Reflexes
One of the principal functions of the spinal cord related to homeostasis is an integrating center for spinal
reflexes. Reflexes are fast, predictable automatic responses to changes in the environment. Reflexes show
stereotypical responses i.e., they do not show variability. A few cranial reflexes involve the brain stem and
cranial nerves. Spinal reflexes, such as somatic reflexes involve contraction of skeletal muscles and autonomic
(visceral) reflexes involve responses of smooth muscle, cardiac muscle and glands. Reflexes are not consciously
perceived.
Reflex Arc and Homeostasis
A series of neurons in a pathway that include at least one synapse within the CNS is known as a reflex arc.
Reflexes permit the body to make exceedingly rapid adjustments to homeostatic imbalances. Reflexes operate to
maintain homeostasis.
A reflex arc has five functional components:
1. Receptor or Sense Organ
The distal end of a sensory neuron (dendrite) or an associated sensory structure serves as a receptor. It
responds to a specific stimulus, a change in the internal or external environment. If the stimulus causes a
threshold level of depolarization, it will trigger nerve impulses.
2. Sensory neuron
The nerve impulses propagate from the receptor through the dendrite, cell body and axon of the sensory
neuron bringing the nerve impulse into the gray matter of the spinal cord or brain stem.
3. Integrating center
This is within the gray matter of the CNS. The nerve impulse passes to the association neurons or motor
neurons. A reflex pathway that has only one synapse in the CNS, thus no association neuron, is a
monosynaptic reflex arc. If there are one or more association neurons between the sensory and motor
neurons a polysynaptic reflex arc is present.
4. Motor neuron
Impulses triggered by the integrating center propagate from the spinal cord along the motor neurons to
that part of the body that will respond.
5. Effector
The part of the body that responds to the motor nerve impulse, such as a muscle or a gland is the effector.
Its action is a reflex to the stimulus. If the effector is skeletal muscle the reflex is a somatic reflex. If the
effector is smooth muscle, cardiac muscle or a gland, the reflex is an autonomic (visceral) reflex.
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Somatic Spinal Reflexes
Physiology of the Stretch Reflex
The stretch reflex is a monosynaptic reflex arc. Only two types of neurons (one sensory and one motor) are
involved and there is only one CNS synapse in the pathway. This reflex operates as a negative feedback
mechanism to control muscle length by causing contraction of a muscle (effector) when it is stretched.
A stretch reflex operates as follows :
1. Slight stretching of a muscle stimulates receptors in the muscle, the muscle spindles.
The muscles spindles monitor changes in the length of the muscle.The muscles spindles are a type of
proprioceptors.
2. In response to being stretched, a muscle spindle generates nerve impulses that propagate along a somatic
sensory neuron through the posterior root of the spinal nerve into the spinal cord.
3. In the spinal cord (integrating center), the sensoy neuron makes an excitatory synapse with and thus
activates a motor neuron in the anterior gray horn of the spinal cord.
4. If the excitation is strong nerve impulses arise in the motor neuron and propagate along its axon from the
spinal cord into the anterior root of the spinal nerve. The axon terminals of the motor neuron form the
neuromuscular junctions (NMJs) with skeletal muscle cells of the same muscle that contains the activated
muscle spindles.
5. Acetylcholine released by nerve impulses at the NMJs triggers muscle action potentials in the stretched
muscle (effector) and the muscle contracts. Thus muscle stretch is followed by muscle contraction which
relieves the stretching of the muscle that had been stretched.
If sensory nerve impulses enter the spinal cord on the same side where motor nerve impulses leave it, this is an
ipsilateral reflex arc. All monosynaptic reflex arc are ipsilateral.
The stretch reflex is monosynaptic (two neurons and one synapse), polysynaptic reflex arc to the antagonistic
muscle occurs at the same time. An axon collateral (branch) from the sensory neuron synapses with an
inhibitory association neuron in the integrating center. The association neuron synapses with and inhibits a
motor neuron that excites the antagonistic muscles. When the stretched muscle contracts during a stretch reflex,
antagonistic muscles that oppose the contraction relax.
This type of neural circuit, which simultaneously causes contraction of one muscle and relaxation of its
antagonists, is termed reciprocal innervation. Reciprocal innervation prevents conflict between opposing
muscles and is vital in coordinating body movements.
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Physiology of the Tendon Reflex
The tendon reflex operates as a negative feedback mechanism to control muscle contraction by causing muscle
relaxation. This protects the muscle against excessive contraction.The tendon reflex is ipsilateral.The receptors
for this reflex are the tendon (Golgi tendon) organs that lie within a tendon near its junction with a muscle.
Whereas muscle spindles are sensitive to changes in muscle length, tendon organs respond to changes in muscle
contraction caused by muscular contraction.
A tendon reflex operates as follows:
1. As the tension applied to a tendon increases, the tendon organ (receptor) is stimulated (depolarized to
threshold).
2. Nerve impulses are generated and propagated into the spinal cord along sensory neuron.
3. Within the spinal cord (integrating center), the sensory neuron activates an inhibitory association neuron
that synapses with a motor neuron.
4. The inhibitory neurotransmitter inhibits (hyperpolarizes) the motor neuron, which then generates fewer
nerve impulses.
5. The muscle attached to the tendon relaxes and relieves excess contraction.
The tendon reflex protects the tendon and muscle from damage due to excessive contraction.
The sensory neuron from the tendon organ also synapses with an excitatory association neuron in the spinal
cord. The excitatory association neuron, in turn, synapses with motor neurons controlling antagonistic muscles.
The tendon reflex brings about relaxation of the muscle attached to the tendon organ, it also brings about
contraction of the antagonists. Thus another example of reciprocal innervation.
Physiology of the Flexor ( Withdrawal ) Reflex
The flexor (withdrawal ) reflex is another example of a polysynaptic reflex. If you step on a tack, you
immediately withdraw your leg.
A flexor reflex operates as follow:
1. Stepping on a tack stimulates pain receptors.
2. The pain receptors then generate nerve impulses which are propagated along the sensory neuron into the
spinal cord.
3. Within the spinal cord (integrating center), the sensory neuron activates association neurons that extend
to several spinal cord segments.
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4. The association neurons activate motor neurons in several spinal cord segments. The motor neurons
generate nerve impulses which are propagated toward the axon terminals.
5. Acetylcholine released by the motor neurons causes the flexor muscles in the thigh (effectors) to
contract, withdrawing the leg. The reflex is protective as contraction of the flexor muscles moves the
limb to avoid pain.
The flexor reflex is ipsilateral. Incoming and outgoing impulses are on the same side of the spinal cord.
The flexor reflex illustrates another feature of polysynaptic reflex arc. When you withdraw the limb from a
painful stimulus, more than one muscle is involved. Several motor neurons simultaneously carry impulses to
several limb muscles. Nerve impulses from one sensory neuron through association neurons ascend and descend
in the spinal cord and activate association neurons in different segments of the spinal cord. This is a
intersegmental reflex arc. Through intersegmental reflex arcs, one sensory neuron can activate several motor
neurons and stimulate more than one effector. This is an example of a divergent neuronal pool. The
monosynaptic stretch reflex involves only muscles receiving nerve impulses from one spinal cord segment.
Physiology of the Crossed Extensor Reflex
Something else happens when you step on a tack. You start to lose your balance as your body weight shifts to
the other foot. Automatically reflexes occur to regain balance so you do not fall. The pain impulses from
stepping on the tack not only initiate the flexor reflex that causes you to withdraw the limb but also initiate the
balance-maintaining crossed extensor reflex.
A crossed extensor reflex occur as follow:
1. Stepping a on a tack stimulates pain receptors in the right foot.
2. The pain receptors generate nerve impluses which are propagated in the sensory neuron, into the spinal
cord.
3. Within the spinal cord (integrating center), the sensory neuron activates several association neurons that
synapse with motor neurons on the left side of the spinal cord in several spinal cord segments. Incoming
pain signals cross to the opposite side through association neurons at that level and several levels above
and below the point of entry into the spinal cord.
4. The association neurons excite motor neurons in several spinal cord segments. The motor neurons
propgate nerve impulses toward the axon terminals.
5. Acetylcholine released by the motor neurons causes extensor muscles in the thigh (effectors) of the
unstimulated left limb to contract, producing extension of the left leg. This allows weight to be placed on
the foot of the limb that must support the entire body.
The crossed extensor reflex involves a contralateral reflex arc, sensory impulses enter one side of the the spinal
cord and motor impulses exit on the opposite side. A crossed extensor reflex synchronizes withdrawal (f1exion)
of the stimulated limb with extension of the contralateral limb.
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Reciprocal innervation also occurs in both the flexor reflex and the cross extensor reflex. In the flexor reflex
the flexor muscles are contracting, the extensor muscles of the same limb are relaxing to some degree.
In the crossed extensor reflex, the flexor muscles of the limb that has been stimulated by the tack are contracting
to produce withdrawal, the extensor muscles of the opposite limb are contracting to help maintain balance.
Some stimulus (stress) disrupts homeostasis
by causing an imbalance in a
Controlled condition
Appropriate muscle length or
or tension or secretion by a gland
Receptors
Distal ends of sensory neurons or
associated sensory structures
Input
Nerve impulses along
sensory neurons
Control center
Integrating center within CNS relays
impluses from sensory neruons to motor neuron
usually through association neurons
Output
Nerve impulses along
motor neurons
Effectors
Muscle or gland that responds
to motor neruon impulses
Response
Action produced by a
muscle or gland
Relationship of Reflexes to Homeostasis
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Return to homeostasis
when reflex brings muscular
or glandular status
(the controlled condition)
back to normal
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