SPINAL REFLEX & PROPERTIES

SPINAL REFLEX & PROPERTIES

• Reflex – Involuntary response to stimulus.

• Mediated through a chain of neurons passing through CNS.

• Sensory impulse is automatically converted into a motor effect through CNS involvement.

COMPONENTS OF REFLEX ARC:

1. Receptor organ:

• Stimulation of receptor organ starts reflex.

• Eg: Stretch reflex starts with stimulation of muscle spindle.

2. An afferent neuron:

• Carries stimulus to CNS.

• Eg: Primary afferent in stretch reflex

3. Interneurons (Internuncial neuron):

• Take signals from sensory (afferent) neuron

• Pass it to either motor (efferent) neuron (in bisynaptic reflex) or to other interneurons (in polysynaptic reflex). 

• An interneuron can be excitatory or inhibitory.

• Interneuron is not involved in monosynaptic reflex (e.g., stretch reflex).

4. An efferent neuron:

• Carries response signal to effector organ.

• Eg: α-motor neuron supplies muscle fibers

5. Effector organ:

• Shows reflex response.

• Eg: Muscle contraction in stretch reflex

DIVISIONS: 

• Components of reflex arc can be divided into 3 parts:

1. Afferent limb:

• Includes receptor organ & afferent (sensory) neuron.

2. Center:

• Part of CNS (e.g., interneuron in spinal cord).

3. Efferent limb:

• Includes efferent (motor) neuron & effector organ.

TYPES OF REFLEXES:

1. Based on number of synaptic junctions involved:

• Reflexes may be divided into 3 types:

• Monosynaptic, Bisynaptic & Polysynaptic.

• Simplest form of reflex arc consisting of two neurons.

• An afferent neuron carries sensory input from receptor to CNS & synapses with motor neuron whose axon supplies an effector organ.

• Contains only one synapse; Hence, “Monosynaptic” reflex.

• Only example of monosynaptic reflex in body is “Stretch reflex” like knee-jerk & ankle jerk.

1b) Bisynaptic:

• When one interneuron is involved in reflex, there are two synapses in the arc.

• One between afferent neuron terminal & proximal end of interneuron.

• Another between interneuron terminal and proximal end of efferent neuron; Hence, Bisynaptic reflex.

– Eg: Golgi-tendon organ reflex (Inverse Stretch Reflex).

1c) Polysynaptic:

• When intervention of multiple interneurons between afferent & efferent is involved.

– Eg: Flexor reflex (“Withdrawal reflex) & Crossed Extensor reflex. 

2. Based on sensory signal pathway:

• In every reflex, the afferent neuron carries the impulse to spinal cord. 

• After entering spinal cord, sensory signals may follow many routes of transmission.

2a) Suprasegmental reflex:

• May ascend to a higher level of CNS through various sensory tracts & reach thalamus & sensory cortex

2b) Local segmental reflex:

• Signals may terminate locally in grey matter of spinal cord at same segment & elicit local response.

– Most of clinical reflexes are this type. Typical example is stretch reflex.

2c) Intersegmental reflex:

• Signals may ascend a few or more segments & then terminate in grey matter to produce intersegmental response. 

2d) Spinal reflexes

• Local segmental reflex & intersegmental reflex are mediated entirely by spinal cord, without any participation of brain.

• These are referred as “Spinal reflex”.

Bell-Magendie Law:

• Principle wherein spinal cord dorsal roots are sensory & ventral roots are motor.

PROPERTIES OF SPINAL REFLEX:

1. Delay:

• Between application of stimulus & start of response, time interval between us referred as “Delay/Reaction time”.

Determined by two type of reaction:

• Peripheral delay – Time taken by impulse transmission in afferent & efferent limbs of reflex.

• Central delay – Time spent by impulse in traversing synapse/synapses in spinal cord.

2. Subliminal fringe:

• Tension produced in a muscle by simultaneous stimulation of two afferents may be more than sum of responses produced separately.

• This is due to subliminal fringe.

3. Summation:

• Summation is due summation of EPSP in synapses (at post-synaptic neuron)

• Types: Temporal & Spatial summation

4. Adaptation:

Habituation & Sensitization:

• Habituation – Repeated application of non-injurious stimuli may show decreased intensity of response in withdrawal reflex.

• Sensitization – Repeated application of injurious stimuli may show intensification of response.

• Neurotransmitter release at synapse is decreased in habituation & increased in sensitization.

5. Fatigue:

• Fatigue of synptic transmission can cause fatigue of stretch reflex, if the muscle spindle k stimulated repetitively.

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