1. Spinal reflexes
The reflex arc
SMS 1084
Dr. Mohanad R. Alwan

2. A few reminders!
The central grey matter contains the cell bodies of
relay and motor neurones.
The outer white matter contains myelinated axons,
which run up and down the spinal cord to and from
the brain.
In the centre of the grey matter is the spinal canal,
through which the nutritive cerebrospinal fluid
cirulates.

3. A few reminders!
Sensory neurones enter the spinal cord through the
dorsal root and the concentration of their cell bodies
forms a swelling called the dorsal root ganglion.
Motor neurones leave the spinal cord via the ventral
root.

5. The Reflex Arc
An automatic, rapid response to an adverse stimulus.
Action is involuntary the brain is not involved in the
event but may be informed of it
Many reflex actions are protective
Some complex actions (swallowing, coughing and
blinking) are coordinated by reflexes.

6. Example of Neuronal Organization:
Reflexes
Reflex arcs – simple neural pathways
Responsible for reflexes
 Rapid, autonomic motor responses
Can be visceral or somatic

7. Five Essential Components to the
Reflex Arc
Receptor – detects the stimulus
Afferent (sensory neuron) – transmits impulses to the
CNS
Integration center – consists of one or more
synapses in the CNS
Efferent (motor neuron) – conducts impulses from
integration center to an effector
Effector – muscle or gland cell
Responds to efferent impulses
 Contraction or secretion

8. Example of the Five Components to the Reflex
Arc

9. Reflex Classification
Monosynaptic or polysynaptic
Spinal or cranial
Somatic or autonomic
Innate or learned

10. Types of Reflexes: Number of Classes
Monosynaptic reflex – simplest of all reflexes
Just one synapse
The fastest of all reflexes
Example – knee-jerk reflex
Polysynaptic reflex – more common type of reflex
Most have a single interneuron between the sensory
and motor neuron
Example – withdrawal reflexes

11. Monosynaptic Reflex

12. Polysynaptic Reflex

13. Spinal vs Cranial Reflexes
Spinal = spinal cord integration center
Ex. Knee-jerk reflex
Cranial = brain as integration center
Ex. Pupillary light reflex

14. Somatic vs Autonomic Reflexes
Somatic = motor neurons to skeletal muscles
Ex. Knee-jerk reflex
Autonomic = autonomic neurons to smooth muscle
and glands
Ex. Pupillary light reflex

15. Innate vs Learned Reflexes
Innate = born-with
Knee-jerk reflex, pupillary reflex
Learned = develops based on experiences
Pavlov’s dogs salivation in response to bell

16. A withdrawal reflex

17. A withdrawal reflex
The stimulus
is the heat from
the hot object

18. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by receptors
in the dermis of the skin. Since these
sensory cells respond to temperature,
they are called thermoreceptors.

19. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve impulses
that pass to the spinal
cord along a sensory
neurone.

20. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone cell
body

21. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter
(neurone
cell bodies)

22. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord

23. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord
white matter
(neurone
axons)

24. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord
white matter (neurone
axons)
The sensory neurone enters the spinal
cord via the dorsal branch of the spinal
nerve and forms a synapse with a
relay neurone, inside the grey matter

25. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord
white matter (neurone
axons)
The sensory neurone enters the spinal
cord via the dorsal branch of the spinal
nerve and forms a synapse with a
relay neurone, inside the grey matter
The short relay neurone
forms a synapse with a
motor neurone that
leaves the spinal cord via
the ventral root of the
spinal cord.

26. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by
receptors in the dermis of the
skin. Since these sensory cells
respond to temperature they are
called thermoreceptors.
The thermoreceptors
initiate nerve
impulses that pass
to the spinal cord
along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord
white matter (neurone
axons)
The sensory neurone enters the spinal
cord via the dorsal branch of the spinal
nerve and forms a synapse with a
relay neurone, inside the grey matter
The short relay
neurone forms a
synapse with a motor
neurone that leaves
the spinal cord via the
ventral root of the
spinal cord.
The motor neurone carries the
impulse to an effector, in this
case muscles in the arm,
which contract to withdraw the
hand from the hot object. This
action is known as the
response.

27. A withdrawal reflex
The stimulus is the heat
from the hot object
The stimulus is detected by receptors in
the dermis of the skin. Since these sensory
cells respond to temperature they are
called thermoreceptors.
The thermoreceptors initiate
nerve impulses that pass to the
spinal cord along a sensory
neurone.
sensory neurone
cell body
grey matter (neurone
cell bodies)
spinal cord
white matter (neurone
axons)
The sensory neurone enters the spinal
cord via the dorsal branch of the spinal
nerve and forms a synapse with a
relay neurone, inside the grey matter
The short relay neurone
forms a synapse with a
motor neurone that
leaves the spinal cord via
the ventral root of the
spinal cord.
The motor neurone carries the
impulse to an effector, in this case
muscles in the arm, which
contract to withdraw the hand
from the hot object. This action is
known as the response.

28. Review of reflex arc.

29. Other Reflexes
Stimulus Response
The aroma of your favorite
food
Salivation
A nasty odor Nausea
A bright light shining in your
eye
Pupils get smaller
An insect flying towards your
eye
Blinking