Transcript Document

Partial Chapters 12, 13 & 16
Circuits, Receptors, and Reflexes
Postsynaptic potentials
LaPointe Fall ’11
Slide # 2
• Most important determinants of neural activity are EPSP
/ IPSP interactions
• EPSP (excitatory postsynaptic potential) =
depolarization
• IPSP (inhibitory postsynaptic potential) =
hyperpolarization
• EPSPs and IPSPs can combine through summation
• Temporal summation
• Spatial summation
• facilitation / inhibition
ESPS and ISPS
LaPointe Fall ’11
Slide # 3
Temporal and Spatial Summation
LaPointe Fall ’11
Slide # 4
Summation (facilitation)
LaPointe Fall ’11
Slide # 5
EPSP – IPSP Interactions (inhibition)
LaPointe Fall ’11
Slide # 6
Martini Figure 12.23
LaPointe Fall ’11
Slide # 7
Summation
Seeley, Stephens and Tate
Spatial summation
Mixed summation
Temporal summation
Presynaptic inhibition and facilitation
LaPointe Fall ’11
Slide # 8
• Inhibition
• GABA release at axoaxonal synapse inhibits opening
calcium channels in synaptic knob
• Reduces amount of neurotransmitter released when
action potential arrives
• Facilitation
• Activity at axoaxonal synapse increases amount of
neurotransmitter released when action potential
arrives
• Enhances and prolongs the effect of the
neurotransmitter
Presynaptic Inhibition
LaPointe Fall ’11
Slide # 9
Information processing
LaPointe Fall ’11
Slide # 10
• Determination of the strength of a stimulus can be coded
through recruitment (more neurons fire); or
• By the rate of generation of action potentials are often used
to interpret the signal.
Neuronal pools
• Functional group of interconnected neurons
• Neural circuit patterns
• Divergence
• Convergence
• Reverberation
• Serial processing
• Parallel processing
LaPointe Fall ’11
Slide # 11
Simple circuits (also see Saladin figs 12.29 & 12.30)
LaPointe Fall ’11
Slide # 12
Martini Figure 13.15
Sensory receptors
LaPointe Fall ’11
Slide # 13
• Receptors are specialized cells or cell processes that
monitor specific conditions (respond to stimuli)
• Act as the interface between the CSN and the internal
and external environments.
• Arriving information into the CNS is a sensation.
• Awareness of a sensation is a perception.
• not all sensations are perceived
• neural input must go to the primary sensory areas for
conscious awareness
Receptor classification
LaPointe Fall ’11
Slide # 14
• Exteroceptors - provide information about the external
environment
• Interoceptors - provide information about visceral
organs and functions
• Propioceptors - provide information about positions and
tension of the joints and skeletal muscles.
Classification of receptors
LaPointe Fall ’11
Slide # 15
• Receptors can also be classified based on the type of
stimulus they respond to
• nociceptors - pain
• thermoreceptors - temperature
• mechanoreceptors - physical distortion
• chemoreceptors - chemical concentrations
• photoreceptors - light
Sensory receptors
LaPointe Fall ’11
Slide # 16
• Nerve fibers fire when an action potential is generated
regardless of what caused it. Thus, they are nonspecific.
• Specificity comes from specialized receptors.
• Receptor cells are generally sensitive to limited types of
stimuli (modality) known as receptor specificity.
• Each receptor cell monitors a specific receptive field.
• The larger the receptive field the less precise
localization is.
Receptors and Receptive Fields
LaPointe Fall ’11
Slide # 17
Figure 15.2
Receptor fields and 2 point discrimination
LaPointe Fall ’11
Slide # 18
Interpretation of sensory information
LaPointe Fall ’11
Slide # 19
 Nerve fibers are non-specific !!! - but they go to specific
regions of the brain along specific tracts or bundles.
• Information is interpreted based on the labeled line
that it travels as to what type of sensation it is
(qualitative processing).
• True sensations cannot be distinguished from false
sensations.
• All other characteristics of a stimulus (strength, duration,
variability, etc) are conveyed by the frequency and pattern
of the action potentials (quantitative processing).
Receptors
LaPointe Fall ’11
Slide # 20
• Tonic receptors
• Always active
• Frequency of firing determines information
• Slow to adapt
• Phasic receptors
• Are normally inactive
• Give a burst of activity when stimulated
• Provide information about the intensity and rate of
change of a stimulus
• Combined receptors
Adaptation
LaPointe Fall ’11
Slide # 21
• Reduction in sensitivity of a receptor in the presence of
a constant stimulus.
• Peripheral adaptation occurs at the level of the
receptor.
• Reduces sensory information entering the CNS
• fast-adapting receptors - phasic receptors, e.g.
temperature
• slow-adapting receptors - tonic receptors, e.g.
pain, proprioception
Adaptation
LaPointe Fall ’11
Slide # 22
• Central adaptation occurs within the CNS and often
involves inhibitory interneurons within the pathway.
• CNS can inhibit sensory pathways for example to “filter
noise”
• Reduces information reaching the cerebral cortex
• Awareness is reduced even though the receptors are
still active.
• Responses may still occur via lower level circuits.
• CNS output can also facilitate transmission i.e. increase
sensory transmission
Sensory receptors
LaPointe Fall ’11
Slide # 23
• Free nerve endings
• dendrites
• not protected by accessory structures
• sensitive to many stimuli (pain, temperature, pressure,
trauma)
• Complex receptors
• Are often not neural cells
• Merkel cells
• rods and cones
Mechanoreceptors
LaPointe Fall ’11
Slide # 24
• Sensitive to distortion of their membrane
• Mechanically sensitive ion channels
• Tactile receptors (six types) - touch, pressure,
vibration
• touch - shape and texture
• pressure - mechanical distortion
• vibration - pulsing or oscillating pressure
• Baroreceptors - monitor pressure changes
• Proprioceptors (three groups) - joint and muscle
movement, position and location
Tactile receptors
LaPointe Fall ’11
Slide # 25
• Crude touch and pressure - have large receptor fields
• Tactile receptors - more narrow fields provide more
information
1. Free nerve endings
2. Root hair plexus- rapid respond to movement
3. Tactile discs (Merkel discs) fine touch myelinated
fibers
4. Tactile corpuscles (Messner’s corpusules and
Krause end bulbs) - fine touch and pressure, low
frequency vibrations. Myelinated fibers adapt within 1
second after contact
Tactile receptors
LaPointe Fall ’11
Slide # 26
5. Lamellated corpuscles (Pacinian corpuscles) Large structures, deep pressure, fast adapting so more
sensitive to vibrations. Seen in viscera such as
mesentaries, in the pancreas, urethra, and urinary
bladder, as well as skin
6. Ruffini corpuscles - pressure and distortion of skin,
located in deep dermis, show little adaptation.
• Itch and tickle sensations use free nerve endings
Proprioception
• Muscle spindle,
• Golgi tendon organs,
• joint kinesthetic receptors
• stretch receptors or joint pressure
LaPointe Fall ’11
Slide # 27
Chemoreceptors
LaPointe Fall ’11
Slide # 28
• Chemoreceptors of the general senses do not send
information to the primary sensory cortex. Thus, there is
no conscious awareness (sensation without perception).
• Exhibit peripheral adaptation after a few seconds and
may exhibit central adaptation
• Carotid bodies and Aortic bodies are sensitive to
pH, CO2 and O2
• chemoreceptors may respond to chemicals released by
damaged tissue
Summary slide
LaPointe Fall ’11
Slide # 29
Summary slide
LaPointe Fall ’11
Slide # 30
LaPointe Fall ’11
Slide # 31
• Note: Somatosensory projection pathways and pain
pages 587-591 will be covered in the pathways and tracts
lecture
Reflexes
LaPointe Fall ’11
Slide # 32
• Reflexes are rapid automatic responses to stimuli
• Neural reflex involves sensory fibers to CNS and motor
fibers to effectors
Reflex arc
LaPointe Fall ’11
Slide # 33
• Five steps
• Arrival of stimulus and activation of receptor
• Activation of sensory neuron
• Integration / Information processing (interneurons)
• Activation of motor neuron
• Response by effector (muscle or a gland)
Reflex classification
LaPointe Fall ’11
Slide # 34
• Named several ways i.e. according to:
• Development (innate or acquired i.e. learned)
• Site of information processing (cranial or spinal)
• Nature of resulting motor response (e.g. flexor reflex)
• Complexity of neural circuit
Reflex classifications
LaPointe Fall ’11
Slide # 35
• Innate reflexes - Result from connections that form
between neurons during development (e.g. chewing, sucking,
tracking).
• Acquired reflexes - Learned, and typically more
complex (e.g. driving skills, bell ringing and leave class, typing)
• Cranial reflexes - Reflexes processed in the brain (e.g.
startle reflex)
• Spinal reflexes - Interconnections and processing events
occur in the spinal cord (e.g. knee jerk reflex)
More reflex classifications
LaPointe Fall ’11
Slide # 36
• Somatic reflexes
• Control skeletal muscle
• They are imprecise and crude (e.g. the knee jerk reflex)
• Provide a rapid response (e.g. pull away from a hot
surface)
• often modified by higher centers
• Visceral reflexes (autonomic reflexes)
• Control activities of other systems (e.g. blood pressure,
urination, defecation)
and even more reflex classifications
LaPointe Fall ’11
Slide # 37
• Monosynaptic reflex
• Sensory neuron synapses directly on a motor neuron
(there is no interneuron)
• Polysynaptic reflex
• At least one interneuron between sensory afferent and
motor efferent
• Because of synaptic delay, the more interneurons
there are the slower the reflex i.e. the longer delay
between stimulus and response
Monosynaptic Reflexes
LaPointe Fall ’11
Slide # 38
• Stretch reflex automatically monitors skeletal muscle
length and tone
• Patellar (knee jerk) reflex
• Sensory receptors are muscle spindles
• Postural reflexes maintains upright position
Stretch Reflex (e.g. patellar reflex)
LaPointe Fall ’11
Slide # 39
Also see Saladin fig 13.21 with steps involved
Figure 13.16
Muscle spindles
LaPointe Fall ’11
Slide # 40
• Specialized muscle regions used as sensory stretch
receptors.
• Extrafusal muscle fibers
• alpha (a) motor neurons
• Intrafusal muscle fibers
• gamma (g) motor neurons
Muscle spindles (also see Saladin fig 13.20)
LaPointe Fall ’11
Slide # 41
Figure 13.15
Intrafusal Fibers
LaPointe Fall ’11
Slide # 42
Figure 13.17
Golgi Tendon Reflex (also see Saladin fig 13.23)
LaPointe Fall ’11
Slide # 43
•Prevents contracting muscles from applying excessive tension to
tendons
•Produces sudden relaxation of the contracting muscle and
activation of the antagonistic muscles
Flexor and Inhibitory Reflexes
Also see Saladin fig 13.21
LaPointe Fall ’11
Slide # 44
Withdraw and crossed extensor reflexes
Also see Saladin fig 13.22
LaPointe Fall ’11
Slide # 45
Reinforcement and inhibition
LaPointe Fall ’11
Slide # 46
• Brain can facilitate or inhibit motor patterns based in
spinal cord
• Complex movements such as walking can work by
having the brain initiate reflex movements
• Reinforcement - facilitation that enhances spinal
reflexes
• Spinal reflexes can also be inhibited
• Babinski reflex replaced by the Planter reflex
The Plantar and Babinski Reflexes
LaPointe Fall ’11
Slide # 47
Figure 13.23