1. The Human Nervous System
A simple reflex arc is the simplest response to a stimuli. There whole arc only involves 3 sections of a nerve pathway. The the response is automatic and very rapid, the reason that it is so fast is because the information does not have to travel or be processed by the brain. The three neurons which normally make up the pathway are
(1) sensory neuron
(2) relay neuron
(3) motor neuron
This is how it works
Stimulus sensor coordinator effector response
Touch pain receptor CNS muscle movement

2. ANS The 2 Divisions
sympathetic
parasympathetic
Pupil dilation tear production not affected
Pupil constricts tear production stimulated
Heart rate increase
Heart rate decrease
Bronchi dilate
Bronchi constrict
Gastric+pancreatic activity inhibited
Gastric+pancreatic activity stimulated
Glycogen converted into glucose
Glucose converted into glycogen
Release of adrenaline+noradrenaline
Release of adrenaline+noradrenaline cease
Peristalsis inhibited
Peristalsis stimulated
Bladder relaxes
Bladder constricts
The roles of the sympathetic and parasympathetic nervous system are opposing. Generally the sympathetic nervous system has a stimulatory affect and prepare the body for action while the parasympathetic system returns the body functions to normal.

3. The autonomic nervous system and the pupil
To operate well in different light conditions the eye must be able to control the amount of light entering the eye it does this by changing the diameter of the pupil. The changes in the size of the pupil are brought about by a reflex arc so it involves no conscious thought. The light receptors in the retina detect the intensity of the light (sensor). E.g too weak or too strong this information is then sent to the brain via the optic nerve(Sensory Neuron). Then impulses from the brain are transmitted to the iris muscles by the nerve fibres.

4. Emptying of the bladder
The human bladder can hold up to 800 cm3 of urine but what happens when its full? Stretch receptors in the bladder wall send a impulse to the CNS alerting it to the urgency the rely neuron send an impulse to the motor neuron which sends an impulse to the sphincter muscles to relax and we urinate. But not always as we humans have learnt hoe to control our sphincter muscles (potty training) so we can go when and where convenient.