7- Sensory Transduction + Reflexes

definition of transduction

sensory systems are made up of receptors that convert various forms of energy into electrical signals

4 forms of transduced energy

1. mechanical: touch, pressure, joint

2. chemical: smell + taste

3. electromagnetic: light on retina

4. thermal

what are proprioceptors

• provide info about the positions of different body parts, needed to coordinate movement

• monitor stretch in locomotory organs

3 types of proprioceptors

1. muscle spindles

2. Golgi tendon organs

3. joint kinesthetic receptors

what are muscle spindles

• measures changing length of a muscle

• imbedded in perimysium between muscle fascicles

what are Golgi tendon organs

• monitors tension within tendons

• located near muscle-tendon junction

what are joint kinesthetic receptors

sensory nerve endings within joint capsules

what’s a generator (receptor) potential

depolarization after reception of signals by sensory neurons

describe the mechanism of a generator potential

sensory signal received → Na⁺ channels open

T/F: generator potentials are graded

true, because increasing a stimulus produces an increase in generator potential amplitude

what’s a coding (spiking-generating) region

region adjacent to receptor region, where voltage-gated channels are located + action potentials are generated if receptor potential exceeds threshold

what is frequency modulated coding

when increased sensory stimulus intensity → increased frequency action potentials

what is adaptation

prolonged stimulus → generator potential gradually decreases → action potential frequency decreases (aka accommodation)

T/F: accommodation can still occur even if generator potential remains the same

true

describe sensory input at the spinal cord level

• neurons within the cord are arranged into neuronal pools w/ varying numbers of cells + each pool receives input from primary afferent neurons

• a neuron in a pool can receive excitatory, inhibitory stimulation or both

what is facilitation

a spinal cord neuron can become transiently more excitable to incoming stimulation after previous stimulation that did not go to threshold → increases probability of postsynaptic firing if another excitatory stimulus comes

2 types of neuronal pools

1. divergent

2. convergent

divergent vs. convergent neuronal pools

1. divergent: single presynaptic neuron branches to affect multiple postsynaptic neuron

2. convergent: many presynaptic neurons converge to a smaller # of post synaptic neurons

2 types of zones within a neuronal pool

1. discharge zone: center neurons that’re more likely to reach threshold b/c they have multiple synaptic connections w/ the presynaptic neuron

2. facilitated zone: peripheral neurons that may be depolarized but not to threshold, more excitable to future stimuli

what are the 2 layers of matter in the spinal cord

1. white matter- outer

2. grey matter- inner

what’s in white matter

myelinated neurons carrying signals up + down the cord

what’s in grey matter

• mostly non-myelinated neurons called interneurons

• grey matter has dorsal + ventral horns

each spinal nerve communicates w/ the spinal cord via

2 pathways:

1. dorsal root

2. ventral root

describe how the mechanism of flexion (withdrawal) reflex starts

pain stimulus carried to spinal cord via 1^o afferent (sensory) neuron → afferent neuron enters cord via dorsal root

what happens once the afferent neuron enters the cord

afferent synapses w/ interneuron → interneuron synapses w/ α-motor neuron (efferent) neuron in ventral horn

where does the efferent neuron go after it synapses w/ interneuron

efferent leaves cord via ventral root + terminates in effector organ → muscle contracts to withdraw from pain stimulus

describe the location of the 1^o afferent (sensory) neuron

• soma located in dorsal root ganglion (an enlarged region in the dorsal root containing the somas of many afferent neurons)

• axon terminates in dorsal horn of grey matter

the arrangement of afferent, efferent + interneuron in the flexion (withdrawal) reflex is called

reflex arc

what’s reciprocal inhibition (innervation)

within peripheral muscle pairs (ex: biceps brachii + triceps):

the agonist is stimulated while the antagonist is simultaneously inhibited by excitatory + inhibitory interneuron activated by the same afferent neuron

interneurons create

neural circuits, enabling communication between sensory + motor neurons + w/ the CNS

are interneurons excitatory or inhibitory

both

3 types of interneurons

1. ipsilateral: connect afferents to efferents on the same side of cord

2. contralateral: connect afferents to efferents on the opposite side of cord

3. intersegmental: ascending/descending, carries afferent signals to efferents located 1 or more spinal segments above/below its origin

2 types of intersegmental interneurons

1. short: typically travel entirely within gray matter of cord

2. long: originate in gray matter but may leave gray matter + travel to distant cord segments within the fasciculus proprius, a region of white matter adjacent to the gray (red in figure)

when long intersegmental interneurons are in the fasciculus proprius they’re called

propriospinal neurons, which are myelinated + re-enter gray matter when they reach their destination

T/F: same afferent signal elicits responses on both sides of the body

true

what are second order (secondary) afferent fibers

• they carry sensory info from primary afferent in cord to cognitive centers in brain

• are myelinated, have rapid conduction velocities

• run in “tracts” in white matter of the cord