Unit 3a - Sensory Systems

Question 1

What is proprioception?

Answer 1

sensory information from skin and muscle/joint receptors

Question 2

What are simple receptors?

Answer 2

neurons with free nerve endings, and may have myelinated or unmyelinated axons

Question 3

What are complex neural receptors?

Answer 3

have nerve endings enclosed in connective tissue capsules

Question 4

What are special sense receptors?

Answer 4

cells that release neurotransmitter onto sensory neurons, initiating an action potential, eg. hair cell

Question 5

What are chemoreceptors?

Answer 5

respond to chemical ligands that bind to the receptor

Question 6

What are mechanoreceptors?

Answer 6

respond to various forms of mechanical energy

Question 7

What are photoreceptors?

Answer 7

respond of light

Question 8

What are thermoreceptors?

Answer 8

respond to temperature

Question 9

What does each sensory receptor have?

Answer 9

an adequate stimulus (type of energy to which it responds best)

although sensors are specific for one form of energy, they respond to most other forms if the intensity is high enough

Question 10

What does the stimulus do to receptors?

Answer 10

stimulus opens or closes ion channels in receptor cell membrane (directly or via second membrane systems)

they mostly open cation channels which results in influx of Na or Ca (depolarization)

sometimes there is an efflux of K (hyperpolarization)

Question 11

What is receptor potential?

Answer 11

change in sensory receptor membrane potential (graded potential)

Question 12

What is a receptive field?

Answer 12

a certain physical area which stimuli must full on to activate somatosensory and visual neurons
eg. photoreceptors - light falling on area of retina

are often defined by neurons further up the pathway (sensory input can then be gathered from more than one primary sensory neuron)

Question 13

What are the two afferent neurons pathways to the brain?

Answer 13

1. first order (primary) sensory neuron which directly associates with stimuli
2. second order (secondary) neuron which relays information from first neuron

Question 14

What do many primary neurons converge onto?

Answer 14

a secondary neuron, so the secondary receptive field is very large

• convergence allows summation of multiple stimuli

Question 15

Does convergence (large receptive fields) have two-point discrimination?

Answer 15

no, the two stimuli fall within same secondary receptive field so only one signal goes to brain and is perceived as a single point

Question 16

Does smaller receptive fields have better two-point discrimination?

Answer 16

yes, the two stimuli activate separate pathway to the brain, so the two points are perceived as distinct stimuli

Question 17

Where is sensory information integrated in?

Answer 17

the CNS
1. olfactory pathways from the nose project through the olfactory bulb to the olfactory cortex
2. most sensory pathways project to the thalamus. the thalamus modifies/processes and relays information to appropriate cortex
3. equilibrium pathways project primarily to the cerebellum

summary: smell smth, goes to the bulb, then the thalamus, then the cortex

Question 18

What are the sensory pathways for visceral sensory information? (neurons that monitor the internal environment and organ systems)

Answer 18

are mostly integrated in brain stem and spinal cord
does not usually reach conscious perception

eg. completely subconscious (blood pressure)

Question 19

How are sensations decoded and processed in the CNS if all stimuli converted to graded potentials to APs are all identical?

Answer 19

CNS must be able to distinguish 4 properties of a stimulus: type of stimulus (modality), location, intensity, and duration

Question 20

How is sensory modality determined?

Answer 20

determined by the type of neuron activated and where the pathway terminates in the brain

Question 21

How is location coded?

Answer 21

according to which receptive fields are activated

eg. touch receptors from a particular part of body project to a specific location in somatosensory cortex

Question 22

What is lateral inhibition and what does it do?

Answer 22

the ability of an excited neuron to reduce the activity of its neighbours (inhibits neurons farther from stimulus)

it enhances contrast and makes a stimulus easier to perceive

eg. the responses of primary sensory neurons A,B,C are proprotional to the intensity of the stimulus in each receptor field, secondary sensory neuron B inhibits secondary neurons A and C, creating grater contrast between B and its neighbours

Question 23

How is stimulus intensity coded?

Answer 23

1. number of receptors activated (population coding)
   - different thresholds for stimulation among group of receptors
   - with low intensity stimulus, most sensitive (lowest threshold) receptors recruited first
   - as stimulus intensifies, more receptors activated
2. frequency of APs coming from individual receptor cells
   - frequency of APs increases with stimulus intensity, up to maximum that the axon can transmit

Question 24

How is duration coded?

Answer 24

longer or stronger stimuli release more neurotransmitter

duration of a series of action potentials is proportional to stimulus duration

Question 25

What are the two types of receptors that adapt (cease to respond) to sustained stimulation?

Answer 25

1. Tonic receptors (slowly adapting, respond throughout stimulus) 2. Phasic receptors (rapidly adapt to a constant stimulus and turn off)

Question 26

What are the types of sensory modalities?

Answer 26

touch, temperature, nociception (pain/itch), proprioception (sensory information from skin and muscle/joint receptors)

Question 27

Where does pain, temperature, and coarse touch cross the midline?

Answer 27

spinal cord

Question 28

Where does fine touch, vibration, and proprioception cross the midline?

Answer 28

in the medulla

Question 29

What is the function of the Merkel's disk receptor?

Answer 29

sensing sustained touch/pressure, texture in the skin

Question 30

What is the function of the Meissner's corpuscle receptor?

Answer 30

sensing beginning and end of fine touch/pressure in the skin

Question 31

What is the function of the Ruffini's corpuscle receptor?

Answer 31

sensing sustained touch/vibration/stretch in the skin

Question 32

What is the function of the Pacinian corpuscle receptor?

Answer 32

sensing beginning and end of touch/vibration in the skin

Question 33

What is the function of the free nerve ending receptors?

Answer 33

sensing pain, temperature, and hair movement in the skin

Question 34

What is nociception?

Answer 34

pain, and is found in many tissues, not just skin

Question 35

What is pain?

Answer 35

a sensation rather than a stimulus

Question 36

How is nociception mediated?

Answer 36

by free nerve endings expressing ion channels that respond to a variety of strong stimuli (causes that have the potential to cause tissue damage)

Question 37

How is pain mediated?

Answer 37

is mediated via release of local chemicals (K+, histamine, prostaglandins, serotonin, substance P) - can either directly activate nociceptors or sensitize them

Question 38

What are TRP (Transient Receptor Potential) Channels?

Answer 38

mediates a variety of sensations including pain, heat/warmth, cold, some tastes, pressure, vision, osmotic pressure, stretch, etc relatively non-selective cation channels (Na, Ca, Mg)

Question 39

What is somatic pain?

Answer 39

information from nociceptors can follow several pathways: spinal reflexes or ascending pathways to cerebral cortex (info is also sent to limbic system and hypothalamus which results in emotional reactions and autonomic responses like nausea, vomiting, sweating) different types of pain travel on different fibre types

Question 40

What is visceral/referred pain?

Answer 40

pain in internal organs that is often sensed on the surface of the body - often poorly localized, and perceived to be in distance parts of body, or on surface

Question 41

How do you classify neural reflexes?

Answer 41

1. according to effector - skeletal muscle (controlled by somatic motor neurons) - smooth and cardiac muscle, glands, adipose tissue (controlled by autonomic neurons) 2. according to integrating centre - spinal reflexes (integrated in spinal cord) - cranial reflexes (integrated in the brain) 3. number of neurons in pathway - monosynaptic reflex (somatic motor reflexes only) - polysynaptic reflex (autonomic reflexes)

Question 42

What are autonomic (visceral) reflexes?

Answer 42

polysynaptic, with at least one synapse in the CNS and another in the autonomic ganglion - some are spinal reflexes and some are integrated in brain - can often be modulated by signals from higher centres - inhibition by higher centres can be a learning response (holding pee) - emotional stimuli can be converted into visceral responses (so nervous you want to pee)

Question 43

What is the skeletal muscle reflex monitor?

Answer 43

proprioception (position of limbs in space, relative position of body parts) - effort exerted in lifting and holding objects

Question 44

What is the efferent pathway of skeletal muscle reflexes?

Answer 44

somatic motor neurons

Question 45

What are proprioceptors?

Answer 45

receptors that sense changes in joint movements, muscle length, muscle tension, and send info to CNS depending on appropriate response, CNS activates motor neurons to make motor units contract, or activate inhibitory interneurons to inhibit motor neurons then muscles relax

Question 46

What does muscle spindles do (a type of proprioceptor)?

Answer 46

monitor muscle stretch, and sends information about muscle stretch to the CNS - they are buried among the extrafusal fibers of the muscle - each spindle consists of 3-12 intrafusal muscle fibres arranged in parallel to extrafusal fibres - almost every muscle in body has spindles - most sensitive to muscle stretch (increased muscle length) -tonically active, sending steady stream of APs even at resting length (CNS is informed about muscle tone) - mediate stretch reflexes (induces contraction when muscle stretched and tends to maintain muscle at a constant length) - unloaded when muscle shortens unless 'tightened up' by contraction of its intrafusal fibres

Question 47

What does Golgi tendon organs do (a type of proprioceptor)?

Answer 47

monitor muscle tension (force of contraction), found between muscle fibres and tendon - whether isotonic or isometric, contraction of muscle causes tendon and GTO to stretch (most sensitive to isometric contraction) - relatively insensitive to muscle stretch (changes in length) - sensory info from GTOs combines with info from spindles and joint receptors in CNS integrating centres (monitors/controls of posture and movement)

Question 48

What is withdrawal (flexion) and crossed extensor reflex?

Answer 48

a reflex that helps maintain balance when one foot is lifted from the ground 1. painful stimulus activates nociceptor 2. primary sensory neuron enters spinal cord and diverges 3 cc to brain to inform on sensation (pain) and postural adjustment 4. flexation and relaxation of appropriate muscles move foot away from pain 5. crossed extensor reflex supports body as weight shifts away from painful stimulus