Date post: | 17-Jan-2016 |
Category: |
Documents |
Upload: | erick-allen |
View: | 215 times |
Download: | 0 times |
Spinal Cord Reflexes
Dr. Shaikh Mujeeb AhmedAssistant ProfessorAlMaarefa College
CNS BLOCK 424
Objectives
• Describe the components of spinal reflexes.• Enumerate different types of spinal cord
reflexes.• Explain the mechanism of spinal cord reflex
occurs and their control.• Explain the clinical conditions which can affect
the spinal cord reflex.
What is a reflex.
• Any response that occurs automatically without conscious effort.
What are the types of reflexes.
Number of synapse
Monosynaptic
Polysynaptic
Location of receptor
Superficial
Deep
Visceral
Development
Simple/Basic
Acquired
Reflex Arc
• Receptor
• Affrent Nerve
• Center
• Efferent Nerve
• Effector organ
Fig 13.1 – Monosynaptic and polysynaptic reflexes Silverthorn 2nd Ed
Stretch Reflex
• Sudden stretch to a muscle leads to contraction of that muscle is known as stretch reflex.
• The basic unit of this activity is a reflex arc comprises of;
1. Sense organ.2. Afferent neuron.3. Center4. Efferent neuron5. Effector organ.
Reflex Arc
Receptor• Muscle spindle
Afferent neuron
• Sensory Nerve
Center• Spinal Segment
Efferent neuron
• Motor Nerve
Effector Organ
• Skeletal Muscle
Receptor
• Muscle spindle
Afferent
neuron
• Ia fiber (primary)• II fiber (secondary)
Center
• Spinal Segment
Efferent
neuron
• Gamma motor neuron
Effector
Organ
• Intrafusal muscle fibers
Reflex arc
Skeletal Muscle Reflexes
• How do they work?– Receptors in muscle send info to CNS– CNS decides should muscle contract (or relax) in
response– CNS sends appropriate signal via somatic motor
neurons• Somatic motor neurons are always excitatory:
– CNS activates if contraction is right response– CNS inhibits if relaxation is right response
Receptors
• Muscle receptors:– Sense muscle length and/or tension
• Two types of stretch receptors:– Muscle spindles– Golgi tendon organs
Muscle Spindle Structure
• Consist of collections of specialized muscle fibers known as intrafusal fibers– Lie within spindle-shaped connective tissue
capsules parallel to extrafusal fibers– Each spindle has its own private efferent and
afferent nerve supply– Play key role in stretch reflex
Fig 13.3 – Sensory receptors in muscle Silverthorn 2nd Ed
Two types of intrafusal fibres:1. Nuclear bag fibres: central
area is dilated filled with group of nuclei (2 / spindle)
2. Nuclear chain fibres: smaller than nuclear bag fibres and have one line of nuclei spread in a chain along the receptor area (3-9 /spindle)
Receptor for the stretch reflex: Muscle Spindle
Muscle spindle
Golgi Tendon Organs
• Composed of:– Nerve fiber endings that wind between collagen
fibers inside connective tissue capsule• If muscle is stretched:
– Free nerve endings are pinched and they fire• Activation of Golgi tendon organs:
– Inhibits alpha motor neurons and decreases muscle contraction
Fig 13.6 – Muscle reflexes Silverthorn 2nd Ed
Muscle Spindle Function
Stretch Reflex
• Primary purpose is to resist tendency for passive stretch of extensor muscles by gravitational forces when person is standing upright
• Classic example is patellar tendon, or knee-jerk reflex
Patellar Tendon Reflex
The Golgi tendon reflex (inverse stretch reflex)
• excessive tension on the muscle (passive stretch of tendon or active muscle contraction) >> muscle relaxes opposite response to stretch reflex.
• The receptors are Golgi tendon organs in muscle tendons stimulated >> muscle contract and pulled on the tendon (tension)
• stimulate golgi organ>> A fibers > spinal cord > excitation of inhibitory interneuron>> inhibit alpha motor neuron > muscle relaxation
• Protect muscle from rupture
The Golgi tendon reflex (inverse stretch reflex)
Withdrawal reflex(flexor reflex)
• Stimulation of pain receptors in hand → impulses to spinal cord via A or C fibres → interneurons → anterior horn cells → stimulate hand flexor muscles → move the hand away from the injurious stimulus.
• Its a polysynaptic reflex. • Stimulation of flexors muscle accompanied by
inhibition of extensors.• Inhibitory inter neurons synapse with extensor
motor neurons known as reciprocal innervations (reciprocal inhibition).
Reciprocal inhibition
Crossed Extensor Reflex
Extension of opposite limb
Flexion & withdrawal of stimulated limb
Painful Stimuli
Fig 13.8 – Flexion reflex and the crossed extensor reflex Silverthorn 2nd Ed
Crossed extensor reflex
Clinical Importance of reflexes
• To test the integrity of reflex arc.• Localization of neurological lesion.• Identifying the type of lesion.• Monitoring the progress of neurological
deficit.
• Deep tendon reflexes are absent in lower motor neuron lesion.
• The become exaggerated in upper motor neuron lesions.
• Pendular jerks are observed in cerebellar lesions.
Clinical Importance of reflexes
Centers for reflexesReflex Center (root value)
Deep tendon reflex
Biceps Jerk and Brachioradialis C5/C6
Triceps Jerk C6/C7
Knee Jerk / Patellar reflex L2-L4
Ankle S1
Superficial reflex
Corneal reflex (blink reflex) 5th and 7th cranial nerve
Abdominal reflex T7 – T-12
Plantar reflex L5-S1