Partial Chapters 12, 13

1
Partial Chapters 12, 13 16

• Circuits, Receptors, and Reflexes

2
Postsynaptic potentials

• 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

3
ESPS and ISPS
4
Temporal and Spatial Summation
5
Summation (facilitation)
6
EPSP IPSP Interactions (inhibition)
Martini Figure 12.23
7
Summation
Seeley, Stephens and Tate
Spatial summation
Mixed summation
Temporal summation
8
Presynaptic inhibition and facilitation

• 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

9
Presynaptic Inhibition
10
Information processing

• 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.

11
Neuronal pools

• Functional group of interconnected neurons
• Neural circuit patterns
• Divergence
• Convergence
• Reverberation
• Serial processing
• Parallel processing

12
Simple circuits (also see Saladin figs 12.29
12.30)
Martini Figure 13.15
13
Sensory receptors

• 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

14
Receptor classification

• 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.

15
Classification of receptors

• 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

16
Sensory receptors

• Nerve fibers fire when an action potential is
  generated regardless of what caused it. Thus,
  they are non-specific.
• 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.

17
Receptors and Receptive Fields
Figure 15.2
18
Receptor fields and 2 point discrimination
19
Interpretation of sensory information

• 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).

20
Receptors

• 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

21
Adaptation

• 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

22
Adaptation

• 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

23
Sensory receptors

• 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

24
Mechanoreceptors

• 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

25
Tactile receptors

• 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 (Messners corpusules and
  Krause end bulbs) - fine touch and pressure, low
  frequency vibrations. Myelinated fibers adapt
  within 1 second after contact

26
Tactile receptors

• 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

27
Proprioception

• Muscle spindle,
• Golgi tendon organs,
• joint kinesthetic receptors
• stretch receptors or joint pressure

28
Chemoreceptors

• 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

29
Summary slide
30
Summary slide
31

• Note Somatosensory projection pathways and pain
  pages 587-591 will be covered in the pathways and
  tracts lecture

32
Reflexes

• Reflexes are rapid automatic responses to stimuli
• Neural reflex involves sensory fibers to CNS and
  motor fibers to effectors

33
Reflex arc

• 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)

34
Reflex classification

• 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

35
Reflex classifications

• 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)

36
More reflex classifications

• 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)

37
and even more reflex classifications

• 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

38
Monosynaptic Reflexes

• Stretch reflex automatically monitors skeletal
  muscle length and tone
• Patellar (knee jerk) reflex
• Sensory receptors are muscle spindles
• Postural reflexes maintains upright position

39
Stretch Reflex (e.g. patellar reflex)
Also see Saladin fig 13.21 with steps involved
Figure 13.16
40
Muscle spindles

• Specialized muscle regions used as sensory
  stretch receptors.
• Extrafusal muscle fibers
• alpha (a) motor neurons
• Intrafusal muscle fibers
• gamma (g) motor neurons

41
Muscle spindles (also see Saladin fig 13.20)
Figure 13.15
42
Intrafusal Fibers
Figure 13.17
43
Golgi Tendon Reflex (also see Saladin fig 13.23)

• Prevents contracting muscles from applying
  excessive tension to tendons
• Produces sudden relaxation of the contracting
  muscle and activation of the antagonistic muscles

44
Flexor and Inhibitory Reflexes
Also see Saladin fig 13.21
45
Withdraw and crossed extensor reflexes
Also see Saladin fig 13.22
46
Reinforcement and inhibition

• 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

47
The Plantar and Babinski Reflexes
Figure 13.23