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

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

Laboratory Lecture

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

“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

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

“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

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

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

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

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

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

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

Resources available… Anatomy and Physiology Laboratory Study Pages; submenu: Brain http://ctle.hccs.edu/biologylabs/AP1/AP1index.html