Title: Chapter 1 A Perspective on Human Genetics
1Chapter 6A The Peripheral Nervous System
Afferent
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2- Describe the components (afferent and efferent)
of the peripheral nervous system. This will be
measured by lecture and laboratory exams.
3Outline
- Pathways, perceptions, sensations
- Receptor Physiology
- Receptors have differential sensitivities to
various stimuli. - A stimulus alters the receptors 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.
4Peripheral 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
5Perception
- 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
6What Do You Perceive?
Proof !
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8Receptors
- 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
9Types 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
10Shaft of hair inside follicle
Skin surface
Epidermis
Dermis
Myelinated neuron
Subcutaneous tissue
Merkels disc light, sustained touch
Ruffini endings deep pressure
Pacinian corpuscle vibrations and deep pressure
Hair receptor hair movement and very gentle touch
Meissners corpuscle light, fluttering touch
Figure 6-5 p190
11Muscle 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.
12Muscle 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
Type lA sensory neuron
Nuclear chain fiber
Nuclei of muscle fibers
Motor end plate
Alpha motor neuron
Extrafusal muscle fibers
Gamma motor neuron
Like pg. 289
13Capsule
Alpha motor neuron axon
Intrafusal (spindle) muscle fibers
Gamma motor neuron axon
Contractile end portions of intrafusal fiber
Afferent neuron axons
Noncontractile central portion of intrafusal fiber
Extrafusal (ordinary) muscle fibers
Fig. 8-25a, p. 289
14Uses 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
15Receptors
- May be
- Specialized ending of an afferent neuron
- Separate cell closely associated with peripheral
ending of a neuron - Stimulus alters receptors 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.
16Afferent terminals
Rate of neurotransmitter release at afferent
terminals
30
Afferent fiber potential (mV)
Afferent fiber
70
Frequency of action potentials in afferent fiber
Sensory receptor
Receptor potential (mV)
Rest
Magnitude of receptor potential
Stimulus strength
Stimulus
On
On
Off
Off
Time (sec)
Stimulus strength
Figure 6-3 p189
17Conversion of Receptor Potentials into Action
Potentials
18Receptors
- 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
19Phasic- Membrane potential drops More rapidly
(intensity i.e pressure)
Tonic -Takes longer for the membrane Voltage to
drop (maintaining the signal i.e position)
Fig. 6-5, p. 185
20Somatosensory 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
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22Table 6-1 p192
23Fig. 5-11, p. 145
24Acuity
- Refers to discriminative ability
- Influenced by receptive field size and lateral
inhibition
25Lateral inhibition
Fig. 6-7, p. 187
26Pain
- 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
27- 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
28Pain
- 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
29Table 6-2 p194
30Pain
- 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
31Higher processing of pain
- Substance P
- Different destinations
- Cortex localizes the pain
- Thalamus- perception of pain
- Reticular formation- increases alertness
- Hypothalamus/limbic system- emotional and
behavioral responses
- Glutamate
- AMPA receptors
- Aps in the dorsal horn
- NMDA receptors
- Ca entry makes dorsal horn neuron more sensitive
32(Localization of pain)
Somatosensory cortex
(Perception of pain)
Higher brain
Thalamus
(Behavioral and emotional responses to pain)
Hypothalamus limbic system
Brain stem
Reticular formation
( Alertness)
Noxious stimulus
Spinal cord
Afferent pain fiber
Dorsal horn excitatory interneurons
Substance P
Nociceptor
(a) Substance P pain pathway
Figure 6-9a p195
33Periaqueductal gray matter
Reticular formation
Medulla
No perception of pain To thalamus
Inhibitory interneuron in dorsal horn
Endogenous opiate
Noxious stimulus
Opiate receptor
Transmission of pain impulses to brain blocked
Afferent pain fiber
Dorsal horn excitatory interneurons
Substance P
Nociceptor
(b) Analgesic pathway
Figure 6-9b p195