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

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.

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.

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.