1. “Nervous System: Reflexes”
Laboratory Exercise 15
“Nervous System: Reflexes”

2. Nervous System – Reflexes
In this exercise, you will become understand the functioning of a reflex, identify the types of reflexes observed in the body, and understand the functioning of a somatic and autonomic reflex
These terms should be familiar to you: reflex arc, somatic, autonomic, withdrawal reflex
This laboratory exercise correlates with Chapter 15 and 16 of your textbook; Exercise 15 of your laboratory manual

3. Laboratory Lecture

4. Reflexes
Neural reflexes are rapid, automatic responses to specific stimuli coordinated within the spinal cord.
They are the basic building blocks of neural function; one neural reflex produces one motor response (much of what we know about how the nervous system functions has come from studying neural reflexes)
Reflexes are processed through interconnected sensory neurons, interneurons and motor neurons
A reflex arc is the wiring of a single reflex, beginning at a receptor and ending at a peripheral effector
In general, a reflex arc opposes the original stimulus (negative feedback mechanism)
Reflexes are typed as simple or complex

5. 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

6. Sensation relayed to the brain by axon collaterals
There are Five Steps in a Neural Reflex
Dorsal root
Arrival of stimulus and activation of receptor
Activation of a sensory neuron
Sensation relayed to the brain by axon collaterals
Information processing in the CNS
REFLEX ARC
Receptor
Stimulus
Response by effector
Effector
Ventral root
KEY
Sensory neuron (stimulated)
Activation of a motor neuron
Excitatory interneuron
Motor neuron (stimulated) 6

7. Four Classifications of Reflexes
By development
By type of motor response
By complexity of neural circuit
By site of information processing

8. Development of Reflexes
Innate reflexes
Basic neural reflexes
Formed before birth
Acquired reflexes
Rapid, automatic
Learned motor patterns

9. Motor Response Nature of resulting motor response Somatic reflexes
Involuntary control of nervous system
Superficial reflexes of skin, mucous membranes
Stretch or deep tendon reflexes (e.g., patellar, or “knee-jerk,” reflex)
Visceral reflexes (autonomic reflexes)
Control systems other than muscular system

10. Complexity of Neural Circuit
Monosynaptic reflex (faster)
Sensory neuron synapses directly onto motor neuron
Polysynaptic reflex (slower)
At least one interneuron between sensory neuron and motor neuron

11. Site of Information Processing
Spinal reflexes
Occur in spinal cord
Cranial reflexes
Occur in brain

12. The Four Classifications of Reflexes
can be classified by
development
nature of response
complexity of circuit
processing site
Innate Reflexes
Somatic Reflexes
Monosynaptic
Spinal Reflexes
• Genetically determined
• Control skeletal muscle contraction
• One synapse
• Processing in
• Basic motor patterns
• Include superficial and stretch
the spinal cord
reflexes
Acquired Reflexes
Visceral (Autonomic) Reflexes
Polysynaptic
Cranial Reflexes
• Learned motor patterns
• Control actions of smooth and
• Multiple synapse
• Processing in
• Rapid and automatic
responses
cardiac muscles, glands, and adipose tissue
(two to several hundred)
the brain
1. What would happen if you touched a hot stove? Classify…
2. What would happen if you were driving your car and another car darted into your lane (No extending middle finger jokes)? Classify… 12

13. Spinal Reflexes
Spinal reflexes range in an increasing order of complexity
Monosynaptic reflexes (i.e. stretch reflex)
Simple polysynaptic reflexes (i.e. flexor reflex)
Complex polysynaptic reflexes (intersegmental reflex arcs) (i.e. crossed-extensor reflex)
Many segments interact, producing a highly variable motor response

14. Monosynaptic Reflexes
A Stretch Reflex has the most rapid response between the sensory input and motor output (~30 msec)
An example is the patellar reflex (shown)
The receptor is called a muscle spindle

15. Muscle spindles are the receptors in stretch reflexes
They are composed of bundles of small, specialized intrafusal muscle fibers that are innervated by sensory and motor neurons
The sensory region is the central region of intrafusal fibers; it is wound with dendrites of sensory neurons
The axon of the sensory neuron enters the CNS in the dorsal root and synapses onto motor neurons (gamma motor neurons) in the anterior gray horn of the spinal cord

16. Gamma efferents are the axons of the motor neurons that complete the reflex arc; they synapse back onto intrafusal fibers
They are important in voluntary muscle contraction; they allow the CNS to adjust the sensitivity of muscle spindles to maximal range
The intrafusal muscle fibers are surrounded by extrafusal muscle fibers, which maintain tone and contract the muscle, facilitating body movement

17. Many postural reflexes are stretch reflexes that help us maintain normal upright posture
In other words, the stretched muscle responds by contracting, automatically maintaining our balance (this is proprioceptive), compensating for changes in load

18. Polysynaptic Reflexes
Polysynaptic reflexes are more complicated than monosynaptic reflexes
Interneurons control more than one muscle group, producing an EPSP or an IPSP
Polysynaptic reflexes can range from simple to complex

19. The Tendon Reflex
The tendon reflex prevents skeletal muscles from developing too much tension, which can cause a tear or break in the connecting tendons
The sensory receptors are unlike muscle spindles or proprioceptors (they have not been identified yet)

20. Withdrawal Reflexes
Withdrawal reflexes move a body part away from the stimulus (pain or pressure)
An example is the flexor reflex, which pulls a hand away from hot stove; the strength and extent of the response depend on the intensity and location of the stimulus
For the flexor reflex to work, the stretch reflex of the antagonistic (extensor) muscle must be inhibited (reciprocal inhibition) by interneurons in the spinal cord
This is maintained by reverberating circuits that are orientated in a ipsilateral manner

21. “simple” Ipsilateral – same side “reciprocal inhibition” 21
Distribution within gray horns to other segments of the spinal cord
Painful stimulus
Flexors stimulated
Extensors inhibited
KEY
Sensory neuron (stimulated)
Motor neuron (inhibited)
Excitatory interneuron
Inhibitory interneuron
Motor neuron (stimulated)
“reciprocal inhibition” 21

22. Crossed Extensor Reflexes
Crossed extensor reflexes occur at the same time; they are coordinated with a flexor reflex
For example - you step on a nail. A flexor reflex causes that leg to retract. At the same time, the crossed extensor reflex extends the other leg to receive body weight
This is maintained by reverberating circuits that are orientated in a contralateral manner

23. “complex” Contralteral – opposite sides KEY
To motor neurons in other segments of the spinal cord
“complex”
Contralteral –
opposite sides
Extensors inhibited
Flexors stimulated
Extensors stimulated
Flexors inhibited
KEY
Sensory neuron (stimulated)
Motor neuron (inhibited)
Excitatory interneuron
Inhibitory interneuron
Painful stimulus
Motor neuron (stimulated) 23

24. General Characteristics of Polysynaptic Reflexes
1. Involve pools of interneurons
2. Are intersegmental in distribution
3. Involve reciprocal inhibition
4. Have reverberating circuits
Which prolong reflexive motor response
5. Several reflexes cooperate
To produce coordinated, controlled response

25. The Brain Can Alter Spinal Reflexes
Reflex behaviors are automatic, but processing centers in the brain can facilitate or inhibit reflex motor patterns based in the spinal cord

26. Voluntary Movements and Reflex Motor Patterns
Neuronal pools in the spinal cord direct the autonomic motor patterns for walking, running and jumping
Higher centers of the brain incorporate these lower, reflexive motor patterns, “fine-tuning” the established patterns (think of an orchestrated dance, rather than a fundamental walk) – you really do learn how to be cool

27. Reinforcement or Inhibiton of Spinal Reflexes
Higher centers reinforce (facilitate) spinal reflexes by stimulating excitatory neurons in the brain stem or spinal cord (the reverse is true as well – inhibition)
Reinforcement results in an enhancement of spinal reflexes, which are too strong to be suppressed consciously

28. Babinski reflexes are normal in infants (toes extend)
As descending motor pathways develop, the response disappears in adults (toes flex with a response)
A Babinski sign in may indicate CNS damage in adults

29. What procedures are we doing?
Conduct several reflex tests
Record reaction time
Recognize how reaction time can be influenced

30. Resources available…
Anatomy and Physiology Laboratory Study Pages; submenu: Brain