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