Somatic Reflex PDF

Preview

Summary

Download Somatic Reflex PDF

Description

Somatic Reflexes 1. Define a reflex and explain how reflexes differ from voluntary movement 2. List and describe the general components of a typical reflex arc 3. Describe the structure and explain the function of muscle spindles 4. Explain and illustrate how the stretch reflex functions by using the patellar tendon reflex as an example 5. Explain how the Golgi tendon reflex functions 6. Explain how the flexor withdrawal reflex functions 7. Explain how the crossed extensor reflex functions 8. Exemplify the combined functioning of the flexor withdrawal and crossed extensor reflex by using the example of stepping on glass with the right foot

Defining a Reflex       

A fast, involuntary and predictable sequence of actions by glands or muscles that occurs in response to a particular stimulus When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex (e.g. patellar tendon reflex) no involvement by the brain: lower motor neurons only When integration occurs in brain stem the reflex is a cranial reflex and involves cranial nerves (e.g. eye tracking movements) If the reflex involves the contraction of skeletal muscle as the effector it is called a somatic reflex If the reflex involves the contraction of smooth muscle, cardiac muscle or glands it is called an autonomic (visceral) reflex Voluntary movement is under our control, can be slow or fast, uses higher and lower motoneurons and is variable and not stereotyped

Somatic Reflexes

2. List and describe the components of a typical reflex arc 1. Sensory receptor • distal end of a sensory neuron (dendrite) 2. Sensory neuron • carries information from receptor to posterior horn of spinal cord or to the brainstem 3. Integrating centre (interneuron) • point of synaptic contact between neurons in gray matter of the spinal cord or brainstem

4. Motor neuron •

carries motor impulses from spinal cord to the skeletal muscles

5. Effector (skeletal muscle) •

the muscles that carry out the reflexive response. In somatic reflexes the effector is always skeletal muscle

3. Describe the structure and explain the function of muscle spindles •

Skeletal muscles are composed of individual muscle fibres that contract when stimulated by a motor neuron •

•

may be referred to as extrafusal fibres

Also other fibres found between extrafusal fibres: •

intrafusal fibres that form the muscle spindle apparatus

Somatic Reflexes

The muscle spindles. •

Stretch receptors embedded in skeletal muscles that monitors length of muscle and how fast muscles change in length

•

Alpha-gamma coactivation

•

Composed of: •

intrafusal muscle fibres (nuclear bag & nuclear chain) which are modified muscle cells that have sarcomeres and contractile ability but only at their two ends

•

sensory neurons

•

•

Primary afferent fibres (inform brain on muscle length & speed of muscle length change)

•

Secondary afferent fibres (inform brain on muscle length)

gamma motor-neurons •

Keep spindle fibres at good length for responding to stretch

•

Motoneurons elsewhere in body called alpha motoneurons

Somatic Reflexes

4. stretch reflex functioning using patellar tendon reflex as an example. The stretch reflex •

When a muscle is stretched, it automatically contracts and maintains increased tone •

helps maintain equilibrium (balance) and posture

•

stabilizes joints by balancing tension in extensors and flexors smoothing muscle actions

•

Examples: •

head starts to tip forward as you fall asleep; muscles contract to raise the head

•

knee-jerk (patellar-tendon) reflex

The Patellar Tendon Reflex Arc

The Stretch Reflex

Somatic Reflexes •

A monosynaptic reflex •

•

Sensory neuron synapses directly onto motor neuron in the ventral horn

Reciprocal inhibition prevents muscles from working against each other • •

Agonist muscle contracted (excitation) Antagonist muscle relaxed

5. Explain how the Golgi tendon reflex functions Golgi Reflex Tendon •

Monitor tension in tendons produced by muscle contraction

•

Sensory neurons activated by these receptors input onto inhibitory interneurons that inhibit alpha motoneurons

•

Prevents excessive muscle contractions

•

Also functions when muscle contracts unevenly

•

Has the opposite action on the agonist muscle (inhibition) when compared to a stretch reflex (excitation)

•

tendon organs – proprioceptors in a tendon near its junction with a muscle •

•

Golgi tendon organ - 1mm long, nerve fibers entwined in collagen fibers of the tendon

tendon reflex – in response to excessive tension on the tendon •

inhibits agonist muscle from excessive tension

•

moderates muscle contraction before it tears a tendon or pulls it loose from the muscle or bone

Somatic Reflexes

Summary of stretch reflex and Golgi tendon reflex actions •

Skeletal muscles send sensory information to CNS about •

muscle length, from spindle apparatus

•

muscle tension, from Golgi tendon organ

•

Stretch of muscle spindle activates stretch reflex arc to correct muscle tone and prevent over-stretching

•

Activation of Golgi organ inhibits alpha motor neurons to relax muscle & prevent overcontraction

6. Explain how the flexor withdrawal reflex functions Flexor (withdrawal) Reflex •

This reflex is to protect from damage in response to a painful stimulus

•

The flexor reflex is a polysynaptic reflex arc which involves interneuron/s in the arc

•

The flexor reflex is ipsilateral as the stimulus (stepping on glass) and response (muscle contraction) are on the same side of the body

•

Muscle spindle apparatus & Golgi tendon organ are not involved

2

Sensory neuron activates multiple interneurons

+ + +

+

+

+

+ +

5 3

Ipsilateral motor neurons to flexor excited

4

Ipsilateral flexor contracts

+

Contralateral motor neurons to extensor excited

6 +

1

withdrawal of right leg (flexor reflex)

Contralateral extensor contracts

Stepping on glass stimulates pain receptors in right foot Extension of left leg (crossed extension reflex)

7. Explain how the crossed extension reflex functions Crossed Extensor Reflex •

Flexor Withdrawal Reflexes only useful if can still maintain balance which is maintained by extending other leg

•

Injured leg – flexors (hamstrings) contract and the extensors (quads) relax to lift the leg from the ground

•

Supporting leg

Somatic Reflexes

– flexors relax and extensors contract to stiffen the leg so it can support weight of the body – The crossed extensor reflex is a polysynaptic reflex arc which involves interneuron/s in the arc

Somatic Reflexes

Stepping On Glass Example •

Stepping on glass with the right foot stimulates the dendrites (pain receptors) of a painsensitive neuron

•

The axon of this neuron then travels to spinal cord through dorsal side (dorsal root then dorsal horn) and synapses on multiple interneurons

•

On ipsilateral side of cord an excitatory interneuron stimulates a motoneuron which travels through ventral side (ventral horn then ventral root) to stimulate flexor muscles (hamstrings) which flex the knee joint and lift the leg away from the painful stimulus

•

Also on ipsilateral side of cord an inhibitory interneuron inhibits the alpha motoneuron to the ipsilateral extensor muscles (quadriceps) thus allowing knee joint to flex

•

Interneurons synapsing on the incoming sensory input also cross-over to opposite side of grey matter

•

Here on the contralateral side of cord inhibitory interneurons synapse on an alpha motoneuron which inhibits (prevents) activity in the flexor muscles (hamstrings) of the contralateral leg

•

Also on the contralateral side of the cord an excitatory interneuron excites an alpha motor neuron which travels out ventral horn and ventral root to activate extensor muscles (quadriceps) on contralateral side which extends the knee joint of the left leg and allows weight to be born on that leg

•

Note that reflex arcs often include reciprocal innervation to stimulate a flexor muscle and inhibit the corresponding extensor (or vice versa). Withdrawal reflexes involve regulation of both ipsilateral and contralateral muscles...