LAB EXERCISE 14 SPINAL REFLEXES

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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