Autonomic nervous system

• The nervous system can be divided anatomically into

• 1. central nervous system (CNS) : it consists of the brain and the spinal cord

• 2. peripheral nervous system: it consists of the peripheral nerves

• It can also be divided physiologically into

• 1. somatic nervous system• It controls voluntary movements by the skeletal muscles

• 2. autonomic nervous system (involuntary or self controlling)• It regulates visceral activity e.g. heart , digestive system ….etc• Both systems have a central and peripheral part . The neuron is the atomic

unit of the nervous system.

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Organization of the nervous system

Nervous system

Central NS Peripheral NS

Autonomic NS Somatic NS

Sympathetic NS Parasympathetic NS

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Somatic pathway

AHC

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LHC

The reflex arc• It is the functional unit of the nervous system. It consists of:• 1. a receptor (sense organ)• It is a specialized structure sensitive to changes inside or outside the body.

It converts different forms of energy into nerve impulses e.g. rods and cones in the eye, taste buds in the mouth.

• 2. an efferent neuron• It carries the nerve impulses from the receptor to the CNS• 3. center• It is present inside the CNS• 4. an efferent neuron• It carries the impulses from the center to the effector organ• 5. an effector organ• It is the structure , which produces the response (e.g. gland or muscle)

Types of reflex arc• 1. somatic reflex arc

• E.g. stretch reflex and withdrawal reflex

• a) stretch reflex• When a skeletal muscle is stretched ; it contracts ( e.g. knee jerk and ankle jerk)

• b) withdrawal reflex• When a painful stimulus is applied to the skin , there will be contraction of the flexor

muscles and inhibition of the extensor muscles e.g. pin prick to the skin leads to sudden withdrawal of the limb.

• 2. autonomic reflex arc

• The autonomic reflex arc differs from the somatic reflex arc mainly in that it has two efferent neurons.

Differences between somatic and autonomic reflex arcs

Somatic reflex arc Autonomic reflex arc

The receptor Usually in the skin Usually in a viscus

The afferent neuron

Passes via a dorsal root or cranial nerve and has its cell body in the dorsal root ganglion.

The same like the somatic reflex arc

The center The anterior horn cell The lateral horn cell

The efferent neuron

It is composed of one neuron only

It is composed of two neurons . It passes with the spinal nerves and relay in autonomic ganglia. Before relay , it is called white ramus communicants (myelinated) . After relay , it is called postganglionic neuron (also called gray ramus communicants) {unmyelinated}

The effector A skeletal muscle Usually a viscus ( plain or cardiac muscle or a gland)

Autonomic ganglia

• These are nerve cells present outside the central nervous system. The axons of the lateral horn cells (preganglionic fibers) make synaptic connections with the cell body of the autonomic ganglia (synapse) then postganglionic fibers carry the impulses to the viscera.

Types of autonomic ganglia

• According to their sites they are classified into

• 1. paravertebral (lateral)• There are two lateral chains on each side of vertebral column. They are sympathetic only. The chain

contains on ganglion for each segment of the spinal cord, except in the upper and lower parts of the chain adjacent ganglia fuse together. Thus in the cervical region there are only 3 ganglia instead of 8 . They are called superior, middle and inferior cervical sympathetic ganglia.

• 2 . collateral• They lie between the sympathetic chain and the viscera. They lie at the origin of big arteries. They may

be sympathetic or parasympathetic.

• 3. terminal• They lie within the wall of organ they supply. They are parasympathetic only.

Function of the autonomic ganglia

• They act as distributing centers because each preganglionic axon diverge to many post ganglionic neurons. It is a relay station.

• - in the sympathetic system , the preganglionic fibers relay in the lateral or collateral ganglia and each preganglionic fiber activates many postganglionic neurons. This allows for widespread distribution of nerve impulses producing generalized sympathetic effects.

• -in the parasympathetic system , the preganglionic fibers relay in collateral or terminal ganglia and each preganglionic fiber activates few postganglionic neurons. This distribution produces localized parasympathetic effects.

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Autonomic ganglia• It is a collection of neurons

outside the CNS.• It is the site of synapse

between the preganglionic and the postganglionic neurons.

• It act as a distributing center.

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Lateral and collateral ganglia

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Parasympathetic NS is characterized by: *The preganglionic fibers synapse in the terminal ganglia.

* Long preganglionic fibers .* The postganglionic effects are mediated

through acetyl choline.

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Sympathetic NS

• Sympathetic division of the ANS is called the fight and flight division.

• It prepare the body to deal with stress.

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Parasympathetic NS

• Parasympathetic NS is called the rest and sleep division of the ANS.

• It allows the body to recover from stress

Sympathetic nervous system

• Thoracolumber:

• It consists of two neurons, pre and post ganglionic . The preganglionic neurons have their cell bodies in the lateral horn of all the thoracic segments and the upper 3 lumber segments of the spinal cord. The postganglionic neurons have their cell bodies in the lateral or collateral ganglia. Then the post ganglionic fibers supply the effector organs.

Effects of stimulation of the sympathetic nervous system

Organ Effects

Head and neck -motor to lator pupillae muscle leading to dilation of the eye pupil. di-secretion of small volume of viscid saliva.-constriction of the skin blood vessels.

Thorax -Heart

-lungs

Stimulation of all properties of the heart:-increased heart rate.-increased force of contraction.-dilation of the coronary vessels.-dilation of the bronchi.-slight constriction of the pulmonary vessels.

Abdomen-Liver-Stomach and intestine-Gall bladder and bile ducts -Adrenal medulla

Constriction of the blood vessels of the viscera.Stimulation of glycogenolysis ( increase in blood glucose level).-decreased motility and decreased secretionIncrease in tone of sphincters i.e. contraction of sphincters..-relaxation of the wall and contraction of sphincter.-stimulation of epinephrine and norepinephrine secretion.

CONTINU.SYMPATHETIC functions

pelvis

-Constriction of the blood vessels of the viscera.-inhibitory to the wall of the urinary bladder , motor to the sphincter causing retention of urine.-inhibitory to the wall of the rectum, motor to the sphincter causing retention.-in the male, motor to the plain muscles of the vas deferens , seminal vesicles and prostate causing ejaculation of semen.

Limbs, thoracic and abdominal

walls.

-constriction of the skin blood vessels.-dilation to the muscle blood vessels.-excess secretion of sweat glands.-contraction of piloerector muscles.-skeletal muscles glycogenolysis , increased strength of contraction

General effects- Increased mental activity- increased BMR ( basal metabolic rate)-increased blood coagulation-Increased blood glucose level.