Sensory Systems

Question 1

What is a sensory system?

Answer 1

Part of the nervous system responsible for processing sensory information

Question 2

The sensory system consists of?

Answer 2

1. sensory receptors that receive stimuli
2. neural pathways that conduct information from receptors to CNS
3. parts of the brain that deal with primarily processing of the information

Question 3

What is sensation?

Answer 3

Sensation is the conscious or subconscious awareness of external or internal stimuli
- Detection of physical energy (a stimulus) from the environment and converting it into neural signals
- Sensory impulses that reach the cerebral cortex allows us to precisely locate and identify specific sensations

Question 4

What is perception?

Answer 4

Perception is the conscious awareness and the interpretation of meaning of sensations
- Selection, organization, and interpretation of our sensations
- We have no perception of some sensory impulses because they never reach the cerebral cortex
e.g. sensory receptors that constantly monitor blood pressure propagate to the cardiovascular centre in the medulla oblongata

Question 5

What are sensory modalities?

Answer 5

• Refers to each type of sensation
e.g. pain, hearing, vision, touch
• A given sensory neuron carries information for one modality only

Question 6

What are classes of sensory modalities?

Answer 6

1. General senses: Include both somatic and visceral senses (tactile, thermal, pain, proprioceptive sensations)
2. Special senses: Include modalities of smell, taste, vision, hearing, and balance

Question 7

What are sensory receptors?

Answer 7

Sensory receptors can either be a specialised cell or dendrites of a sensory neuron that monitors particular condition (stimuli) in the internal or external environment

Question 8

How can different receptors be excited to cause receptor potentials?

Answer 8

1. by mechanical deformation of the receptor, which stretches the receptor membrane and opens ion channels
2. by application of a chemical to the membrane, which also opens ion channels
3. By change of the temperature of the membrane, which alters the permeability of the membrane
4. by the effects of electromagnetic radiation, such as light on a retinal visual receptor

Question 9

What is signal transduction?

Answer 9

• Conversion of a stimulus into a receptor potential (depolarization or hyperpolarization)
• In special senses, stimulus from specialised receptor cells trigger release of an excitatory neurotransmitter onto the sensory neuron that leads to generation of action potential

Question 10

How is an impulse generated?

Answer 10

• When graded potential in a sensory neuron (1st order neuron) reaches threshold, it triggers one or more action potentials that propagate impulses from the PNS into the CNS

Question 11

What happens to the sensory input?

Answer 11

A particular Region of the CNS receives and integrates the sensory nerve impulses

Question 12

How are sensory receptors classified based on structure?

Answer 12

1. Free nerve endings
2. Encapsulated nerve endings at the dendrites
3. Specialized separate cells

Question 13

Describe sensory receptors with free nerve endings?

Answer 13

bare dendrites that often have no visible structural specialisation
e.g. receptors for pain, thermal, tickle, itch

Question 14

Describe sensory receptors with encapsulated nerve endings?

Answer 14

dendrites are enclosed in a connective tissue capsule that has distinct microscopic structure
e.g. lamellated (Pacinian) corpuscles, receptors for somatic and visceral sensations like touch, pressure and vibration are encapsulated

Question 15

Describe the sensory receptors that have specialized separate cells?

Answer 15

Sensory receptors for the special senses of vision (photoreceptors), hearing and equilibrium (hair cells), and taste (gustatory receptor cells) consists of specialised separate cells that synapse with first order neurons

Question 16

Names and describe sensory receptors classified on location?

Answer 16

1. Exteroreceptors
   - Located in or near the external surface of the body and detect changes in the external environment
   e.g. receptors for touch (Meissner’s corpuscles, Merckel’s discs), cold, heat and pain, vision, smell, vision, vibration etc.
2. Interoreceptors
   - Transmit impulses from the visceral organs, blood vessels
   - Usually not consciously perceived
3. Telereceptors (distance receptors)
   - Detect stimuli reaching us from remote sources
   e.g. smell, sound
4. Proprioceptors
   - Supply info containing movements and position of the body in space
   e.g. Golgi tendon organs, muscle spindles, and hair cells in the labyrinth of the inner ear

Question 17

Name and describe sensory receptors classified based on their stimulus type?

Answer 17

1. Mechanoreceptors - Mechanical forces
   e.g. Touch, pressure, stretch, vibrations
2. Thermoreceptors - temperature changes
3. Chemoreceptors - Chemicals in solution: Detect chemicals in the mouth and body fluids
   e.g. Taste, smell, blood chemistry
4. Photoreceptors - Respond to light
5. Nociceptors - Harmful stimuli (pain) from physical or chemical damage to tissue

Question 18

Describe sensory receptors classified based on their distribution?

Answer 18

1. General senses
   - Scattered throughout the body, simple in structure
   e.g. Temperature, pain, touch, pressure, proprioception, chemicals in blood
2. Special senses
   - Receptors located in sense organs in the head where they are protected by surrounding tissues
   e.g. Olfaction (smell), vision, gustation (taste), equilibrium (balance) and hearing

Question 19

What is sensory adaptation?

Answer 19

adaptation either partially or completely to any constant stimulus after a period of time
- When a continuous sensory stimulus is applied, the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potentials decreases to very few or often to none at all
• E.g. cutaneous cold receptors when taking a cold shower

Question 20

What is peripheral adaptation?

Answer 20

When receptors or sensory neurons alter their level of activity
i.e. reduces amount of info that reaches the CNS

Question 21

What is central adaptation?

Answer 21

• Occurs in the CNS
• Involves inhibition of nuclei along a sensory pathway

Question 22

Describe the sensory receptors classified based on response?

Answer 22

1. Tonic receptors adapt slowly and inform about the presence and strength of a stimulus
   - always active and adapt little or not at all
   e.g. pain receptors, proprioceptors
2. Phasic receptors adapt rapidly and therefore inform about the rate of change of a stimulus
   - normally inactive and become active for a short time when stimulus strength changes
   e.g. temp, touch receptors

Question 23

Describe the function of first order neurons?

Answer 23

• Conduct impulses from sensory receptors to the CNS
• Cell bodies are located in dorsal root ganglion if the spinal cord
• Impulses from the face, mouth, eyes, teeth propagate along cranial nerves into brainstem
• Impulses from the neck, trunk, limbs and posterior aspect of head propagate along spinal nerves into spinal cord

Question 24

What are dermatomes?

Answer 24

• Each spinal nerve contains sensory* neurons that serve a specific predictable segment of the body
• A dermatome is the area of the skin that provides sensory input to the CNS via one pair of spinal nerves
• There are 8 cervical nerves (note C1 has no dermatome), 12 thoracic nerves, 5 lumbar nerves and 5 sacral nerves.
• The nerve supply in the adiacent dermatomes overlap
• Knowing which spinal cord segments supply each dermatome makes it possible to locate damaged regions of spinal cord e.g. from radiculopathy (pinched nerve)

Question 25

Describe the function is second order neurons?

Answer 25

• Second order neurons
Interneurons on which first order neurons synapse
• Located in the spinal cord or brain stem
• Conduct impulses from the brain stem and spinal cord to thalamus
• Axons of second order neurons decussate (cross over to opposite side) in the brainstem or spinal cord before ascending to the ventral posterior nuclei of thalamus
All somatic sensory info from one side of body reaches the
thalami

Question 26

Describe the function of third order neutrons?

Answer 26

• Third order neurons
• Second order neurons synapse on third order neurons located in thalamus
• Axons of third order. neurons synapse with neurons of the primary sensory cortex from the thalamus
The thalamus is a relay station

Question 27

The dorsal column transmits what?

Answer 27

Light
Touch
Proprioception
Vibration information
- to the sensory cortex

Question 28

Describe the dorsal column-medial lemniscus pathway?

Answer 28

• The pathways carries signals upwards to the medulla mainly in the dorsal columns of the spinal cord.
• In the spinal cord, the pathway is divided into two tracts:
1. The fasciculus gracilis that conveys sensations from nerve endings below the mid-thorax level
2. The fasciculus cuneatus that conveys sensations from nerve endings above the mid-thorax.
• After the signals synapse and cross to the other side of the medulla, they continue upward through the brainstem to the thalamus via the medial lemniscus (continuation of the dorsal column in the brain stem).

Question 29

What does the spinocerebellar pathway transmit?

Answer 29

Transmits proprioceptive information from
Golgi tendon organs
tactile receptors
joint receptors
muscle spindles
motor signals to the spinal cord (rubrospinal and corticospinal tracts)

Question 30

Describe the spinocerebellar tract?

Answer 30

• Axons terminate in cerebellar cortex
• Important for coordinated movement
• Does not reach our conscious

Question 31

Describe the posterior (direct) spinocerebellar pathway?

Answer 31

• Carries proprioception, cutaneous touch and pressure information from the lower limbs and trunk
• Ascends on the ipsilateral side (does not decussate)
• Enters the cerebellum via the inferior peduncle and terminates as mossy fibers in the cerebellar cortex

Question 32

Describe the anterior spinocerebellar pathway?

Answer 32

• Carries proprioception from the lower limbs
• Crosses over twice: ascends contralaterally in the spinal cord, enters the cerebellum contralaterally through the superior cerebellar peduncle then decussates back to the ipsilateral side of the input within the cerebellum
• Fibers of the anterior spinocerebellar tract are less numerous than those of the posterior spinocerebellar one and are also larger in size

Question 33

Function of spinothalamic tract?

Answer 33

Carry pain, temperature, non distributive touch and pressure information to the thalamus

Question 34

Describe the spinothalamic tracts?

Answer 34

spinal cord from the dorsal spinal nerve roots, 1st order neurons synapse in grey matter of the cord.
• 2nd order neurons cross within the grey matter before ascending to the thalamus through the anterior and lateral white columns of the cord to terminate in the thalamus.

Question 35

Describe the lateral spinothalamic tracts?

Answer 35

• Carries pain and temperature sensation.
• A useful mnemonic to remember the modalities of the lateral spinothalamic tract is “Pa-Te-La” (Pain, Temperature via Lateral spinothalamic).

Question 36

Describe the anterior spinothalamic tract?

Answer 36

Carrie’s light touch and pressure sensation

Question 37

Describe the spinoretucular tract?

Answer 37

Alertness and arousal in response to pain

Question 38

Describe the trigeminothalamic tract?

Answer 38

Ine sensory opntnalmic nerve
(V1), the sensory maxillary nerve
(V2), and the mixed sensory and motor mandibular nerve (V3)
Pain, temperature, vibration, fine and crude touch and
proprioception sensations from the head

Question 39

Describe the spinomesencephalic/spinotectal tract?

Answer 39

• Terminates in different areas in the midbrain e.g. periaqueductal gray
Transmits nociceptive information and facilitation of reflexive head movement towards noxious stimuli.