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Reflex Arcs
SNAB Topic 8 Grey Matter
Lesson 3
Learning Objectives
Describe the path of nerve impulses in
reflex arcs
 Describe the pupil reflex
 Explain the process by which the pupil
reflex is controlled

Key Notes
Interesting Facts
Questions
Reflex Actions
Many of the actions we carry out every
day are done without conscious thought
 These are call Reflex Actions
 The nerve impulses in reflex actions do
not follow the same path as those in
conscious actions
 The path they follow is called the Reflex
Arc

GCSE Review Question

What neuron is unique to the Reflex Arc?
A.
B.
C.
D.
Effector
Motor
Relay
Sensory
Examples of Reflex Arcs
Knee Jerk Reaction – Involved in keeping
standing
 Pupil Dilation – Protect eyes from light
 Vomit Reflex
 Cough Reflex
 Sneeze Reflex
 Eye tracking Reflex

Nervous Impulse Pathway
in a Reflex Arc
Interlink/Relay Neurons



These neurons are
the vital part of the
reflex arc
They connect the
Sensory neuron to the
Motor/Effector neuron
They can be found in
the grey matter of the
spinal cord and in the
midbrain
Advantage of Reflexes


The reflex arc allows
a much quicker
response to stimuli
This is especially
important where the
stimuli is a source of
potential damage e.g.
a hot object, a sharp
pin
Inhibition of the Reflex Response
The reflex response is considerably
inhibited in several situations:
 Exposure to Noradrenalin
(Parasympathetic nervous system)
 Tiredness
 High Blood Alcohol
 Effect from Sedative Drugs (e.g.
Cannabis)

Stimulation of the Reflex Response
The speed of the reflex response can be
increase by several factors:
 Exposure to adrenaline (Sympathetic
Nervous System)
 Exposure to stimulant drugs (Caffeine,
Beta Amphetamines/Speed)

The Pupil Reflex Response
The pupil response allows the iris to
contract and dilate
 This changes the size of the pupil
 In the eye the pupil acts like an aperture
 It controls the amount of light which enters
the eye..
 … and protect the retina from
“overexposure”

Bright Light, Bright Light



When light levels are
high Circular
muscles in the iris
contract
This causes the pupil
to become smaller
This is controlled by
the parasympathetic
nervous system
In the Dark…



When light levels are
low Radial muscles
contract
This causes the pupil
to increase in size
This is caused by the
sympathetic nervous
system
Diagram of Pupil Response
Control of the Pupil Reflex: In
Detail
When high levels of light strike
photoreceptors in the retina they cause
nervous impulses to be sent along the
Sensory neuron (Optic Nerve)
 These impulses are co-ordinated by relay
neurons in the Midbrain
 Signals are then sent along the
parasympathetic nervous system

Diagram of the Pupil
Reflex Arc
Eye Dilation Videos
http://video.google.co.uk/videoplay?docid=
-4513396283977692296&q=pupil
 http://video.google.co.uk/videoplay?docid=
-2586168005397990196&q=pupil
 http://video.google.co.uk/videoplay?docid=
-6096473483443032157&q=pupil+reflex

Atropine
Atropine is a sedative drug made from the
deadly nightshade plant (Atropa
belladonna)
 It was apparently used in the middles ages
by women to dilate there pupils
 This is the source of the word belladonna
in Latin and Belle in French

Check Point

What muscle contracts when the pupil
dilates?
A.
B.
C.
D.
Optic
Radial
Ocular Motor
Circular
Check Point

What nervous system is involved in pupil
contraction?
A.
B.
C.
D.
Somatic
Sympathetic
Central
Parasympathetic
Correct!
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