Exam 2

• Afferent Vs. Efferent

  • Axon Length: short vs. long
  • Receptors: present vs absent
  • Dendrons: Single long vs. many short
  • Multipolar or unipolar: unipolar vs. multipolar
• Sensory System represents the afferent arm of the nervous system and provides the brain with information about the internal and external environment.
• The sensory system consists of

  1. sensory receptors that receive stimuli from the environment
  2. neural pathways that conduct information from the receptors to the brain/spinal cord
  3. parts of the brain that process information
• All parts of the sensory system (hearing, vision, equilibrium, taste) travel to the thalamus except for smell, which goes directly to the olfactory cortex
• Transduction


  Conversion of one form of energy into another
• The sensory system functions by:And the motor system is the inverse

  Threshold by physical stimulus -> transduction by receptors -> conduction by neurons -> processing by the brain -> perception!
• 5 types of receptors and sensations:

  1. Photoreceptors (light)
  2. Mechanoreceptors in skin, joints, muscle, inner ear (touch, pressure, hearing, balance)
  3. Thermoreceptors (temperature)
  4. Chemoreceptors (smell, taste)
  5. Nociceptors (Pain)
• Sensory receptors are transducers which are:


  A device that transforms energy from one type to another.
• Sensory receptors transfer stimulus energy into electrical signals known as:

  Receptor potentials (which upon reaching threshold trigger action potentials)
• The receptor potential is a graded (proportional to stimulus intensity) response that may be depolarizing or hyperpolarizing & can work to trigger an AP within the same cell or adjacent cell via neurotransmitter release.
• The receptor potential is propagated passively towards the neuron's trigger zone
• Primary afferent neuron firing frequency reflects the magnitude of the receptor potential which in turn reflects the magnitude of the stimulus
• Coding


  The conversion of stimuli into the action potentials in the CNS and the AP frequencies
• Key questions of coding:

  What is it? (quality/modality)

  How strong is it? (intensity)

  How long is it? (Timing)

  Where is it? (Localization)
• The specific form of energy of a stimulus (ie. temperature, sound, pressure, light, pain, taste) is its modality
• The modality to which the receptor responds to best is the

  Adequate stimulus
• Stimulus intensity is coded by the frequency of APs generated in the first order neuron
• Stronger stimuli excite more receptors and afferents

  Recruitment of sensory units
• The typical response to a constant stimulus will decrease over time

  Sensory adaptation
• Receptor adaptation can be phasic/rapid (respond to change) or tonic/slow (track duration)
• Labelled line code connects the peripheral receptor to the cortex via a series of neurons
• Nerve terminals have 1st order neurons which send signals up to the 2nd order neurons in the spinal cord which sends them to the 3rd order neurons in the thalamus
• Lateral inhibition involves the inhibitory neurons between 1st and 2nd order neurons that allow the body to "focus" on the are that is being stimulated by preventing the unnecessary activation of nearby neurons
• Sensory neuron disease (SND)


  PNS diseases caused by degeneration of short & long peripheral axons and afferent sensory projections
• Sensory processing disease examples:

  Synesthesia

  Prosopagnosia (face blindness)
• Acetylcholine is released in the neuromuscular junction to activate a motor neuron
• Motor neurons are the final common pathway out of the CNS
• Motor neurons send their axons to innervate the muscles which allows action potentials to travel down the motor neuron and branch into many terminals near their target
• Motor pathways descend from brain through the pons, to the spinal cord, then out to the effector organ
• A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals
• Innervation ratio


  Number of fibers innervated by an axon
• Almost all motor behaviors involve both voluntary and involuntary actions
• Actions become more involuntary as the actions are repeated
• Three classes of motor movements:

  Reflexes

  • Simplest
  • Largely involuntary
  • Spinal cord

  Ex. sneezing, knee-jerk, flexor withdrawal
• Three classes of motor movements:

  Rhythmic Motor Patterns

  • Starting and stopping are voluntary
  • Sequence of stereotyped, repetitive movements
  • Thalamus, basal ganglia, cerebellum

  Ex. Walking, running
• Three classes of motor movements:

  Voluntary movements

  • Goal-directed and purposeful
  • Largely learned and improve with practice
  • Cerebral cortex motor areas

  Ex. Playing piano, writing
• Motor control hierarchy: High

  Pre-command level : Planning

  Controls voluntary movements

  Cortical association cortex (idea) leads to cerebellum, basal ganglia, and motor cortex/brain stem
• Motor control hierarchy: Middle

  Projection level : instructions

  Controls rhythmic motor patterns

  Cerebellum sends to spinal cord and motor cortex/brain stem, the basal ganglia sends to the motor cortex/brain stem which sends to the spinal cord
• Motor control hierarchy: Low

  Segmental level : Central pattern generators

  Controls reflexes

  Spinal cord leads to movement
• Interneurons can be inhibitory or excitatory and can function as switches to turn motor neurons "on" and "off" (also constitute 90% of spinal cord neurons)