2: Physiology - Primary sensory neurons and their modalities

Question 1

Which senses is the somatosensory system concerned with?

Answer 1

Touch (inc. fine, firm, pressure, vibration)

Pain

Temperature

Itch

Proprioception

Question 2

Which structures are classed as the

a) CNS
b) PNS?

Answer 2

a) Brain and spinal cord

b) Nerves outside the brain and spinal cord

Question 3

In which nervous system are the bodies of first-order neurons of the somatosensory system found?

Answer 3

PNS

Question 4

Where in the PNS are most first-order neurons found?

Answer 4

Dorsal root ganglion

Question 5

Most first-order neurons are found at the dorsal root ganglion. Where are they found in the head and neck?

Answer 5

Cervical ganglia

Question 6

Which cranial nerve supplies sensory fibres to the anterior head?

Answer 6

CN V

Trigeminal nerve

Question 7

Where do pre-ganglionic sensory fibres from the trigeminal nerve synapse?

Answer 7

Cervical ganglia

Question 8

Second-order neurons of the somatosensory system can be found in either the dorsal horn or the medulla.

Which sensory pathways have second order neurons in the

a) dorsal horn
b) medulla?

Answer 8

a) Dorsal column - medial lemniscus system

b) Spinothalamic tract

Question 9

What sensations are picked up by the

a) DC/ML system
b) spinothalamic tract?

Answer 9

a) Fine touch, proprioception, vibration

b) Firm touch, pressure, temperature, pain

Question 10

Where are third-order neurons of the somatosensory system found?

Answer 10

Thalamus

No matter which pathway

Question 11

In the

a) DC/ML pathway
b) spinothalamic tract

which structures are connected by the first-order neurons?

Answer 11

a) Dorsal root ganglia TO medulla

b) Dorsal root ganglia TO spinal cord

Question 12

As the intensity of a sensory stimulus increases, what happens to the amplitude of the receptor’s potential?

Answer 12

As intensity increases, receptor potential amplitude increases

Question 13

A somatic receptor will generate an action potential once a ___ potential has been reached.

Answer 13

threshold

Question 14

To generate a sensory signal, a stimulus must have enough ___ to overcome the threshold potential of the receptor.

Answer 14

intensity

Question 15

In relation to sensory receptors, what is meant by the term adequate stimulus?

Answer 15

Receptors are only activated by the stimulus they are programmed for

e.g a mechanoreceptor won’t generate an action potential in response to heat

Question 16

Which type of receptors sense touch, pressure and vibration?

Where are they found?

Answer 16

Mechanoreceptors

Skin

Question 17

Which type of receptors are responsible for proprioception?

Where are they found?

Answer 17

Mechanoreceptors

Joints and muscles

Question 18

Which type of receptors are responsible for detecting temperature?

Answer 18

Thermoreceptors

Question 19

Which type of receptors are responsible for the sensation of pain?

Answer 19

Nociceptors

Question 20

What is an adequate stimulus?

Answer 20

A stimulus matching the type of receptor which senses it

Question 21

Different receptors respond to increasing ___ of stimulus.

Answer 21

intensities

Question 22

What happens to the frequency of action potentials generated by a receptor as the intensity of a stimulus increases?

Answer 22

APs become more frequent

Question 23

Which receptors respond to

a) low intensity stimuli
b) high intensity, damaging stimuli?

Answer 23

a) Thermoreceptors, mechanoreceptors, etc.

b) Nociceptors

Question 24

What is the difference between low intensity receptors and high intensity nociceptors?

Answer 24

Different threshold potentials

i.e each is activated by different intensities of stimulus

Question 25

In **slowly adapting** **receptors,** what is the relationship between stimulus intensity and firing rate?

Answer 25

**Directly proportional** The greater the stimulus intensity, the greater the firing rate

Question 26

In **fast adapting receptors**, what increases the **firing rate**?

Answer 26

**Greater RATE of change in stimulus** i.e a sharp increase will increase the firing rate more than a slow one

Question 27

Which two factors affect the **conduction velocity** of an axon?

Answer 27

**1. Diameter of axon** (which is difficult to change in real time) ## Footnote **2. Degree of myelination**

Question 28

Which type of receptors tend to have the a) fastest b) slowest conduction velocities?

Answer 28

**a) Proprioceptors** (body needs constant feedback on where its bits are) **b) Pain fibres** (why there's a delayed response between burning your hand on something and feeling it)

Question 29

**Sensory axons** are arranged in groups from **A** to **C**. What happens to the **axon diameter** and **degree of myelination** as you go from A to C?

Answer 29

**Axon diameter decreases** **Degree of myelination** **decreases** So conduction velocity decreases

Question 30

What is the term for a **sensory nerve's territory on the skin**?

Answer 30

**Receptive field**

Question 31

What is a **receptive field**?

Answer 31

**Area on the skin corresponding to the territory of ONE sensory neuron**

Question 32

How are sensory nerves arranged in areas of ## Footnote **a) low acuity** **b) high acuity?**

Answer 32

**a) Few neurons, big receptive fields** **b) Loads of neurons each with their own small receptive fields**

Question 33

Sensory nerves have numbered **types** depending on whether they have **small receptive fields** or **large receptive fields**. What are these types?

Answer 33

**Type 1** - small sensory fields (high acuity) **Type 2** - large sensory fields (lower acuity)

Question 34

From Dermatology, which skin cells/structures are notable for being prone to a **high mortality cancer**?

Answer 34

**Merkel cells**

Question 35

Which sensory receptors are found as complexes with **Merkel cells**? What **a) adaptation** and **b) receptive field** properties do they have? What do they sense?

Answer 35

**Merkel cell-neurite complexes** **Slow adapting, small receptive fields** (so SA1 for short) Pressure, texture and object edges

Question 36

What do **Meissner corpuscles** sense?

Answer 36

**Light touch**

Question 37

What do **Pacinian corpuscles** sense?

Answer 37

**Pressure**

Question 38

What do **Ruffini endings** detect?

Answer 38

**Shearing forces**

Question 39

What is detected by a) Merkel cell-neurite complexes b) Ruffini endings c) Pacinian corpuscles d) Meissner corpuscles?

Answer 39

**a) Fine touch** **b) Shearing forces** (skin stretching)) **c) _Pressure_, high frequency vibration** **d) Low frequency vibration**

Question 40

What is a **dermatome**?

Answer 40

**Area of skin supplied with sensory fibres from a root of a spinal nerve** So anterior root does the anterior dermatome, posterior root does the posterior dermatome

Question 41

The skin of **most of the body** is supplied with sensory fibres by **spinal nerves**. Which part of the body is supplied with sensory fibres by **CN V?**

Answer 41

**Face**

Question 42

Which **ganglion** becomes infected by varicella zoster to cause **shingles** in a dermatomal pattern?

Answer 42

**Dorsal root ganglion**