Download Ch. 15 – Sensory Pathways and the Somatic Nervous System

Ch. 15 – Sensory Pathways
and the Somatic Nervous System
• Pathways = nerves, nuclei, and tracts that link the
processing centers of the CNS with the rest of the
body
• Discussed in Ch. 15:
– 1. Sensory pathways
• Both somatic sensory pathways and visceral sensory pathways
– 2. Motor pathways
• Only somatic motor pathways of the somatic nervous system (SNS)
are discussed in Ch. 15
– (Visceral motor pathways of the autonomic nervous system (ANS) are
discussed in Ch. 16)
Another way to look at Ch. 15
Fig. 12-1, p. 387
1
An overview of events occurring
along sensory and motor pathways
Fig. 15-1, p. 510
Sensation and receptors
• Sensory receptors = specialized cells or cell processes that
detect stimuli about conditions inside or outside of the body,
and send that info to the CNS
– The job of a receptor is transduction = the translation of a stimulus
into action potentials (the “language” of the NS)
• Sensation = the info arrives in the CNS
– Vs. perception = you are consciously aware of the sensation (only ~ 1% of
sensations are perceived!)
– Receptor specificity = (modality specificity) = receptors are
specialized to detect different types of stimuli (modalities), e.g. light,
sound, chemicals, physical distortion (pressure), etc.
• Senses:
– General senses: touch, pressure, vibration, temperature, pain, and
proprioception (body position)
• Receptors for the general senses are distributed throughout the body and
are relatively simple in structure
– Special senses (Ch. 17): olfaction (smell), vision (sight), gustation
(taste), equilibrium (balance), and hearing
• Receptors for the special senses are found in special sense organs and
are more complex in structure
2
Receptive fields
• Receptive field = the area monitored by a single receptor
– Smaller receptive fields → more precise localization of the stimulus
(e.g. touch by the fingertips)
– Larger receptive fields → less precise localization of the stimulus (e.g.
touch on the back)
Fig. 15-2, p. 511
The interpretation of sensory info
• Labeled lines and cortical mapping
– Sensory info is sent from a specific receptor in a specific
part of the body to a specific area of the cerebral cortex
• Labeled line = the neuronal pathway from the receptor to the
specific part of the cortex
• The sensory cortex can then be mapped (see #1 and #2 below)
• The sensory areas of the cerebral cortex interpret
the:
– 1. Type of stimulus (modality) based upon the cortical
destination of the labeled line (e.g. touch → primary
sensory cortex, light → visual cortex, sound → auditory
cortex, etc.)
– 2. Location of stimulus based upon exactly where in
particular sensory cortex the labeled line arrives (e.g.
consider the sensory homunculus)
– 3. Intensity, duration, and other characteristics of the
stimulus based upon the action potential frequency and
pattern
3
Sensory adaptation
•
Adaptation = ↓ sensitivity to a constant, painless stimulus
– 1. Peripheral adaptation: ↓ receptor sensitivity, ↓ the amount of info that
reaches the CNS
• Only phasic (fast-adapting) receptors can peripherally adapt (see figure below)
– 2. Central adaptation: occurs within the CNS, along sensory pathways, ↓ the
amount of info that reaches the cerebral cortex
• It may be subconscious or conscious inhibition
– E.g. getting used to a smell – subconscious inhibition of olfactory centers in the brain
– E.g. “tuning out” background noise on a busy street – subconscious or conscious inhibition
of auditory pathways
• The CNS may also sometimes subconsciously or consciously ↑ sensitivity
(facilitation) to certain stimuli
– E.g. “listening carefully” – conscious facilitation of auditory pathways
E.g. pain receptors and proprioceptors
E.g. temperature receptors and deep pressure receptors
Fig. 15-3, p. 512
The general senses – receptor types
• General sense receptors can be classified by:
– 1. What/where they’re monitoring for changes:
• Interoceptors – internal organ systems (e.g. sensing heart rate,
blood pressure, deep pressure/pain, etc.)
• Exteroceptors – the external environment (e.g. sensing ambient
temperature, light, touch, sound, etc.)
• Proprioceptors – the position and movement of muscles and joints
(i.e., body position)
– 2. The nature of the stimulus that excites them:
• Nociceptors – pain (see the next slide)
• Thermoreceptors – temperature (see the next slide)
• Mechanoreceptors – physical distortion (see the two slides after
that)
• Chemoreceptors – the concentration of dissolved chemicals (e.g.
H+, CO2, O2) in certain body fluids
– This information is NOT perceived by the cerebral cortex; it is sent to
lower brain centers for subconscious homeostatic adjustments
4
Nociceptors and thermoreceptors
• Nociceptors (pain receptors) = relatively nonspecific
receptors that are sensitive to extreme temperature,
mechanical damage, or chemicals released by damaged
cells
– They’re tonic, so they exhibit very little peripheral adaptation…
• However, central adaptation may decrease the perception of pain via the
release of endorphins and enkephalins (which are structurally similar to
morphine)
– There are 2 general types of pain:
• 1. Fast pain (prickling pain) – carried by Type A fibers; it is more
pinpoint/localizable pain (e.g. deep cut, pin prick)
• 2. Slow pain (burning and aching pain) – carried by Type C fibers; it is
more general/diffuse pain (e.g. stomach ache, uterine cramps)
• Thermoreceptors (temperature receptors) – include both
cold receptors and warm receptors
– They’re both exteroceptors (e.g. in the skin) and interoceptors (e.g. in
the hypothalamus)
– They’re phasic (fast-adapting), so they are very active only when the
temperature is changing
Mechanoreceptors
• Are sensitive to stimuli that
physically distort their cell
membranes – e.g. stretching,
compression, twisting, etc.
– The membranes of mechanoreceptors
contain mechanically gated ion
channels
• There are 3 classes of
mechanoreceptors:
– 1. Tactile receptors – sense touch,
pressure, and vibration
– 2. Baroreceptors – sense pressure
changes in the walls of hollow organs
– 3. Proprioceptors – sense the position
of joints and muscles
Fig. 12-11c, p. 403
5
• Sense touch, pressure,
and vibration
• There are 2 main types
of tactile receptors:
Tactile receptors
– 1. Fine touch and
pressure receptors –
more superficial
• They are precisely
localized due to
smaller receptive
fields
• E.g. free nerve
endings, tactile
discs (which include
Merkel cells), tactile
(Meissner’s)
corpuscles
– 2. Crude touch and
pressure receptors –
deeper
• They are poorly
localized due to larger
receptive fields
• E.g. lamellated
(pacinian) corpuscles
and Ruffini corpuscles
Fig. 15-4, p. 515
Baroreceptors and proprioceptors
• Baroreceptors – monitor pressure in the walls of hollow
organs (i.e., they monitor the stretch of the walls due to the
pressure of the fluid or gas within the organ)
– They’re phasic (fast-adapting) receptors that are found in the walls of
the digestive tract, urinary bladder, carotid arteries, aorta, and lungs
• Proprioceptors – monitor joint position, tension in tendons
and ligaments, and the general state of muscular
contraction
– They’re tonic – they continuously send info,
and do not adapt
– Most proprioceptive info is processed
subconsciously (e.g. by the cerebellum)
– There are 3 main types:
• 1. Muscle spindles (shown here) –
detect the length or degree of stretch
of a muscle
• 2. Golgi tendon organs – detect the
tension on a muscle and its tendon
• 3. Joint capsule receptors – detect
pressure, tension, and movement at
a joint
Fig. 13-16, p. 453
6
Somatic sensory pathways
and ascending tracts in the spinal cord
•
•
Carry sensory info from the skin and skeletal musculature
There are 3 major somatic sensory pathways:
– 1. Posterior column pathway
• Tracts: fasciculus cuneatus and fasciculus gracilis
– 2. Spinothalamic pathway
• Tracts: anterior spinothalamic tract and lateral spinothalamic tract
– 3. Spinocerebellar pathway
• Tracts: anterior spinocerebellar tract and posterior spinocerebellar tract
See Table 15-1 for
much more detail
than what we’ll
cover in lecture
Fig. 15-4, p. 503
The organization
of somatic sensory
pathways
• Somatic sensory pathways
usually consist of 3 neurons:
– 1. First-order neuron = a sensory
neuron that sends info to the CNS
– 2. Second-order neuron = an
interneuron
• At some point, it crosses over to the
opposite side
– 3. Third-order neuron = an
interneuron
• It travels from the thalamus to the
postcentral gyrus (primary sensory
cortex)
• It must send an AP if the sensation
is to reach conscious awareness
Remember
the idea of
a “labeled
line”!
Fig. 15-6, p. 520
7
The spinothalamic
pathway
Fig. 15-6, p. 520
Fig. 15-6, p. 521
The posterior
column pathway
8
Fig. 15-6, p. 521
The spinocerebellar
pathway
•
Note that there is no thirdorder neuron; processing of
this proprioceptive info always
occurs subconsciously (in the
cerebellum)
Visceral sensory pathways
•
•
•
•
Sensory info from abdominopelvic interoceptors enter the dorsal horn of the spinal
cord (SC) via the dorsal root (shown here)
Sensory info from interoceptors in the mouth, pharynx, larynx, and thoracic viscera
enter the brain stem via cranial nerves (not shown here)
Either way, visceral sensory info is sent to the solitary nucleus of the
medulla oblongata, which relays it to the appropriate lower (subconscious) brain
destinations (e.g. cardiovascular centers, respiratory centers, etc.)
Extremely strong visceral pain sensations can cross-stimulate somatic pain
interneurons located at the same level of the SC or brain stem, so you feel pain in
the corresponding part of the body surface
= referred pain (e.g. heart attack,
appendicitis, etc.)
Fig. 15-7 p. 522
Fig. 13-8 p. 440
9
Somatic motor pathways and
descending tracts in the spinal cord
•
There are 3 major somatic motor pathways:
– 1. Corticospinal pathway
The names give it away!
• Tracts: corticobulbar tract (which terminates at the brain stem), lateral corticospinal
tract, and anterior corticospinal tract
– 2. Medial pathway
• Tracts: vestibulospinal tract, tectospinal tract, and reticulospinal tract
– 3. Lateral pathway
• Tracts: rubrospinal tract
•
See Table 15-2 for
much more detail
than what we’ll
cover in lecture
Remember:
– The somatic nervous system
(SNS) sends (mostly)
voluntary motor commands to
skeletal muscles
• Vs. the autonomic nervous
system (ANS), or visceral
motor system, which sends
involuntary motor commands
to cardiac muscle, smooth
muscle, glands, and
adipocytes – Ch. 16
– Somatic motor pathways are
monitored and adjusted by the
basal nuclei and cerebellum
Fig. 15-8, p. 524
Fig. 15-9, p. 525
The
organization of
somatic motor
pathways
• Somatic motor pathways
consist of at least 2
neurons:
– 1. Upper motor neuron
(UMN) – it’s actually an
interneuron
• It facilitates or inhibits a
lower motor neuron
– 2. Lower motor neuron
(LMN) – it’s a true motor
neuron
• It stimulates the muscle
fibers of a motor unit
10
Fig. 15-9, p. 525
The
corticospinal
pathway
• Carries out conscious
control of skeletal
muscles
• The vast majority of
corticospinal pathway
UMNs decussate at the
pyramids of the medulla
oblongata
Fig. 15-8, p. 524
The medial
and lateral
pathways
• Carry regulatory subconscious somatic motor commands
from lower brain levels (e.g., the basal nuclei, diencephalon,
or brain stem)
• Modify or direct skeletal muscle contractions by stimulating,
facilitating, or inhibiting LMNs
– Medial pathway: subconsciously helps control muscle tone and
gross movements of neck, trunk, and proximal limb muscles
– Lateral pathway: subconsciously helps control muscle tone and
more precise movements of distal limb muscles
11