The Reflex Arc
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Reflexes are an automatic and rapid response
to a particular stimulation
If the command centre for the reflex is located
in the brain – cerebral reflex
If the command centre for the reflex is located
in the spinal cord – spinal reflex
Autonomic and Somatic Reflexes
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Autonomic reflexes are mediated by the
autonomic division of the nervous system
Involve activation of smooth and cardiac
muscles as well as glands
Regulate bodily functions such as digestion,
elimination, blood pressure, salivation, and
sweating
Somatic Reflexes involve stimulation of
skeletal muscles
The Reflex Arc
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The name given to
the pathway along
which the initial
stimulus and the
corresponding
response message
travel
There are 5 parts to a reflex arc:
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1. Receptor: receives the initial stimulus i.e. A
loud noise, pinprick to the skin)
2. Sensory nerve: carries the impulse to the
spinal column or brain (aka afferent nerve)
3. Intermediate nerve fibre: interprets the
signal and issues and appropriate response
Cont...
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4. Motor nerve: carries the response message
from the spinal cord to the muscle or organ (aka
efferent nerve)
5. Effector organ: carries out the response
(i.e. Skeletal muscle – moving your hand away
from danger)
Proprioceptors and the Control of
Movement
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Proprioceptors – specialized receptors found
in tendons, muscles, and joints
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Provide sensory information about the state of
muscle contraction, the position of limbs, and body
posture and balance
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This feedback is provided primarily by afferent
(sensory) input from two sensory receptors: tendon
organs and muscle spindles
Golgi Tendon Organs
(Tension Reflex)
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Sensory receptors that terminate where
tendons joint to muscle fibres
Since they are aligned with muscle, any
stretching of the muscle also stretches the GTO
The job of GTO is to detect increased tension
exerted on the tendon
GTO cont...
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When a change in tension is detected an
impulse is sent along afferent (sensory)
neurons to the CNS
The efferent (motor) neurons transmit an
impulse causing the muscle to relax
This prevents injury
Muscle Spindles
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Lie parallel to the
muscle fibre
Send constant signals
to the spinal cord
Help maintain muscle
tension
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Unlike GTO, they are
sensitive to changes
in muscle length
rather than tension
Muscle Spindles cont...
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Contains two afferent and one efferent nerve
fibres
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Two sensory nerves explains high level of
sensitivity as well as the critical role they play in
regulating muscle contraction
Responds to changes in length by sending a
message to the spinal cord
Resulting contraction allows the muscle to
maintain proper muscle tension or tone (i.e.
Erect posture)
The Stretch Reflex
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Simplest spinal reflex
Depends only on the
single connection
between primary
afferent fibres and
motor neurons of the
same muscle
i.e. Knee-jerk test
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1. The receptor muscle senses the action of
the hammer against the patella ligament
through the muscle spindle's sensory neuron
2. The message is transmitted along the
afferent (sensory) nerve axon to the spinal cord
3. The afferent neuron synapses with the
efferent pathway (motor neuron) of the same
muscle
4. An impulse is transmitted along the efferent
pathway (motor neuron) to the muscle
5. The motor units contract (knee-jerk to
accomodate additional stretch)
Reciprocal Inhibition
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During a reflex, the opposing muscle group is
simultaneously stimulated
In a knee-jerk reflex the quadriceps contract to
extend the knee while the hamstrings are
inhibited in a slightly delayed response (they do
not flex)
Polysynaptic Reflexes
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With other spinal reflexes, one or more
interneurons lie between the primary sensory
fibres and the motor neurons
The more interactions involved, the more
complex and slow the reflex
The Withdrawal Reflex
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The withdrawal of a body part from a painful
stimulus
Reflex action involves transferring the impulse
from a sensory neuron to a motor neuron
through a connecting interneuron in the spinal
cord
Extremely rapid, brain doesn't have time to
interpret information