1. LAB EXERCISE 14
SPINAL REFLEXES

2. Reflexes Reflexes Automatic responses coordinated within spinal cord
The wiring of a single reflex
Through interconnected sensory neurons, motor neurons, and interneurons

3. Reflexes Reflexes Produce simple and complex reflexes
Beginning at receptor
Ending at peripheral effector
Generally opposes original stimulus
Negative feedback

4. Five Steps in a Neural Reflex
Reflexes
Five Steps in a Neural Reflex
Step 1: Arrival of stimulus, activation of receptor
Physical or chemical changes
Step 2: Activation of sensory neuron
Graded depolarization
Step 3: Information processing by postsynaptic cell
Triggered by neurotransmitters
Step 4: Activation of motor neuron
Action potential
Step 5: Response of peripheral effector
Triggered by neurotransmitters

5. The Classification of Reflexes
can be classified by
development
response
response
complexity of circuit
processing site
Innate Reflexes
Somatic Reflexes
Monosynaptic
Spinal Reflexes
• Genetically
• Control skeletal muscle
• One synapse
• Processing in
determined
contractions
the spinal cord
• Include superficial and stretch
reflexes
Acquired Reflexes
Visceral (Autonomic) Reflexes
Polysynaptic
Cranial Reflexes
• Learned
• Control actions of smooth and
• Multiple synapse
• Processing in
cardiac muscles, glands, and adipose tissue
(two to several hundred)
the brain 5

6. Classification by Development
Learned Reflex
Voluntary & premeditated action
Slower than an inborn reflex
Varied response (learning occurs)
Enhanced by repetition
Meter stick catch
Driving
Inborn Reflex
Involuntary & unlearned action
Prevent us from having to think about things like maintaining posture or controlling visceral activities
Patterned response (same every time)
Tendon reflex
Pupillary reflex
Swallowing
Withdrawal reflex
Sucking

7. Reflexes Motor Response Nature of resulting motor response
Somatic reflexes
Involuntary control of skeletal system
Superficial reflexes of skin, mucous membranes
Stretch or deep tendon reflexes
*Patellar, or “knee-jerk,” reflex
Visceral reflexes (autonomic reflexes)
Control or adjust activities of smooth & cardiac muscle, glands, and adipose tissues
Control systems other than muscular system

8. Reflexes Complexity of Neural Circuit Monosynaptic reflex
Simplest reflex arc involving one sensory and one motor neuron
Faster response time due
to only one synapse

9. Reflexes Polysynaptic reflex
Involve at least one interneuron one sensory neuron, and one motor neuron
Longer delay between stimulus and response due to increased number of synapses
Produce more complex reflexes

10. Site of Information Processing
Reflexes
Site of Information Processing
Spinal Reflexes
Occur in nuclei of spinal cord
Two types
Single segmental
Within one spinal segment
Intersegmental
Multiple spinal segments
Cranial reflexes
Occur in nuclei of brain

11. Spinal Reflexes Spinal Somatic Reflexes
Integration center is in the spinal cord
Most do not involve higher learning centers
The brain remains “advised” of spinal reflex activity and can facilitate, inhibit or adapt it if needed.
Effectors are skeletal muscle

12. Spinal Reflexes
Somatic spinal reflexes include
The stretch reflex
Tendon reflex
Flexor (withdrawal) reflex
Crossed extensor reflex

13. Spinal Reflexes Monosynaptic Reflexes A stretch reflex
Have the least delay between sensory input and motor output
For example, stretch reflex (such as patellar reflex)
Completed in 20–40 msec
Receptor is muscle spindle

14. Stretch and Golgi Tendon Reflexes
For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary
Muscle spindles inform the nervous system of the length of the muscle to maintain healthy muscle tone
Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

15. Spinal Reflexes Muscle Spindles The receptors in stretch reflexes
Bundles of small, specialized intrafusal muscle fibers
Innervated by sensory and motor neurons
Surrounded by extrafusal muscle fibers
Which maintain tone and contract muscle

16. The Stretch Reflex
Simplest reflex arc involving one sensory and one motor neuron

17. Monosynaptic Reflex Postural reflexes
Many stretch reflexes that help maintain upright posture
Postural muscles generally have firm muscle tone and extremely sensitive stretch receptors
Allow for very fine, subconscious adjustments

18. Monosynaptic Reflex Postural reflexes
Coordinated activities of opposing muscles to keep body’s weight over feet
Example: leaning forward stretches calf muscle receptors which stimulate the muscles to increase tone
Returns body to upright position

19. Spinal Reflexes Polysynaptic Reflexes
More complicated than monosynaptic reflexes
Responsible for automatic actions involved in complex movements
Examples: walking and running

20. Spinal Reflexes Polysynaptic Reflexes
Interneurons control more than one muscle group
Produce either EPSPs or IPSPs

21. Polysynaptic Reflexes
May involve sensory and motor responses on the same side of body or opposite sides
Same side: ipsilateral reflexes
Examples: stretch reflex, withdrawal reflex
Opposite sides: contralateral reflexes
Example: crossed extensor reflex

22. Spinal Reflexes The Tendon Reflex Prevents skeletal muscles from:
Developing too much tension
Tearing or breaking tendons
Golgi Tendons
Sensory receptors unlike muscle spindles or proprioceptors

23. Spinal Reflexes Withdrawal Reflexes
Move body part away from stimulus (pain or pressure)
Show tremendous versatility because sensory neurons activate many pools of interneuron
Strength and extent of response
Depend on intensity and location of stimulus
Example:
Flexor reflex
Crossed extensor reflexes

24. Polysynaptic Reflexes
Withdrawal reflex example: flexor reflex
Grabbing an unexpectedly hot pan causes pain receptors in hand to be stimulated
Sensory neurons activate interneurons in spinal cord
Interneurons
Activate motor neurons in anterior gray horn to contract flexor muscles
Activated inhibitory interneurons keep extensors relaxed
= Reciprocal inhibition

25. Quadriceps (extensors) Hamstrings (flexors)1
Quadriceps strongly
contracts. Golgi tendon
organs are activated.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors) +
Excitatory synapse –
Inhibitory synapse
Figure 13.18, step 1

26. Quadriceps (extensors) Hamstrings (flexors)1
Quadriceps strongly
contracts. Golgi tendon
organs are activated. 2
Afferent fibers synapse
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors) +
Excitatory synapse –
Inhibitory synapse
Figure 13.18, step 2

27. Quadriceps (extensors) Hamstrings (flexors)1
Quadriceps strongly
contracts. Golgi tendon
organs are activated. 2
Afferent fibers synapse
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors) 3a
Efferent impulses
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension. +
Excitatory synapse –
Inhibitory synapse
Figure 13.18, step 3a

28. Quadriceps (extensors) Hamstrings (flexors)1
Quadriceps strongly
contracts. Golgi tendon
organs are activated. 2
Afferent fibers synapse
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors) 3a
Efferent impulses
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension. 3b
Efferent
impulses to
antagonist
muscle cause
it to contract. +
Excitatory synapse –
Inhibitory synapse

29. Flexor (withdrawal) Reflex
Initiated by a painful stimulus
Step on tack
Pain fibers send signal to spinal cord
Interneurons branch to different spinal cord segments
Motor fibers in several segments are activated
More than one muscle group activated to lift foot off of tack
Causes automatic withdrawal of the threatened body part
Ipsilateral and polysynaptic

30. Spinal Reflexes Crossed Extensor Reflexes
Example: stepping on a tack
Flexor reflex pulls injured foot away (ipsilateral)
Flexor muscles stimulated
Extensor muscles inhibited
Crossed extensor reflex straightens uninjured leg and supports shifting weight
(Contralateral)
Activated by collaterals of excitatory and inhibitory interneurons
Extensor muscles stimulated
Flexor muscles inhibited
Maintained by reverberating circuits

31. Diagnostics using Reflexes
Babinski reflex
Stroking lateral side of sole of foot
Positive response:
Toes fan due to lack of inhibitory control of reflex response from descending motor pathways
Normal in infants
Can indicate damaged higher centers or descending tracts in adults
Negative response:
Toes curl due to development and normal reflex response in adults
= Plantar reflex