autonomic nervous system

 Autonomic Nervous System

Autonomic or involuntary part of nervous system control "automatic" function of body. (Initiated in brain below level of cerebrum).

Effect of autonomic cavity are rapid and effector organs are -

1. Smooth muscle of internal organ; changes in blood vessel diameter.

2. Cardiac muscle; change in rate and force of heartbeat.

3. Glands; increasing or decreasing gastrointestinal (salivary and sweat glands) secretion.

Division of Autonomic nervous system

1. Sympathetic nervous system

Activity- helps body adjust stressful

situation

Originate in thoracolumbar region of spinal cord

2. Parasympathetic nervous system

Activity- during rest

Emerge from brain and sacral spinal cord (craniosacral)

Autonomic pathway

In ANS each division has two efferent neurons, in its peripheral pathway between CNS and effector organ.

The two neurons in pathway synapse in autonomic ganglia, which are outside CNS.

Ganglia containing cell bodies of neuron

1. Preganglionic neurons

2. Postganglionic neurons

Cell body of preganglionic neurons is in brain or spinal cord.

It's axon terminals synapse with cell body of postganglionic neurons in autonomic ganglion outside the CNS.

Postganglionic neurons conducts impulses to effector organ.

Sympathetic nervous system

Detects body's rapid involuntary response to dangerous or stressful situation.

Since preganglionic neurons originate in spinal cord at throcaic and lumbar level, name thoracolumbar outflow.

1. Preganglionic neuron 

It's cell body in brainbrain is in brain stem or spinal cord and it's myelinated axon courses through cranial or spinal nerve.

1st neuron= preganglionic, 2nd neuron = postganglionic 

 Acetylcholine                              Noradrenaline

Axon terminates at synapses with dendrites and cell body of one or more neurons in an autonomic ganglion.

Acetylcholine is neurotransmitter at sympathetic ganglion.

2. Postganglionic neurons 

This has its cell body in ganglion and terminates in organ or tissue supplied.

Noradrenaline (norepinephrine) is usually neurotransmitter at sympathetic efforts organs.

3. Sympathetic ganglia

1. Sympathetic chain ganglia

2. Prevertebral ganglia

Sympathetic chain ganglia

This chains extend from upper cervical level to sacrum, one chain lying on each side of vertebral bodies.

Ganglia are attached to each other by nerve fibres.

Preganglionic neurons that emerge from spinal cord may synapse with cell body of post ganglionic neurons at same level or they may pass up or down the chain through one or more ganglia before synapsing.

For example, nerve that dilates pupil of eye leaves cord at level of 1st thoracic vertebrae and passes up chain to superior cervical ganglion before it synapses with cell body of postsynaptic neurons. The postganglionic neurons then pass to eye.

Arrangement of ganglia allows excitation of nerve at multiple levels very quickly, providing a rapid and widespread sympathetic response.

As sympathetic chain ganglia are near spinal cord. Most sympathetic preganglionic axon are short and postganglionic axons are long.

Prevertebral ganglia

These three prevertebral ganglia situated in abdominal cavity close to large abdominal arteries.

1. Coeliac ganglion

2. Superior mesenteric ganglion

3. Inferior mesenteric ganglion

Preganglionic sympathetic fibers pass through the lateral chain to reach these ganglia.

2. Parasympathetic Nervous System; rest and digest system

(Conserves energy as it slow heart rate, increase intestinal and gland activity, and relax sphincter muscle in gastrointestinal track.

Two neurons preganglionic and postganglionic are involved in transmission of impulse from their sources to effector organ.

Neurotransmitter at both synapses is acetylcholine.

1. Preganglionic neurons

Arise from brain stem in brain or sacral region of spinal cord.

Those originating in brain are cranial nerves -

Oculomotor nerve III

Facial nerve VII

Glassopharyngeal nerve IX

Vagus nerve X

Arising from nuclei in mid brain and brain stem, and their nerve fibers terminate at or near effector organ.

Cranial Outflow

Oculomotor nerve III

Parasympathetic fibers of oculomotor nerves innervate smooth muscle (cillary and pupillary constrictor muscles) in eye that cause lense to bulge and pupil to constrict to close vision.

Facial nerve VII

Parasympathetic fibers of facial nerve stimulate may gland in head-

Nasal glands

Lacrimal gland of eye

Submandibular and sublingual salivary glands

Glossopharyngeal nerves IX

Parasympathetic fibres in glossopharyngeal nerve activate the parotid salivary gland anterior to ear.

Vagus Nerve X

Parasympathetic cranial outflow via vagus (X) nerve organ.

Innervate liver, gall bladder, stomach, small intestine, kidney, Pancreas, large intestine.

To neck

To thoracic organ - thorax - cardia plexus

Pulmonary plexus serving lungs and bronchi and esophageal plexus supplying esophagus.

Cardia plexus supply fibres to heart .

To abdominal cavity

In abdomen vagus nerves, enters the abdominal cavity through the esophageal hiatus.

Sacral outflow- 

Their fibers leave spinal cord in sacral segment 2,'3 and 4 and synapse with postganglionic neurons in walls of pelvic organs.

2. Postganglionic neurons

This is usually very short and has its cell body either in ganglion or more often, in wall of organ supplied.

Physiology of Autonomic Nervous System

1st neurons                     2nd neurons

Preganglionic.                 Postganglionic

ANS neurotransmitter and receptors

Integration and control of ANS (reflexes)

Effect ANS

Autonomic function

                 

Receptor in cell membrane combination of neurotransmitter and receptor function signal to cell.

Autonomic Reflexes

Autonomic reflexes are responses that occur when nerve impulses pass through autonomic reflex arc.

A neural pathway that control action reflex that affect inner organs and in somatic reflex arc that affect muscles.

Visceral sensory neurons, which send information concerning chemical changes action stretch, irritation of viscera pain, are first link in autonomic reflexes.

Autonomic reflexes are have-

Sensory receptor

Sensory neurons; conduct impulses

Association neurons; signal from sensory neurons to motor neurons.

Motor neurons

Effector

 Involuntary; smooth muscle, cardiac muscle, gland secretion

Level of ANS control

CNS activity is regulated by CNS controls in -

Spinal cord

Brain stem

Hypothalamus

Cerebral cortex

Physiological effects of ANS

(Autonomic nerve stimulation)

Sympathetic stimulation prepares body to deal with exciting and stressful situation. For example; strengthening its defence in times danger and extremes of environmental temperature.

Range of emotion States- fear, embarrassment, anger

Cause sympathetic stimulation

Adrenal glands are stimulated to secrete hormone adrenaline (epinephrine) and noradrenaline (norepinephrine) into bloodstream - these hormone potentiate and sustain effects of sympathetic stimulation.

Parasympathetic stimulation

Has tendency to slow down body processes except digestion and absorption of food and function of genitourinary system.

Normally two function system together, maintaining regular heartbeat, normal temperature and internal environment compatible with immediate external surroundings.

Effects of Autonomic Stimulation

1. Cardiovascular system

Sympathetic

A. Accelerates firing of SA node in heart, increasing rate and force of heartbeat.

B. Dilates coronary arteries, increasing blood supply to cardiac muscle.

C. Dilate blood vessel supplying skeletal muscle, increasing supply of oxygen and nutritional materials and removal of metabolic waste product, thus increasing capacity of muscle to work.

D. Constrict blood vessels in secretory gland of digestive system. This raises volume of blood available for circulation in dilated blood vessels.

E. Accelerates blood coagulation because of constriction (narrowing of blood vessel resulting contraction of muscular wall of vessels.

Parasympathetic stimulation

A. Decrease rate and force of hearbeat.

B. Constrict coronary arteries, reducing blood supply to cardiac muscle.

2. Respiratory system

Sympathetic stimulation

This causes smooth muscle relaxation, so

A. Dilation of bronchioles, allowing a greater amount of air to enter lungs at each inspiration, increase respiration rate.

B. In conjunction with increased heart rate, oxygen intake and carbon dioxide output of body are increased to deal with fight or flight situation.

Parasympathetic stimulation

Causes contraction of smooth muscle in leading to bronchoconstriction.

3. Digestive and Urinary stimulation

Sympathetic stimulation

A. Liver increase conversion of glycogen to glucose, making more carbohydrate immediately available to provide energy.

B. Stomach and small intestine

Smooth muscle contraction (peristalsis) and secretion of digestive juices are inhibited, delaying digestion, onward movement and absorption of food, and tone of sphincter muscle is increased.

C. Adrenal (suprarenal) gland are stimulated to secrete adrenaline (epinephrine) and noradrenaline (norepinephrine) which potentiate and sustain effect of sympathetic stimulation throughout body.

D. Urethral and anal sphincter 

The muscle tone of sphincter increased, inhibiting micturition and defecation.

E. Bladder wall relaxes.

F. Metabolic rate is greatly increased.

Parasympathetic stimulation

Liver secretion of bile is increased.

Stomach and small intestine motility and secretion are increased, together with rate of digestion and absorption of food.

Pancreas, secretion of pancreatic juice is increased.

Urethral and anal sphincters

Relaxation of internal urethral sphincter is by contraction of muscle of bladder wall, and micturition occurs.

Relaxation of internal anal sphincter is by contraction of muscle of rectum and defecation occurs. 

There is voluntary relaxation of external sphincter.

Adrenal gland- no effect

Metabolic rate- no effect

5. Eye

Sympathetic stimulation

A. This causes contraction of radiating muscle fibres of iris, dilating pupil.

B. Ciliary muscle that adjusts thickness of lens is slightly relaxed, facilitating distant vision.

Parasympathetic stimulation

This contract the circular muscle fibres of iris, constricting pupil.

Ciliary muscle contract, facilitating near vision.

6. Skin

Sympathetic stimulation

Increase sweat secretion, leading to increased heat loss from body.

Contract muscle in hair follicles of skin, giving appearance of goose flesh. Goose bumps

Constrict peripheral blood vessels, increasing blood supply available to active organs. For example; heart and skeletal muscle.

Parasympathetic stimulation

There is no parasympathetic nerve supply to skin.