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Chapter 6A
The Peripheral Nervous System:
Afferent
Divisionhttp://www.brainline.org/multimedia/interactive_brain/the_hu
man_brain.html?gclid=CJroxvfmjaACFVth2godUkI6eA
• Describe the components (afferent and efferent) of
the peripheral nervous system. This will be
measured by lecture and laboratory exams.
Outline
• Pathways, perceptions, sensations
• Receptor Physiology
– Receptors have differential sensitivities to various stimuli.
– A stimulus alters the receptor’s permeability, leading to a graded
receptor potential.
– Receptor potentials may initiate action potentials in the afferent
neuron.
– Receptors may adapt slowly or rapidly to sustained stimulation.
– Each somatosensory pathway is “labeled” according to modality
and location.
– Acuity is influenced by receptive field size and lateral inhibition.
– PAIN
– Stimulation of nociceptors elicits the perception of pain plus
motivational and emotional responses.
– The brain has a built-in analgesic system.
Peripheral Nervous System
• Consists of nerve fibers that carry information
between the CNS and other parts of the body
• Afferent division
– Sends information from internal and external
environment to CNS
• Visceral afferent
– Incoming pathway for information from internal viscera
(organs in body cavities)
• Sensory afferent
– Somatic (body sense) sensation
» Sensation arising from body surface and
proprioception
– Special senses
» Vision, hearing, taste, smell
Perception
• Conscious interpretation of external world derived
from sensory input
• Why sensory input does not give true reality
perception
– Some information is not transduced
– Some information is filtered out
– Cerebral cortex further manipulates the data
– Sensation vs. perception
What Do You Perceive?
Proof !
Receptors
• Structures at peripheral endings of afferent neurons
• Detect stimuli (change detectable by the body)
• Convert forms of energy into electrical signals
(action potentials)
– Process is called transduction
Types of Receptors
• Photoreceptors
– Responsive to visible wavelengths of light
• Mechanoreceptors
– Sensitive to mechanical energy
• Thermoreceptors
– Sensitive to heat and cold
• Osmoreceptors
– Detect changes in concentration of solutes in body fluids and
resultant changes in osmotic activity
• Chemoreceptors
– Sensitive to specific chemicals
– Include receptors for smell and taste and receptors that detect
O2 and CO2 concentrations in blood and chemical content of
digestive tract
• Nociceptors
– Pain receptors that are sensitive to tissue damage or distortion of
tissue
Shaft of hair inside follicle
Skin surface
Epidermis
Dermis
Myelinated
neuron
Subcutaneous
tissue
Hair
receptor:
hair
movement
and very
gentle touch
Merkel’s
disc: light,
sustained
touch
Pacinian corpuscle:
vibrations and deep
pressure
Ruffini endings:
deep pressure
Meissner’s
corpuscle:
light,
fluttering
touch
Figure 6-5 p190
Muscle Receptors
• Two types of muscle receptors.
• Both are activated by muscle stretch, but monitor
different types of information.
• Muscle spindles monitors muscle length.
• Golgi tendon organs detect changes in tension.
Muscle spindle (proprioceptor)
regulates rate of change of length,
And length
Golgi tendon organ
Type II
sensory neuron
Spinal
cord
Intrafusal muscle
fibers
Nuclear bag
fiber
Nuclear chain
fiber
Type lA
sensory
neuron
Nuclei of muscle
fibers
Motor end plate
Extrafusal muscle
fibers
Alpha motor neuron
Gamma motor neuron
Like pg. 289
Capsule
Alpha motor
neuron axon
Intrafusal (spindle)
muscle fibers
Gamma motor
neuron axon
Afferent neuron
axons
Contractile end
portions of intrafusal
fiber
Noncontractile
central portion
of intrafusal
fiber
Extrafusal (“ordinary”)
muscle fibers
Fig. 8-25a, p. 289
Uses For Perceived Information
• Afferent input is essential for control of efferent
output
• Processing of sensory input by reticular activating
system in brain stem is critical for cortical arousal
and consciousness
• Central processing of sensory information gives rise
to our perceptions of the world around us
• Selected information delivered to CNS may be
stored for further reference
• Sensory stimuli can have profound impact on our
emotions
Receptors
• May be
– Specialized ending of an afferent neuron
– Separate cell closely associated with peripheral ending of
a neuron
• Stimulus alters receptor’s permeability which leads to graded
receptor potential
• Usually causes nonselective opening of all small ion channels
• This change in membrane permeability can lead to the influx
of sodium ions. This produces receptor (generator)
potentials.
• The magnitude of the receptor potential represents the
intensity of the stimulus.
• A receptor potential of sufficient magnitude can produce an
action potential. This action potential is propagated along an
afferent fiber to the CNS.
Afferent
fiber
Afferent fiber potential (mV)
Afferent
terminals
Rate of neurotransmitter release at afferent terminals
+30
–70
Sensory
receptor
Stimulus
Rest
Stimulus strength
Receptor potential (mV)
Frequency of action potentials in afferent fiber
Magnitude of receptor potential
On
Stimulus strength
On
Off
Time (sec)
Off
Figure 6-3 p189
Conversion of Receptor Potentials into Action Potentials
Receptors
• May adapt slowly or rapidly to sustained stimulation
• Types of receptors according to their speed of
adaptation
– Tonic receptors
• Do not adapt at all or adapt slowly
• Muscle stretch receptors, joint proprioceptors
– Phasic receptors
• Rapidly adapting receptors
• Tactile receptors in skin
Tonic -Takes longer for the membrane
Voltage to drop (maintaining the signal i.e position)
Phasic- Membrane potential drops
More rapidly (intensity i.e pressure)
Fig. 6-5, p. 185
Somatosensory Pathways
• Pathways conveying conscious somatic sensation
• Consists of chains of neurons, or labeled lines,
synaptically interconnected in particular sequence to
accomplish processing of sensory information
– First-order sensory neuron
• Afferent neuron with its peripheral receptor that first
detects stimulus
– Second-order sensory neuron
• Either in spinal cord or medulla
• Synapses with third-order neuron
– Third-order sensory neuron
• Located in thalamus
Table 6-1 p192
Fig. 5-11, p. 145
Acuity
• Refers to discriminative ability
• Influenced by receptive field size and lateral
inhibition
Lateral inhibition
Fig. 6-7, p. 187
Pain
• Primarily a protective mechanism meant to bring a
conscious awareness that tissue damage is
occurring or is about to occur
• Storage of painful experiences in memory helps us
avoid potentially harmful events in future
• Sensation of pain is accompanied by motivated
behavioral responses and emotional reactions
• Subjective perception can be influenced by other
past or present experiences
• Cortex
– Higher processing
• Basal nuclei
– Control of movement, inhibitory, negative
• Thalamus
– Relay and processing of sensory information
– Awareness, a positive screening center for information
• Hypothalamus
– Hormone secretion, regulation of the internal environment
• Cerebellum
– Important in balance and in planning and executing voluntary
movement
• Brain Stem
– Relay station (posture and equilibrium), cranial nerves,
control centers, reticular integration, sleep control
Pain
• Presence of prostaglandins (lower nociceptors threshold for
activation) greatly enhances receptor response to noxious
stimuli
– Role of asprin
• Nociceptors do not adapt to sustained or repetitive stimulation
• Three categories of nociceptors
– Mechanical nociceptors
• Respond to mechanical damage such as cutting, crushing, or
pinching
– Thermal nociceptors
• Respond to temperature extremes
– Polymodal nociceptors
• Respond equally to all kinds of damaging stimuli
Table 6-2 p194
Pain
• Two best known pain neurotransmitters
– Substance P
• Activates ascending pathways that transmit nociceptive
signals to higher levels for further processing
– Glutamate
• Major excitatory neurotransmitter
• Brain has built in analgesic system
– Suppresses transmission in pain pathways as
they enter spinal cord
– Depends on presence of opiate receptors
• Endogenous opiates – endorphins, enkephalins,
dynorphin
Higher processing of pain
• Substance P
– Different destinations
• Cortex – localizes the pain
• Thalamus- perception of
pain
• Reticular formationincreases alertness
• Hypothalamus/limbic
system- emotional and
behavioral responses
• Glutamate
– AMPA receptors
• Ap’s in the dorsal horn
– NMDA receptors
• Ca entry makes dorsal
horn neuron more
sensitive
Somatosensory
cortex
Higher
brain
(Localization of pain)
(Perception of pain)
Thalamus
Hypothalamus; (Behavioral and
limbic system emotional responses
to pain)
Brain
stem
Reticular
formation
( Alertness)
Noxious
stimulus
Spinal
cord
Dorsal horn
Substance P
excitatory
interneurons
(a) Substance P pain pathway
Afferent pain fiber
Nociceptor
Figure 6-9a p195
Periaqueductal
gray matter
No perception of pain
To thalamus
Endogenous
opiate
Medulla
Reticular
formation
Inhibitory
interneuron
in dorsal horn
Noxious
stimulus
Opiate
receptor
Transmission
of pain
impulses to
brain blocked
Dorsal horn
excitatory
interneurons
Afferent pain fiber
Substance P
Nociceptor
(b) Analgesic pathway
Figure 6-9b p195