modulated(e.g.,

Autoinhibitory Feedback)

From Postsynaptic/Postjunctional

Site (e.g., Trans-synaptic/

Transjunctional Inhibitory Feedback)

From Nearby Nerve

Terminal (Cross-

talk)

From Remote Site via

Circulation(e.g., Renin

Release)

Sources of Modulators of

Neurotransmission

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Neuromodulation and presynaptic interactions

• As well as functioning directly as neurotransmitters, chemical mediators may regulate:

• - presynaptic transmitter release

• - neuronal excitability.

• Both are examples of neuromodulation and generally involve second messenger regulation of membrane ion channels.

• Presynaptic receptors may inhibit or increase transmitter release, the former being more important.

• Inhibitory presynaptic autoreceptors occur on noradrenergic and cholinergic neurons, causing each transmitter to inhibit its own release (autoinhibitory feedback).

• Many endogenous mediators (e.g. GABA, prostaglandins, opioid and other peptides), as well as the transmitters themselves, exert presynaptic control (mainly inhibitory) over autonomic transmitter release.

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Mechanisms of Neuromodulation

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SOMATIC NERVOUS SYSTEM

• The somatic nervous system, or voluntary nervous system, is part of the peripheral nervous system that regulates body movement through control of skeletal (voluntary) muscles and also relates the organism with the environment through the reception of external stimuli, such as through the senses of vision, hearing, taste, and smell.

• The somatic nervous system controls such voluntary actions as walking and smiling through the use of efferent motor nerves, in contrast with the function of the autonomic nervous system, which largely acts independent of conscious control in innervating cardiac muscle and exocrine and endocrine glands.

• It is the somatic nervous system that allows individuals to receive sensory information and consciously react to environmental changes.

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Overview of somatic nervous system

• The somatic nervous system consists of cranial and spinal nerves that innervate skeletal muscle tissue and are more under voluntary control (as well as the sensory receptors).

• The somatic nervous system includes all the neurons connected with muscles ,skin , and sense organs.

• The somatic nervous system processes sensory information and controls all voluntary muscular systems within the body, with the exception of reflex arcs.

• The somatic nervous system consists of efferent nerves responsible for sending brain signals for muscle contraction.

• In humans, there are 31 pairs of spinal nerves and 12 pairs of cranial nerves.

• The 31 pairs of spinal nerves emanate from different areas of the spinal cord and each spinal nerve has a ventral root and a dorsal root.

• The ventral root has motor (efferent) fibers that transmit messages from the central nervous system to the effectors, with the cell bodies of the efferent fibers found in the spinal cord gray matter.

• The dorsal root has sensory (afferent) fibers that carry information from the sensory receptors to the spinal cord

• The 12 pairs of cranial nerves transmit information on the senses of sight, smell, balance, taste, and hearing from special sensory receptors.

• They also transmit information from general sensory receptors in the body, largely from the head. This information is received and processed by the central nervous system and then the response travels via the cranial nerves to the skeletal muscles to control movements in the face and throat, such as swallowing and smiling .

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Motor Pathway of Somatic Nervous System

Somatic division:•Cell bodies of motor neurons reside in CNS (brain or spinal cord)•Their axons (sheathed in spinal nerves) extend all the way to their skeletal muscles

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Nerve Signal Transmission

•The nerve signals in the efferent somatic nervous system involves a sequence that begins in the upper cell bodies of motor neurons (upper motor neurons) within the precentral gyrus (primary motor cortex). •Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract, via axons to control skeletal (voluntary) muscles. •These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord, and across synapses to be received by the sensory receptors of alpha motor neuron (large lower motor neurons) of the brainstem and spinal cord.•Upper motor neurons release a neurotransmitter, acetylcholine, from their axon terminal knobs, which are received by nicotinic receptors of the alpha motor neurons. •In turn, alpha motor neurons relay the stimuli received down their axons via the ventral root of the spinal cord. These signals then proceed to the neuromuscular junctions of skeletal muscles.•From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers

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Sensory neurons

• General visceral sensory neurons monitor:

– Stretch, temperature, chemical changes, and irritation

• Cell bodies are located in the dorsal root ganglia

1. Pain Receptors.

2. Thermo receptor

3. Mechanoreceptor

4. Chemoreceptor

5. Photoreceptor

6. Hearing and balance

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Pain Receptors

• Throughout body; except brain

• Respond to chemical released by damaged cells

• Important to recognize– Danger

– Injury

– Disease

Thermoreceptors

• In skin, body core, hypothalamus

• Detect variations in body temperature

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Mechanoreceptors

• Skin, skeletal muscle, and inner ears

• Sensitive to – Touch

– Pressure

– Stretching of muscles

– Sound

– motion

Chemo receptors

• Chemoreceptors pick up chemical reception in nose and mouth

• Smell – olfactory bulb

• Taste – taste buds– Salty

– Bitter

– Sour

– Sweet

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Photoreceptors

• Eyes

• Sensitive to Light

Vision• Cornea

– Helps focus light– Filled with aqueous humor

• Iris– Back of cornea– Colored part of eye

• Pupil– Tiny muscles regulate the size– Regulates amount of light

• Lens– Small muscles change its shape to

focus on object near and far away– Behind lens eye filled with vitreous

humor• Retina

– Has photoreceptors– No photoreceptors where optic nerve

passes through the back of the eye; blind spot

– Two types1. Rods – black and white2. Cones – color

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Hearing and Balance

Hearing

• Ear

• Two Functions– Hearing

– Detecting Positional change to movement.

• Cochlea

• Hammer, Anvil, Stirrup

• Tymapnum

• Auditory canal

• Sound

Balance

• Semicircular Canals– 3 canals that form half circles

– Filled with fluid and hairs that detect motion of head in relation to gravity

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Differences Between somatic & Autonomic Nervous System

Differences Somatic Autonomic

No of neurons in efferent path way

1 2

Neurotransmitter / receptor at neuron target synapse

Ach/nicotinic Ach/ Muscarinic (or) N.E/ Alpha or beta

Target tissue Skeletal muscle Smooth & cardiac muscle, some endocrine & exocrineGlands , some adipose tissue.

Neurotransmitter released from

Axon terminals Varicosities & axon terminals

Effects on target tissue Excitatory only,Muscle contracts.

Excitory or inhibitory.

Peripheral components found outside the cns

Axons only Preganglionic axons, ganglia, postganglionic neurons.

Summary of functions Posture & movement Visceral functions , including Movement in internal organs &secretion , control of metabolisium. 51

References

GOODMAN & GILMAN'S THE PHARMACOLOGICALBASIS OF THERAPEUTICS (11th Ed.) .

PHARMACOLOGY BY H.P.RANG, M.M.DALE(6th Ed.).

ESSENTIALS OF MEDICAL PHARMACOLOGY BYKD.TRIPATHI(5th Ed.)

LIPPINCOTT’S ILLUSTRATED REVIEWSPHARMACOLOGY.

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