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Chapter 1 A Perspective on Human Genetics

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Title: Chapter 1 A Perspective on Human Genetics Author: Penn State Altoona Last modified by: Fayetteville State University Created Date: 9/4/2004 10:37:46 PM – PowerPoint PPT presentation

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Title: Chapter 1 A Perspective on Human Genetics


1
Chapter 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.

3
Outline
  • 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.

4
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

5
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

6
What Do You Perceive?
Proof !
7
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8
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

9
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

10
Shaft 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
11
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.

12
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
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
13
Capsule
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
14
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

15
Receptors
  • 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.

16
Afferent 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
17
Conversion of Receptor Potentials into Action
Potentials
18
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

19
Phasic- 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
20
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

21
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22
Table 6-1 p192
23
Fig. 5-11, p. 145
24
Acuity
  • Refers to discriminative ability
  • Influenced by receptive field size and lateral
    inhibition

25
Lateral inhibition
Fig. 6-7, p. 187
26
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

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

28
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

29
Table 6-2 p194
30
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

31
Higher 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
33
Periaqueductal 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
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