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 endingsor other kinds of cells which are associated

with nerve endingsWhen 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 adaptationThe only receptors that don’t adapt are:

pain receptors

Somatic Senses:Exteroceptive senses – changes at

body surfaceProprioceptive 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 touchreceptors 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 Funresponsive above, but pain receptors

fire = burning

Cold receptors –respond primarily between 10 – 20 o C

or 50 - 68 o Funresponsive 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 deficiencyof 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 painSeldom continues after stimulus stops

Chronic pain fibers (C fibers) – thin, unmyelinated fibers (conduct up to 2 meters per second)Dull, aching and widespread painMay 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 contractionOf the muscle to maintain position.

Golgi tendon organs sense stretching of tendons and cause the muscle to relax to prevent damage to the tendon.