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Nervous System
Exercises 22 and 23
Reflexes
• Reflexes are fast, predictable, automatic,
subconscious responses to changes
inside or outside the body.
• Somatic reflexes involve contraction of
skeletal muscle
• Autonomic reflexes involve responses of
smooth muscle, cardiac muscle and
glands.
• Pathway – route followed by a series of
nerve impulses from origin to destination
• Reflex arc – simplest type of pathway
– Receptor – distal end of a sensory neuron or
associated structure
– Sensory neuron
– Integrating center
• Synapse of sensory neuron with motor
neuron = monosynaptic reflex arc
• One or more interneurons (association
neurons) = polysynaptic reflex arc
– Motor neuron
– Effector – part of the body that responds
• The action of the effector is the reflex
• We can use reflexes to test pathways for
damage
• Somatic reflexes are usually easy to test
• Autonomic reflexes are more difficult –
exception is the pupillary light reflex
Knee-jerk (patellar tendon or stretch)
reflex
• Monosynaptic reflex
• Can be tested at elbow, wrist, knee or ankle
joints
• Stretch of tendon stimulates receptors called
muscle spindles which monitor changes in the
length of the muscle
• Impulses enter cord through sensory neuron
• Sensory neuron synapses with motor neuron
in anterior gray horn
• If excitation is great enough, motor neuron send
impulses out through ventral root to same
muscle that activated the spindle and causes it
to contract.
• Since nerves enter and leave the same side of
the spinal cord, this is an ipsilateral reflex arc.
All monosynaptic reflexes are ipsilateral.
• A polysynaptic reflex occurs to the
antagonistic muscle at the same time. The
sensory neuron also synapses with an
inhibitory association neuron for the
antagonistic muscle. This is called
reciprocal innervation.
• Axon collaterals from the sensory neuron
also synapses with cells that carry
information about the state of this muscle
to the brain.
The Flexor (withdrawal) reflex
• Polysynaptic reflex
• Stepping on a tack stimulates pain receptors
• Sensory neuron synapses with association
neuron in spinal cord.
• Association neuron activates motor neurons in
several spinal cord segments, which leave
through anterior root and innervate several
muscles, causing flexor muscles in the thigh to
contract, withdrawing foot from source of pain.
• This reflex is also ipsilateral.
• Because it activates association neurons
in several segments of the spinal cord, it is
an intersegmental reflex arc.
• Also exhibits reciprocal innervation.
Crossed Extensor Reflex
• Stimulation of pain sensing neuron in right foot.
• Sensory neuron sends impulses into spinal cord
• In cord, neuron activates several association
neurons that synapse with motor neurons on
the left side of the spinal cord in several spinal
segments.
• Association neurons activate motor neurons
that cause the extensor muscles of the left leg
to contract to support the body.
• Contralateral reflex arc
• Also reciprocal innervation.
General Senses
All senses work the same way:
Receptors collect information
– stimulate neurons
-- information is sent to the brain
– the cerebral cortex integrates the information
with that from other senses
-- forms a perception (a person’s particular
view of the stimulus)
Receptor types:
• Pain receptors or nociceptors – respond to
tissue damage due to mechanical, electrical,
thermal or chemical energy
• Thermoreceptors
– respond to temperature change
•Mechanoreceptors – respond to mechanical forces,
such as pressure or fluid movement; changes
usually deform the receptor
Proprioceptors – sense changes in muscles
and tendons
Baroreceptors – in blood vessels – detect
changes in pressure
Stretch receptors – in lungs – sense degree of
inflation
Photoreceptors
-respond to light – as little as one photon
Chemoreceptors
– sensitive to chemical concentration
of various substances
Receptors are structured in two basic ways:
receptors can be nerve endings
or other kinds of cells which are associated
with nerve endings
When these are stimulated, they produce graded
potentials. If hit threshold, nerve fires.
A sensation or perception occurs when the brain
interprets the incoming nerve impulses.
All impulses coming into the brain are alike.
The sensation depends on which part of the brain
is stimulated.
Synesthesia – tasting colors, etc.
Sensory adaptation
The only receptors that don’t adapt are:
pain receptors
Somatic Senses:
Exteroceptive senses – changes at
body surface
Proprioceptive senses – changes in
muscles and tendons and body
position
Visceroceptive senses – changes in
viscera
Touch and pressure senses:
1. Free nerve endings – touch and pressure
2. Meissner’s corpuscles – light touch
receptors are connective tissue
3. Pacinian corpuscles – heavy pressure
and vibrations
receptors are connective tissue
Itch and Tickle:
Receptors are free nerve endings
Temperature senses:
Free nerve endings in skin
Heat receptors –
respond primarily between 25 – 45 o C
or 77 – 113 o F
unresponsive above, but pain receptors
fire = burning
Cold receptors –
respond primarily between 10 – 20 o C
or 50 - 68 o F
unresponsive below, but pain receptors
fire = burning
Pain :
Also free nerve endings
Most pain receptors can be stimulated by more than
one stimulus, although some are more sensitive to
mechanical damage, and others to extreme
temperature, or chemicals.
Deficiency of blood flow (ischemia) and thus a deficiency
of oxygen (hypoxia) can stimulate pain receptors.
Visceral pain:
Pain receptors are the only receptors in the viscera
that produce sensations.
Tends to be referred pain – feels as though in comes
from elsewhere – due to common nerve pathways
Phantom pain – comes from a limb that has been amputated.
Pain fibers are of two types:
Acute pain fibers ( A or delta fibers) – thin, myelinated fibers
(Conducts up to 30 meters/sec)
Sharp, localized pain
Seldom continues after stimulus stops
Chronic pain fibers (C fibers) – thin, unmyelinated fibers
(conduct up to 2 meters per second)
Dull, aching and widespread pain
May continue for some time after stimulus
Stretch receptors:
We know how our body parts are moving
through our proprioceptive or kinesthetic sense.
These receptors adapt only slightly
Keep brain informed of the status of body
parts to insure coordination.
Use specialized receptors that sense tension in
tendons and muscles.
No sensation occurs when these are stimulated.
Muscle spindles sense stretching
of muscle, and cause contraction
Of the muscle to maintain position.
Golgi tendon organs sense stretching of
tendons and cause the muscle to relax
to prevent damage to the tendon.