Download Sensory Pathways and the Somatic Nervous System

Neural Integration
Sensory Pathways and the
Somatic Nervous System
General Senses
• Sensation of temperature, pain, touch, pressure,
vibration, and proprioception.
Sensation: Information provided by sensory receptors to
the CNS.
Perception: The conscious awareness of sensation.
Signals
Sensation and perception : elements that balance and
complement one another. They work together for us to be
able to identify and create meaning from stimuli-related
information. Without sensation, perception will not be
possible
Special Senses
• More complex than general senses,
having sensory organs that protect
receptors.
– Olfaction (smell)
– Vision (Sight)
– Gustation (taste)
– Equilibrium (balance)
– Hearing (audotory)
Sensory Receptors
Free Nerve Endings
• Simplest type of Sensory receptor
• Dendrites of Sensory Neurons detect stimuli
• No accessory structures, so they detect
numerous types of stimuli (low receptor
specificity)
Sensory Receptors
• Detects stimuli and translates into action
potential that can be conveyed to the CNS in a
process called transduction.
– Can humans transduce all stimuli?
• receptor potential
Transduction
– transduction begins when a
stimulus changes the
transmembrane potential of a
sensory neuron.
– has a threshold in stimulus
amplitude that must be
reached before a response is
generated
• graded effect: stronger
stimulus = larger receptor
potential.
• taste bud
•generator potential
graded response to a stimulus and
is capable of producing an action
potential in the afferent axon in the
nearby sensory neuron.
Transduction
Special Senses General Senses
Tonic and Phasic Receptors
• Tonic Receptors
– always active, indicative of background level of stimulation
– change in frequency of action potentials indicates increase or
decrease of stimulation
• Phasic Receptors
– normally inactive
– activated for short period by a stimulus
– provide information about the intensity and rate of change of
stimuli
Phasic receptors respond quickly to stimuli, while tonic receptors
adapt more slowly and over a more sustained period of time.
Receptive Field
The receptive field of an individual sensory neuron is
the particular part of the body surface in which a
stimulus will trigger the firing of that neuron.
• The area is monitored by a single receptor cell
• The larger the receptive field, the more difficult it is
to localize a stimulus
Sensory Adaptation
• Reduction in sensitivity of a receptor in the
presence of a constant stimulus
– Peripheral Adaptation:
• level of receptor activity decreases
– Central Adaptation:
• signal decreases along sensory pathway after receptor
• usually inhibition of nuclei along pathway by other neurons
Sensory Adaptation
• fast-adapting receptors
– phasic receptors
– high level of peripheral adaptation
– example is thermoreceptors
• you are not constantly aware of
temperature, only change in
temperature
Sensory Adaptation
General Senses
Classification by location
1. exteroreceptors
•
touch, pressure, pain,
special senses
2. interoreceptors
•
chemical changes,
stretching of tissues,
temperature
3. proprioceptors
•
•
Location of self
only in skeletal muscle,
tendons, joints, ligaments,
& CT coverings of bones
& muscles
Classification by sensation
1. nociceptors
•
pain
2. thermoreceptors
•
temperature
3. mechanoreceptors
•
•
deformed by force
touch, pressure (BP),
vibration, stretch, itch
4. chemoreceptors
•
•
chemicals in solution
Smell, taste, blood
chemistry
Nociceptors
•
Free nerve endings with large
receptive fields
•
May be sensitive to:
1.
2.
3.
1.
extremes of temperature
mechanical damage
dissolved chemicals, such as
chemicals released by injured cells
Very strong stimuli may activate
all 3 receptor types.
Fast Pain
•
•
•
•
carried by large myelinated Type A fibers
reach CNS quickly and trigger fast reflex
specific activation of primary sensory cortex
easy to localize where pain occurred
Slow Pain
•
•
•
•
carried by small unmyelinated Type C fibers
reaches CNS slowly
generalized activation of the thalamus
difficult to localize area of pain
Neurotransmitters and Pain
• Sensory Neurons
– glutamate and Substance P
• excitatory
• facilitate neurons along pain pathway
Neurotransmitters and Pain
• Natural painkillers
– endorphins (in small red neuron)
– inhibit the perception of pain by the CNS by inhibiting
the release of Substance P from sensory neurons
Keep in mind:
The perception of pain may be
gone, but the stimulus remains.
Tactile Receptors
• Range in complexity from free nerve endings to
specialized sensory complexes.
– Fine touch and pressure receptors
• source of stimulus, exact location, shape, size, texture, and
movement
• small receptive field
– Crude touch and pressure receptors
• poor localization (and that’s it really)
• large receptive field
Tactile Receptors
•
Six types in skin
1. free nerve endings
2. root hair plexus
3. tactile (Merkel’s)
discs
4. tactile (Meissner’s)
corpuscles
5. lamellated
(Pacinian)
corpuscles
6. Ruffini corpuscles
Tactile Receptors
• Free Nerve Endings
–
–
–
–
sensitive to touch and pressure
tonic receptors with small receptive fields
situated between epidermal cells
no difference in structure between these and thermoreceptors or
nociceptors
Tactile Receptors
• Root Hair Plexus
– movement of hair distorts the dendrites surrounding the hair
follicle producing action potentials
– phasic receptors that adapt quickly to a stable stimulus
– For this reason you are not aware of your clothing at all times!
Tactile Receptors
• Tactile (Merkel’s) Discs
– fine touch and pressure receptors
– extremely sensitive and tonic receptors
– close to stratum germinativum
Tactile Receptors
• Tactile (Meissner’s) Corpuscles
– fine touch, pressure, and low-frequency vibration
– dendrites coiled and surrounded by schwann cells and then by
fibrous capsule
– abundant in the eyelids, lips, fingertips, nipples, and genitalia
Tactile Receptors
• Lamellated (Pacinian) Corpuscles
–
–
–
–
sensitive to deep pressure
fast-adapting tonic receptors
single dendrite inside collagen fibers and supporting cells
found in dermis, mesentery, pancreas, urethra, and bladder
Tactile Receptors
• Ruffini Corpuscles
–
–
–
–
also sensitive to pressure and distortion of the skin
located in the reticular (deep) dermis
capsule filled with a network of dendrites and collagen fibers
tonic receptors
• little adaptation
Tickle and Itch
• These sensations are not well understood, but closely
related to touch and pain
• receptors are free nerve endings and are carried over
Type C fibers
• Tickle sensation is usually described as pleasurable.
• Itch sensations can be more unpleasant than pain.
Baroreceptors
• free nerve endings that branch within elastic tissues of
extendable organs
– blood vessels, digestive tract, urinary tract
• fast-adapting phasic receptors
• Blood pressure monitored by baroreceptors in:
– carotid artery
– aorta
Proprioceptors
3 major groups (tonic non-adapting receptors)
– Muscle Spindles
• monitor skeletal muscle length (stretch
reflex)
– Golgi tendon organs
• located at the junction between skeletal
muscle and its tendon
• stimulated by tension in tendon
• monitor external tension developed
during muscle contraction
– Joint capsule receptors
• free nerve endings detect pressure,
tension, and movement at the joint
Chemoreceptors
• Respond to soluble substances
dissolved in surrounding fluid.
• Receptors are fast-adapting phasic
type
• Located in the:
– carotid bodies
– aortic bodies
• Receptors monitor Ph, carbon
dioxide, and oxygen levels in arterial
blood
Somatic Sensory Pathways
•
carry sensory information from the skin and
musculature of the body wall, head, neck, and limbs
1.
2.
3.
posterior column pathway
anterolateral pathway
spinocerebellar pathway
Sensory Homunculus
• electrical stimulation of
human brains was used to
create a functional map of the
primary sensory cortex
• the homunculus has a huge
face because the face has
the most sensory receptors
and thus give the most
sensory input
The Anterolateral Pathway
• conscious, poorly localized
sensations of touch, pressure,
pain and temperature
• first-order neurons
– synapse on second-order
neurons in the posterior gray
horn of the spinal cord
The Anterolateral Pathway
• second-order neurons (soma
in spinal cord)
– fibers ascend in
• anterior spinothalamic tract
– touch and pressure
• lateral spinothalamic tract
– pain and temperature
The Anterolateral Pathway
• third-order neurons
– soma in ventral nuclei of the
thalamus
– arriving information is sorted
• nature of stimulus
• region of body involved
– the axon travels to a specific
region of the primary sensory
cortex
•
Phantom Limb Pain
• An individual can experience painful sensations
from an amputated limb, or if they were born
without the limb.
• The sensory neurons are still firing and the
signal reaches the primary cortex and thus is
perceived as a real stimulus on an absent limb.
Referred Pain
• Perception of pain that is
incorrectly localized.
• Sensations arriving at segment
of spinal cord can stimulate
interneurons that are part of
anterolateral pathway
– heart attack
• referred pain in left arm
– appendicitis
• referred pain in
the naval
Visceral Sensory Pathways
• sensation in visceral tissues and organs,
primarily in thoracic and abdominopelvic
cavities.
• interoceptors:
–
–
–
–
–
nociceptors
thermoreceptors
tactile receptors
baroreceptors
chemoreceptors
The Somatic Nervous System
• controls contractions of the skeletal muscles
• under voluntary control
Motor neurons of the SNS
• SNS pathways have at least
2 motor neurons:
– Upper motor neuron
• cell body is in the CNS
• may facilitate or inhibit LMN
– Lower motor neuron
• cell body is in brainstem or
spinal cord
• activation triggers muscle
contraction
Pathways of the SNS
• Corticospinal Pathway
• Medial Pathway
• Lateral Pathway
• Activity in these pathways is monitored
and adjusted by the basal nuclei and
cerebellum.
Motor Homunculus
Sensory
Motor
proportions of homunculus relate to number of motor units in region
hand and mouth large because they have many motor units for fine motor control
Medial and Lateral Pathways
• historically called extrapyramidal pathway
• subconscious motor control
– Medial Pathway
• controls gross movements of trunk and proximal limb
muscles
– Lateral Pathway
• controls distal limb muscles perform precise movments
• Upper motor neurons in this pathway stimulate
the same lower motor neurons as the
corticospinal pathway
The Role of the
Basal Nuclei and Cerebellum
• help to coordinate conscious and
unconscious muscle contractions
– Basal Nuclei - provide background patterns of
movement involved in voluntary motor activities
– Cerebellum - monitors:
• proprioceptive (position) sensations
• visual information from the eyes
• vestibular (balance) sensations from inner ear as
movements are under way