MOTOR SYSTEM
REFLEXES
Definition - A reflex is a stereotype reaction of the CNS to sensory stimulus
Types - There are 3 types of reflexes; Monosynaptic, Bisynaptic and polysnaptic
Reflex arc
This is the basic unit of integrated neural activity
Basic Components of a Reflex Arc
All spinal reflexes share a fundamental wiring pattern called the reflex arc:
1. Receptor – detects the stimulus (e.g., muscle spindle, pain receptor).
2. Afferent (sensory) neuron – carries the signal into the spinal cord via the dorsal root.
3. Integration center – usually interneurons in the gray matter of the spinal cord (except in
monosynaptic reflexes where there is direct synapse).
4. Efferent (motor) neuron – sends the command out of the ventral root to the effector organ.
5. Effector – usually a skeletal muscle that contracts or relaxes.
N/B In humans, the connection between the afferent and efferent neuron is in the spinal cord or brain
Bell Magendie's Law - This is the principle which states that in the spinal cord, the dorsal roots are
sensory and the ventral roots are motor.
At the synapse, receptor and myoneural junction in the arc, there is a non-propagated graded
response that is proportional to the magnitude of the stimulus. Whereas, in the portion of the arc
specialized for transmission (i.e axons and muscle membranes) the response is all or none action
potential
Classification of Spinal Reflexes
Spinal reflexes can be classified according to:
Number of synapses:
Monosynaptic reflexes – one synapse (e.g., stretch reflex)
Bisynaptic reflexes – two synapses (rare).
Polysynaptic reflexes – more than two synapses (e.g., withdrawal reflex).
Function:
, Somatic reflexes – act on skeletal muscle.
Autonomic reflexes – act on smooth muscle, cardiac muscle, or glands.
Laterality:
Ipsilateral reflexes – response on the same side as stimulus.
Contralateral reflexes – response on the opposite side (e.g., crossed extensor reflex).
Monosynaptic Reflexes
Definition
A monosynaptic reflex is the simplest type of reflex arc involving only one synapse between a sensory
(afferent) neuron and a motor (efferent) neuron. There are no interneurons in the pathway.
Physiological Example
Stretch Reflex (aka. myotatic reflex / deep tendon reflex)
This reflex maintains muscle tone and posture by resisting sudden muscle length
changes.
Type: Monosynaptic, ipsilateral.
Stimulus: Stretch of a muscle.
Receptor: Muscle spindle.
Response: Contraction of the same muscle to resist further stretch.
Example: Knee jerk (patellar) reflex.
Function: Maintains muscle tone and posture.
Circuit Pathway
1. Stimulus: Sudden stretch of a muscle (e.g., tapping the patellar tendon stretches quadriceps).
2. Receptor: Muscle spindle detects stretch.
3. Afferent pathway: Group Ia sensory fibers carry the impulse via dorsal root ganglion to the spinal
cord.
4. Central synapse: Ia fiber makes a direct excitatory synapse with the alpha motor neuron in the
anterior horn.
5. Efferent pathway: Alpha motor neuron sends an impulse back to the same muscle.
REFLEXES
Definition - A reflex is a stereotype reaction of the CNS to sensory stimulus
Types - There are 3 types of reflexes; Monosynaptic, Bisynaptic and polysnaptic
Reflex arc
This is the basic unit of integrated neural activity
Basic Components of a Reflex Arc
All spinal reflexes share a fundamental wiring pattern called the reflex arc:
1. Receptor – detects the stimulus (e.g., muscle spindle, pain receptor).
2. Afferent (sensory) neuron – carries the signal into the spinal cord via the dorsal root.
3. Integration center – usually interneurons in the gray matter of the spinal cord (except in
monosynaptic reflexes where there is direct synapse).
4. Efferent (motor) neuron – sends the command out of the ventral root to the effector organ.
5. Effector – usually a skeletal muscle that contracts or relaxes.
N/B In humans, the connection between the afferent and efferent neuron is in the spinal cord or brain
Bell Magendie's Law - This is the principle which states that in the spinal cord, the dorsal roots are
sensory and the ventral roots are motor.
At the synapse, receptor and myoneural junction in the arc, there is a non-propagated graded
response that is proportional to the magnitude of the stimulus. Whereas, in the portion of the arc
specialized for transmission (i.e axons and muscle membranes) the response is all or none action
potential
Classification of Spinal Reflexes
Spinal reflexes can be classified according to:
Number of synapses:
Monosynaptic reflexes – one synapse (e.g., stretch reflex)
Bisynaptic reflexes – two synapses (rare).
Polysynaptic reflexes – more than two synapses (e.g., withdrawal reflex).
Function:
, Somatic reflexes – act on skeletal muscle.
Autonomic reflexes – act on smooth muscle, cardiac muscle, or glands.
Laterality:
Ipsilateral reflexes – response on the same side as stimulus.
Contralateral reflexes – response on the opposite side (e.g., crossed extensor reflex).
Monosynaptic Reflexes
Definition
A monosynaptic reflex is the simplest type of reflex arc involving only one synapse between a sensory
(afferent) neuron and a motor (efferent) neuron. There are no interneurons in the pathway.
Physiological Example
Stretch Reflex (aka. myotatic reflex / deep tendon reflex)
This reflex maintains muscle tone and posture by resisting sudden muscle length
changes.
Type: Monosynaptic, ipsilateral.
Stimulus: Stretch of a muscle.
Receptor: Muscle spindle.
Response: Contraction of the same muscle to resist further stretch.
Example: Knee jerk (patellar) reflex.
Function: Maintains muscle tone and posture.
Circuit Pathway
1. Stimulus: Sudden stretch of a muscle (e.g., tapping the patellar tendon stretches quadriceps).
2. Receptor: Muscle spindle detects stretch.
3. Afferent pathway: Group Ia sensory fibers carry the impulse via dorsal root ganglion to the spinal
cord.
4. Central synapse: Ia fiber makes a direct excitatory synapse with the alpha motor neuron in the
anterior horn.
5. Efferent pathway: Alpha motor neuron sends an impulse back to the same muscle.
