Download Sensory Pathways and the Somatic Nervous System

FUNDAMENTALS OF ANATOMY AND PHYSIOLOGY
Mrs. Willie Grant, Instructor
[email protected]
(210) 485-2870
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Chapter 15
Sensory Pathways
and the Somatic
Nervous System
Lecture Presentation by
Lee Ann Frederick
University of Texas at Arlington
© 2012 Pearson Education, Inc.
An Introduction to Sensory Pathways and the Somatic Nervous System
Learning Outcomes
15-1 Specify the components of the afferent and efferent divisions of the
nervous system, and explain what is meant by the somatic nervous
system.
15-2 Explain why receptors respond to specific stimuli, and how the
organization of a receptor affects its sensitivity.
15-3 Identify the receptors for the general senses, and describe how they
function.
15-4 Identify the major sensory pathways, and explain how it is possible to
distinguish among sensations that originate in different areas of the
body.
15-5 Describe the components, processes, and functions of the somatic
motor pathways, and the levels of information processing involved in
motor control.
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15-1 Sensory Information
Sensory Receptors
Specialized cells that monitor specific conditions in the body or external
environment
When stimulated, a receptor passes information to the CNS in the form of
action potentials along the axon of a sensory neuron
Sensory Pathways
Deliver somatic and visceral sensory information to their final destinations
inside the CNS using: Nerves
Nuclei
Tracts
1 Specialized cells that monitor specific conditions in the body or the external
environment are called ____________.
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15-1 Sensory Information
Somatic Motor Portion of the Efferent Division
Controls peripheral effectors
Somatic Motor Commands
Travel from motor centers in the brain along somatic motor pathways of:
Motor nuclei
Tracts
Nerves
Somatic Nervous System (SNS)
Motor neurons and pathways that control skeletal muscles
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15-2 Sensory Receptors
The Detection of Stimuli
Receptor specificity—each receptor has a characteristic sensitivity
Receptive field—area is monitored by a single receptor cell
The larger the receptive field, the more difficult it is to localize a
stimulus
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15-2 Sensory Receptors
Adaptation is the reduction in sensitivity of a constant stimulus
Your nervous system quickly adapts to stimuli that are painless and constant
Tonic receptors (always active, slow-adapting receptors that show little
peripheral adaptation)
Remind you of an injury long after the initial damage has occurred
Phasic receptors (normally inactive, fast-adapting receptors that become
active whenever a change occurs)
Provide information about the intensity and rate of change of a stimulus
2 A reduction in sensitivity in the presence of a constant stimulus is
__________________.
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15-2 Sensory Receptors
General Senses
Describe our sensitivity to:
Temperature
Pressure
Pain
Vibration
Touch
Proprioception
Sensation
The arriving information from these senses
Perception
Conscious awareness of a sensation
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15-2 Sensory Receptors
Special Senses
Olfaction (smell)
Vision (sight)
Gustation (taste)
Equilibrium (balance)
Hearing
The Special Senses
Are provided by special sensory receptors located in sense organs
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15-2 Sensory Receptors
The Interpretation of Sensory Information
Arriving stimulus reaches cortical neurons via labeled line (link between
peripheral receptor and cortical neuron)
Takes many forms (modalities)
Physical force (such as pressure)
Dissolved chemical
Sound
Light
Sensations
Taste, hearing, equilibrium, and vision provided by specialized
receptor cells
Communicate with sensory neurons across chemical synapses
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15-3 Classifying Sensory Receptors
Classifying Sensory Receptors
Exteroceptors provide information about the external environment
Proprioceptors report the positions of skeletal muscles and joints and
provide a purely somatic sensation.
No proprioceptors in the visceral organs of the thoracic and
abdominopelvic cavities. (You cannot tell where your spleen, appendix,
or pancreas is at the moment).
Interoceptors monitor visceral organs and functions
General Sensory Receptors (divided by nature of the stimulus)
Nociceptors (pain) Thermoreceptors (temperature) Mechanoreceptors
(physical distortion) Chemoreceptors (chemical concentration)
3 Receptors that monitor visceral organs and functions are simply classified as ______.
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_________
15-3 Classifying Sensory Receptors
1 Nociceptors (Pain Receptors)
Are common in the superficial portions of the skin, joint capsules, periostea of bones
and around walls of blood vessels
May be sensitive to: Temperature extremes, mechanical damage, dissolved
chemicals, such as chemicals released by injured cells
Are free nerve endings with large receptive fields.
Two types of axons - Type A (myelinated for fast pain or prickling pain)
Type C fibers (slow pain or burning and aching pain)
2 Thermoreceptors
Also called temperature receptors
Are free nerve endings located in: The dermis, Skeletal muscles, the liver, and the
hypothalamus
Temperature sensations carry pain sensations sent to: The reticular formation, the
thalamus, and the primary sensory cortex
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15-3 Classifying Sensory Receptors
3 Mechanoreceptors
Sensitive to stimuli that distort their plasma membranes
Contain mechanically gated ion channels whose gates open or close in response to:
Stretching, Compression, Twisting, and Other distortions
Three Classes of Mechanoreceptors
Tactile receptors (provide the sensations of touch, pressure, vibration, shape,
texture)
Pressure sensations indicate degree of mechanical distortion
Vibration sensations indicate pulsing or oscillating pressure
Baroreceptors (detect pressure changes in the walls of blood vessels and in
portions of the digestive, reproductive, and urinary tracts)
Proprioceptors (monitor the positions of the joints and muscles)
The most structurally and functionally complex of general sensory receptors
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15-3 Classifying Sensory Receptors
Tactile Receptors
Fine touch and pressure receptors
Are extremely sensitive. Have a relatively narrow receptive field.
Provide detailed information about a source of stimulation including
exact location, shape, size, texture, movement.
Crude touch and pressure receptors
Have relatively large receptive fields. Provide poor localization. Give
little information about the stimulus.
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15-3 Classifying Sensory Receptors
Six Types of Tactile Receptors in the Skin
1 Free nerve endings which are sensitive to touch and pressure situated
between epidermal cells. The nerve endings providing touch sensations
are tonic receptors with small receptive fields.
2 Root hair plexus nerve endings monitor distortions and movements
across the body surface whereever hairs are located
3 Tactile discs (Merkel discs) are fine touch and pressure receptors that
are extremely sensitive to tonic receptors and have very small receptive
fields.
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15-3 Classifying Sensory Receptors
Six Types of Tactile Receptors in the Skin
4 Tactile corpuscles (Meissner’s corpuscles) perceive sensations of fine
touch, pressure and low-frequency vibration. They adapt to stimulation
with 1 second after contact.
Most abundant in the eyelids, lips, fingertips, nipples, and external genitalia
5 Lamellated corpuscles (Pacinian corpuscles) are sensitive to deep
pressure. They are fast-adapting and are most sensitive to pulsing or high
frequency vibrating stimuli
6 Ruffini corpuscles
Also sensitive to pressure and distortion of the skin
Located in the reticular (deep) dermis
Tonic receptors that show little if any adaptation
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Figure 15-3 Tactile Receptors in the Skin
4 Fine touch and pressure receptors are called __________.
5 Temperature receptors are called ___________________.
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15-3 Classifying Sensory Receptors
Baroreceptors—Monitor change in pressure
Consist of free nerve endings that branch within elastic tissues within walls of
distensible organs (blood vessels). Respond immediately to a change in
pressure, but adapt rapidly.
Proprioceptors
Monitor position of the joints, tension in the tendons and ligaments as well as the
state of muscular contraction
Three Major Groups of Proprioceptors
Muscle spindles (monitor skeletal muscle length and trigger stretch reflexes)
Golgi tendon organs (receptors in tendon that monitors the external tension
during muscle contraction)
Receptors in joint capsules (receptors that detect pressure, tension, and
movement at the joint).
6 Proprioceptors that monitor skeletal muscle length and trigger stretch reflexes are the
_____________.
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15-3 Classifying Sensory Receptors
Chemoreceptors
Respond only to water-soluble and lipid-soluble substances dissolved in
surrounding fluid
Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial
blood are located in: Carotid bodies (within carotid arteries) and
Aortic bodies (between branches of the aortic arch)
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15-4 Sensory Pathways
Somatic Sensory Pathways
Carry sensory information from the skin and musculature of the body wall,
head, neck, and limbs
Three major somatic sensory pathways
The spinothalamic pathway (provides conscious sensations of
poorly localized (“crude”) touch, pressure, pain and temperature)
The posterior column pathway (carries sensations of highly
localized (“fine”) touch, pressure, vibration and proprioception).
The spinocerebellar pathway (sends proprioceptive information
about the position of skeletal muscles, tendons, and joints to the
cerebellum).
7 Conscious sensations of poorly localized “crude” touch, pressure, pain and
temperature are provided by the __________.
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15-4 Sensory Pathways
Feeling Pain (Lateral Spinothalamic Tract)
Referred pain (Pain felt at a site other than where the cause is situated)
The pain of a heart attack is frequently felt in the left arm
The pain of appendicitis is generally felt first in the area around the
navel and then in the right, lower quadrant
8 Where is the pain of a heart attack felt?
9 Which visceral organ has the broadest area
for referred pain?
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15-4 Sensory Pathways
Sensory Information
Most somatic sensory information is relayed to the thalamus for
processing
A small fraction of the arriving information is projected to the cerebral
cortex and reaches our awareness
The Somatic Nervous System (SNS)—the somatic motor system
Controls contractions of skeletal muscles
The Autonomic Nervous System (ANS)—the visceral motor system
Controls visceral effectors, such as smooth muscle,
cardiac muscle, and glands
10 What is ALS?
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Levels of Processing and Motor Control
All sensory and motor pathways involve a series of synapses, one after the other. Along
the way, the information is distributed to processing centers operating at the subconscious
level.
Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that
preserve homeostasis over the short term.
Voluntary responses are more complex and require more time to prepare and execute.
Cranial and spinal reflexes control the most basic motor activities. Integrative centers in the
brain perform more elaborate processing.
As you move from the medulla oblongata to the cerebral cortex, the motor patterns become
more complex and variable. The primary motor cortex of the cerebral hemispheres directs
the most complex and variable motor activities.
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Clinical Case Living with Cerebral Palsy
Name the principal descending motor pathways that carry information that would be
Affected by the brain damage causing Michael’s cerebral palsy.
Besides aspiration pneumonia, what other difficulties could result from poor motor
Control of the muscles used in swallowing?
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Figure 15-5 Somatic Sensory Pathways (Part 4 of 6)
POSTERIOR COLUMN PATHWAY
SPINOCEREBELLAR PATHWAY
The posterior column pathway carries
sensations of highly localized (”fine”) touch,
pressure, vibration, and proprioception. This
pathway, also known as the dorsal
column-medial lemniscus, begins at a
peripheral receptor and ends at the primary
sensory cortex of the cerebral hemispheres.
The cerebellum receives proprioceptive information
about the position of skeletal muscles, tendons, and
joints along the spinocerebellar pathway. The
posterior spinocerebellar tracts contain axons
that do not cross over to the opposite side of the
spinal cord. These axons reach the cerebellar cortex
via the inferior cerebellar peduncle of that side. The
anterior spinocerebellar tracts are dominated by
axons that have crossed over to the opposite site of
the spinal cord.
Ventral nuclei
in thalamus
PONS
Midbrain
Cerebellum
Nucleus
gracilis and
nucleus
cuneatus
Medial
lemniscus
Medulla
oblongata
Fasciculus
gracilis and
fasciculus
cuneatus
Medulla
oblongata
Spinocerebellar
pathway
Spinal
cord
Anterior
spinocerebellar
tract
Dorsal root
ganglion
Fine-touch, vibration, pressure, and
proprioception sensations from right
side of body
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Posterior
spinocerebellar
tract
Proprioceptive input from Golgi
tendon organs, muscle spindles,
and joint capsules
Figure 15-5 Somatic Sensory Pathways (Part 1 of 6)
SPINOTHALAMIC PATHWAY
The spinothalamic pathway provides conscious sensations of
poorly localized (“crude”) touch, pressure, pain, and temperature. In
this pathway, the axons of first-order neurons enter the spinal cord
and synapse on second-order neurons within the posterior gray
horns. The axons of these interneurons cross to the opposite side of
the spinal cord before ascending to the thalamus. The third-order
neuron synapses in the primary sensory cortex.
KEY
Axon of firstorder neuron
Second-order
neuron
Third-order
neuron
A Sensory Homunculus
A sensory homunculus
(“little human”) is a functional
map of the primary sensory
cortex. The proportions are
very different from those of
any individual because the
area of sensory cortex devoted
to a particular body region is
proportional to the number of
sensory receptors it contains.
Midbrain
The anterior
spinothalamic
tracts of the
spinothalamic
pathway carry crude
touch and pressure
sensations.
Midbrain
Medulla
oblongata
Medulla
oblongata
Anterior
spinothalamic
tract
Crude touch and pressure sensations
from right side of body
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Spinal
cord
The lateral
spinothalamic
tracts of the
spinothalamic
pathway carry pain
and temperature
sensations.
Lateral
spinothalamic
tract
Pain and temperature sensations
from right side of body