chapter 48

Question 1

What are somatic senses?

Answer 1

Somatic senses are the body’s way of collecting sensory information from all over—not just from the head or specific organs. They are different from special senses, which include vision, hearing, smell, taste, and balance.

Question 2

Main types of somatic senses?

Answer 2

1) Mechanoreceptive senses – include touch and position sensations, triggered by physical movement or pressure.
2) Thermoreceptive senses – detect temperature changes.
3) Pain sense – responds to tissue damage.

Question 3

Classifications based on origin?

Answer 3

1) Exteroreceptive sensations – from body surface.
2) Proprioceptive sensations – internal physical state, like body position, muscle stretch, and sometimes balance.
3) Visceral sensations – from internal organs.
4) Deep sensations – from muscles, bones, fascia; involve pressure, pain, vibration.

Question 4

Difference between touch, pressure, and vibration?

Answer 4

They share many receptors but differ in stimulus: Touch – from surface contact; Pressure – deeper tissue deformation; Vibration – rapid, repeated movement.

Question 5

Touch receptor types?

Answer 5

1) Free nerve endings – in skin and tissues, detect light touch and pressure.
2) Meissner’s corpuscles – in non-hairy skin, detect movement and low-frequency vibration, adapt quickly.
3) Merkel’s discs – in skin and hair, detect steady pressure and texture, adapt slowly.
4) Hair end-organs – detect hair movement, adapt quickly.
5) Ruffini’s endings – in deep layers and joints, detect continuous pressure and stretch, adapt slowly.
6) Pacinian corpuscles – in deep tissues, detect rapid vibration, adapt extremely fast.

Question 6

Function of free nerve endings?

Answer 6

Found in skin and other tissues, detect light touch and pressure; present in sensitive areas like the cornea; use Aδ or C fibers.

Question 7

Function of Meissner’s corpuscles?

Answer 7

Found in non-hairy areas (e.g., fingertips, lips); connected to myelinated Aβ fibers; sensitive to movement and low-frequency vibration; adapt rapidly.

Question 8

Function of Merkel’s discs?

Answer 8

Found in hairy and non-hairy skin; detect steady pressure and continuous touch; grouped in ‘touch domes’; connected to a single Aβ fiber; give strong initial signal then sustained weak one.

Question 9

Function of hair end-organs?

Answer 9

Each hair is wrapped with a nerve fiber; detects movement or initial contact; adapts quickly.

Question 10

Function of Ruffini’s endings?

Answer 10

Located in deep skin and joints; respond to stretch and continuous pressure; adapt slowly; help detect joint rotation.

Question 11

Function of Pacinian corpuscles?

Answer 11

Located in deep skin and connective tissue; detect high-frequency vibration and rapid compression; adapt extremely fast.

Question 12

Which fibers carry touch signals?

Answer 12

Meissner’s, Merkel’s, Pacinian, Ruffini, and hair end-organs use large myelinated Aβ fibers (30–70 m/s); free nerve endings use smaller Aδ fibers (5–30 m/s) or unmyelinated C fibers (<2 m/s).

Question 13

What sensations use fast fibers?

Answer 13

Fine touch, vibration, and precise location use fast Aβ fibers.

Question 14

What sensations use slow fibers?

Answer 14

Dull pressure, tickle, and itch use slower Aδ or C fibers.

Question 15

How is vibration sensed?

Answer 15

All touch receptors contribute. Pacinian corpuscles detect high-frequency (30–800 Hz) due to rapid adaptation and fast Aβ fibers. Meissner’s corpuscles detect lower-frequency vibrations.

Question 16

What detects tickle and itch?

Answer 16

Free nerve endings in skin surface detect tickle and itch; fast-adapting; signals sent via unmyelinated C fibers.

Question 17

Why do we scratch an itch?

Answer 17

Scratching activates pain fibers, which block itch signals in the spinal cord via lateral inhibition.

Question 18

How do sensory signals enter the CNS?

Answer 18

Sensory signals like touch and temperature enter through the dorsal roots of spinal nerves, then travel to the brain via two main pathways: 1) Dorsal column–medial lemniscal system, 2) Anterolateral system.

Question 19

Steps in dorsal column–medial lemniscal system?

Answer 19

1) Signal enters dorsal spinal cord.
2) Travels up to medulla.
3) Synapses in medulla.
4) Crosses to opposite side.
5) Ascends via medial lemniscus.
6) Reaches thalamus.

Question 20

Steps in anterolateral system?

Answer 20

1) Signal enters spinal cord.
2) Synapses in dorsal horn.
3) Crosses over.
4) Travels in anterior/lateral columns. 5) Reaches brainstem and thalamus.

Question 21

Difference in fibers of the two pathways?

Answer 21

Dorsal column–medial lemniscal: large, fast, myelinated fibers (30–110 m/s). Anterolateral: small, slower fibers (few m/s to ~40 m/s).

Question 22

Difference in spatial orientation?

Answer 22

Dorsal column–medial lemniscal system: high spatial orientation (precise location). Anterolateral system: low spatial orientation (less precise).

Question 23

What does the dorsal column–medial lemniscal system carry?

Answer 23

Carries fine touch, vibration, and proprioception. Fast and precise.

Question 24

What does the anterolateral system carry?

Answer 24

Carries pain, temperature, and crude touch. Broader range but less detail.

Question 25

Pathway of medial branch in dorsal column system?

Answer 25

Large myelinated fibers enter dorsal roots, medial branch ascends in dorsal column on same side to brain.

Question 26

Pathway of lateral branch in dorsal column system?

Answer 26

Lateral branch enters dorsal horn and splits: 1) Some fibers ascend to brain via dorsal columns. 2) Some form short spinal reflexes. 3) Some go to spinocerebellar tracts for balance and coordination.

Question 27

Role of spinocerebellar fibers?

Answer 27

Part of lateral branch of dorsal column system; send signals to cerebellum to help coordinate movement and balance; separate from thalamic path but vital for motor control.

Question 28

Spatial orientation in the dorsal column–medial lemniscal system

Answer 28

As sensory information travels from the body to the brain, it stays neatly organized. In the spinal cord, signals from the lower body (legs and feet) are located more centrally in the dorsal columns. As signals from the upper body (arms, neck) enter, they're added more laterally. This spatial layout continues up to the thalamus (ventrobasal complex), where the legs are represented laterally and the face medially. Because fibers cross in the medulla, each brain hemisphere processes sensations from the opposite body side.

Question 29

Where somatosensory signals are processed in the brain

Answer 29

Sensory signals are processed in the somatosensory cortex, located behind the central sulcus in the parietal lobe. It interprets sensations like touch, pressure, and body position.

Question 30

Parts of the somatosensory cortex

Answer 30

It has two parts: Somatosensory Area I (S1) and Somatosensory Area II (S2).

Question 31

Somatosensory Area I (S1)

Answer 31

Located in the postcentral gyrus; includes Brodmann areas 3, 1, and 2. It has a precise body map. Regions like the lips, face, and fingers have large areas due to high receptor density, while the back and legs take up less space.

Question 32

Somatosensory Area II (S2)

Answer 32

Smaller and located more laterally than S1. It has a less detailed body map, receives input from both sides of the body, from S1, and from visual and auditory cortices. If S2 is damaged, S1 still functions. If S1 is removed, S2 stops functioning.

Question 33

Sensory homunculus representation

Answer 33

A distorted map of the body in the somatosensory cortex showing how much cortical area is devoted to each body part. Areas like lips, face, tongue, and thumb take up large regions due to high sensitivity. Trunk, hips, and legs take up less space. On the cortex surface, face and mouth are lateral, arms/shoulders/legs are medial. Each hemisphere processes the opposite body side.

Question 34

Number of layers in the cerebral cortex

Answer 34

There are six layers (I–VI) from the surface inward. Each has distinct functions in processing sensory signals.

Question 35

First layer activated by incoming sensory signals

Answer 35

Layer IV is activated first. Signals then spread to upper and lower layers.

Question 36

Function of Layers I and II

Answer 36

Receive input from lower brain areas to regulate overall cortical activity and prepare the cortex for action.

Question 37

Function of Layers II and III

Answer 37

Send signals to the opposite brain hemisphere through the corpus callosum for inter-hemispheric communication.

Question 38

Function of Layers V and VI

Answer 38

Send signals to deeper parts of the nervous system. Layer V sends large signals to basal ganglia, brainstem, spinal cord. Layer VI sends signals to the thalamus and interacts with incoming sensory data.

Question 39

Columnar organization of neurons in somatosensory cortex

Answer 39

Neurons are arranged in vertical columns spanning all six layers. Each column handles a specific sensory modality (e.g., pressure, stretch, touch).

Question 40

Column diameter and neuron count

Answer 40

Each column is about 0.3–0.5 mm wide and contains ~10,000 neurons.

Question 41

Function of Layer IV columns

Answer 41

Columns at Layer IV (where input first enters) work almost independently.

Question 42

Function of upper layers of columns

Answer 42

Neurons work together to analyze and interpret sensory information more integratively.

Question 43

Columns in area 3A

Answer 43

Located at the front of somatosensory cortex, these columns respond to stretch receptors in muscles, tendons, joints. Helps with body position awareness and movement control.

Question 44

Columns further back in the cortex

Answer 44

Respond more to skin signals. Slow-adapting touch receptors are processed centrally, deep pressure more posteriorly.

Question 45

Directional columns in the posterior cortex

Answer 45

About 6% of columns respond only to stimuli moving in a specific direction across the skin. These help detect motion and refine touch perception.

Question 46

Effect of damage to Somatosensory Area I

Answer 46

If Somatosensory Area I is damaged or removed, a person may still feel basic sensations like touch, pain, and temperature, but loses precise sensory judgment. Effects include: 1) Loss of precise localization—can't pinpoint where the sensation is felt. 2) Difficulty judging pressure—can’t determine how strong pressure is. 3) Difficulty judging weights—can’t tell object weight by holding. 4) Loss of form recognition—can't recognize object shapes by touch (called astereognosis). 5) Difficulty judging texture—can’t distinguish materials' textures, especially with fingers. Pain and temperature are still felt but poorly localized.

Question 47

Somatosensory association areas

Answer 47

Brodmann areas 5 and 7, located in the parietal cortex just behind Somatosensory Area I, help us interpret deeper meanings of sensory inputs. They integrate information from Somatosensory Area I and other brain regions. Stimulation can cause the feeling of touching complex objects like a ball or knife.

Question 48

Inputs to somatosensory association areas

Answer 48

They receive signals from: 1) Somatosensory Area I, 2) Ventrobasal nuclei of the thalamus, 3) Other thalamic regions, 4) Visual cortex, 5) Auditory cortex.

Question 49

Amorphosynthesis from association area damage

Answer 49

If the somatosensory association area is removed from one side of the brain, the person loses the ability to recognize complex shapes or forms felt on the opposite side of the body. They may lose awareness of that body side, not use it for movement, and may only perceive one side of objects. This condition is called amorphosynthesis.

Question 50

Dorsal column–medial lemniscal signal transmission

Answer 50

This pathway is the primary route for sensory signals to the brain. In the cortex, central neurons respond most to weak stimuli. As stimulus strength increases, more neurons fire, with central ones responding faster. This helps distinguish spatial stimuli and aids in sensory analysis like two-point discrimination.

Question 51

Two-point discrimination test

Answer 51

A test to evaluate tactile sensitivity. Two needles are pressed on the skin to see if a person can distinguish one point or two. On fingertips, people detect two points only 1–2 mm apart. On the back, points must be 30–70 mm apart to be felt separately. This difference is due to higher tactile receptor density in sensitive areas like fingertips.

Question 52

Role of dorsal column in two-point discrimination

Answer 52

The dorsal column system supports two-point discrimination by transmitting detailed spatial information. Higher receptor density and precise neural processing in certain areas (e.g., fingertips) allow fine spatial resolution of touch stimuli."Question

Question 53

Somatosensory cortex response to two-point stimulation

Answer 53

When two nearby points on the skin are stimulated at once, the somatosensory cortex shows two distinct peaks of neural activity (as seen in Figure 48-10). This separation of activity helps the brain recognize that two separate locations are being touched.

Question 54

Function of lateral inhibition

Answer 54

Lateral inhibition enhances sensory precision by having excited neurons inhibit their neighboring neurons. This reduces surrounding noise and strengthens the central signal, helping the brain localize stimuli more accurately. It occurs at multiple levels: 1) Dorsal column nuclei in the medulla, 2) Ventrobasal nuclei of the thalamus, 3) Somatosensory cortex.

Question 55

Purpose of lateral inhibition in sensory processing

Answer 55

Lateral inhibition helps the brain sharpen distinctions between closely spaced sensory inputs by enhancing contrast between them. This allows better spatial resolution, such as in two-point discrimination.

Question 56

Dorsal column and rapidly changing signals

Answer 56

The dorsal column system efficiently detects rapidly changing or repetitive stimuli, even those changing in 1/400th of a second. It is especially suited for sensations like vibration, which require fast signal transmission.

Question 57

Types of vibratory signals and their receptors

Answer 57

High-frequency vibrations (up to 700 cycles per second or more) are detected by Pacinian corpuscles in the skin and deeper tissues. Low-frequency vibrations are detected by Meissner’s corpuscles located in the skin. Both types are transmitted via the dorsal column system.

Question 58

Clinical test for dorsal column function

Answer 58

Neurologists use vibration tests (like placing a vibrating tuning fork on the skin) to evaluate the dorsal column pathway. Since vibratory signals travel only through this system, it provides a reliable test for its function."Question

Question 59

Why must sensory systems handle a wide range of intensities?

Answer 59

Sensory systems must detect a wide range of stimulus intensities to accurately represent both the internal condition of the body and the external environment. For example, the auditory system handles sound intensities that differ by more than 10 billion times, while the skin detects pressure changes up to 100,000 times, and the eyes perceive light changes over a half-million-fold range.

Question 60

How do receptors like Pacinian corpuscles adapt to stimulus intensity?

Answer 60

Pacinian corpuscles are highly sensitive to small intensity changes at low stimulus levels, but as stimulus intensity increases, the receptor requires a much larger change to generate the same amount of response. This helps them handle a wide range of intensities.

Question 61

How does the auditory system detect sound intensity?

Answer 61

Weak sounds activate only a few hair cells in the cochlea, while louder sounds activate more hair cells and increase the firing rate of each auditory nerve fiber. This combined increase in the number of active fibers and their firing rates allows the system to represent a broad range of sound intensities.

Question 62

What does the range of sensory reception enable in real life?

Answer 62

It allows humans to adapt naturally to different environments, like bright or dim lighting, without needing tools (e.g., a light meter for photography), showing how advanced and adaptable human sensory systems are compared to machines.

Question 63

What is the Weber-Fechner Principle?

Answer 63

The Weber-Fechner Principle states that perceived stimulus strength follows a logarithmic relationship to actual stimulus intensity. This means that the change a person notices depends on the ratio of added intensity to the original. For example, increasing weight from 30g to 31g is noticeable, but at 300g, a 10g increase is needed. Formula: Interpreted signal = Log(Stimulus) + Constant.

Question 64

When does the Weber-Fechner principle work best?

Answer 64

It applies well to high-intensity sensations like vision, hearing, and touch but is less accurate for other types of sensory input.

Question 65

What is the Power Law of sensory intensity?

Answer 65

The Power Law describes perceived intensity as: Interpreted signal = K × (Stimulus)^y. K and y vary by sensation type. It often fits sensory data better than the Weber-Fechner law and becomes linear when plotted on logarithmic coordinates.

Question 66

What are the two types of position sense (proprioception)?

Answer 66

1) Static position sense: Awareness of the position of body parts relative to each other. 2) Rate of movement sense (kinesthesia): Awareness of the speed or rate at which a body part moves.

Question 67

What receptors contribute to proprioception?

Answer 67

Position sense uses both skin and deep joint receptors. In fingers, skin receptors provide about half the input, while in large joints (e.g., knees), deep receptors like muscle spindles, Pacinian corpuscles, Ruffini endings, and Golgi tendon-like receptors dominate.

Question 68

What is the role of muscle spindles in proprioception?

Answer 68

Muscle spindles are key for detecting joint position during normal movement. They sense changes in muscle stretch and send signals to the spinal cord and brain, helping coordinate joint positioning and muscle control.

Question 69

What receptors are important at the ends of joint motion?

Answer 69

At joint extremes, ligament and tissue receptors such as Pacinian corpuscles, Ruffini endings, and Golgi tendon-like receptors provide position feedback, especially in response to stretch or compression.

Question 70

What receptors detect fast movement?

Answer 70

Pacinian corpuscles and muscle spindles detect fast movement, making them key for sensing the rate of motion (kinesthesia).

Question 71

How is position sense processed in the brain?

Answer 71

Position sense signals are processed in the dorsal column–medial lemniscal pathway. In the thalamus, some neurons respond most when joints are fully rotated, while others respond best when joints are in minimal rotation. This pattern helps the brain judge how far a joint has moved.

Question 72

What sensations are transmitted by the anterolateral pathway?

Answer 72

The anterolateral pathway transmits pain, temperature, crude touch, pressure, tickle, itch, and sexual sensations. These are less precise but still essential for sensory perception.

Question 73

Where does the anterolateral pathway begin and where does it go?

Answer 73

It begins in the dorsal horn of the spinal cord (laminae I, IV, V, VI), where sensory fibers from the dorsal roots terminate. Fibers then cross the spinal cord via the anterior commissure and ascend in the anterior and lateral white columns as spinothalamic tracts. These end in the brainstem’s reticular nuclei and two thalamic regions: the ventrobasal complex and intralaminar nuclei.

Question 74

Where do tactile and pain signals go in the anterolateral pathway?

Answer 74

Tactile signals go to the ventrobasal complex of the thalamus (same as dorsal column input) and then to the somatosensory cortex. Pain signals first reach the brainstem’s reticular nuclei, then are relayed to the thalamus’s intralaminar nuclei.

Question 75

What are key differences between the dorsal column and anterolateral pathways?

Answer 75

1) Anterolateral pathway has slower signal speeds (8–40 m/sec) than dorsal columns. 2) It has weaker spatial localization. 3) It detects only about 10–20 intensity levels, versus 100 in dorsal columns. 4) It poorly detects rapid signal changes. Despite this, it is the only route for pain, temperature, tickle, itch, sexual sensations, and crude touch.

Question 76

What does damage to the somatosensory cortex cause?

Answer 76

It leads to loss of fine touch and tactile detail, but crude touch can still be sensed. Pain and temperature perception remain mostly unaffected, as these are processed by the thalamus, brainstem, and basal brain areas, which evolved earlier than the cortex.

Question 77

What is the role of corticofugal signals?

Answer 77

Corticofugal signals are inhibitory feedback signals from the brain to the thalamus, medulla, and spinal cord. They 1) sharpen perception by limiting signal spread to nearby neurons and 2) balance sensory sensitivity to prevent overload or under-responsiveness. All sensory systems use this feedback.

Question 78

What are dermatomes?

Answer 78

Dermatomes are areas of skin supplied by sensory fibers from specific spinal nerves. Although dermatome maps show distinct areas, there is significant overlap. For instance, the anal region is associated with dermatome S5, and leg regions are served by L2 to S3. Dermatome mapping helps identify spinal nerve function.