chapter 48 Flashcards
What are somatic senses?
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.
Main types of somatic senses?
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.
Classifications based on origin?
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.
Difference between touch, pressure, and vibration?
They share many receptors but differ in stimulus: Touch – from surface contact; Pressure – deeper tissue deformation; Vibration – rapid, repeated movement.
Touch receptor types?
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.
Function of free nerve endings?
Found in skin and other tissues, detect light touch and pressure; present in sensitive areas like the cornea; use Aδ or C fibers.
Function of Meissner’s corpuscles?
Found in non-hairy areas (e.g., fingertips, lips); connected to myelinated Aβ fibers; sensitive to movement and low-frequency vibration; adapt rapidly.
Function of Merkel’s discs?
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.
Function of hair end-organs?
Each hair is wrapped with a nerve fiber; detects movement or initial contact; adapts quickly.
Function of Ruffini’s endings?
Located in deep skin and joints; respond to stretch and continuous pressure; adapt slowly; help detect joint rotation.
Function of Pacinian corpuscles?
Located in deep skin and connective tissue; detect high-frequency vibration and rapid compression; adapt extremely fast.
Which fibers carry touch signals?
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).
What sensations use fast fibers?
Fine touch, vibration, and precise location use fast Aβ fibers.
What sensations use slow fibers?
Dull pressure, tickle, and itch use slower Aδ or C fibers.
How is vibration sensed?
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.
What detects tickle and itch?
Free nerve endings in skin surface detect tickle and itch; fast-adapting; signals sent via unmyelinated C fibers.
Why do we scratch an itch?
Scratching activates pain fibers, which block itch signals in the spinal cord via lateral inhibition.
How do sensory signals enter the CNS?
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.
Steps in dorsal column–medial lemniscal system?
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.
Steps in anterolateral system?
1) Signal enters spinal cord.
2) Synapses in dorsal horn.
3) Crosses over.
4) Travels in anterior/lateral columns. 5) Reaches brainstem and thalamus.
Difference in fibers of the two pathways?
Dorsal column–medial lemniscal: large, fast, myelinated fibers (30–110 m/s). Anterolateral: small, slower fibers (few m/s to ~40 m/s).
Difference in spatial orientation?
Dorsal column–medial lemniscal system: high spatial orientation (precise location). Anterolateral system: low spatial orientation (less precise).
What does the dorsal column–medial lemniscal system carry?
Carries fine touch, vibration, and proprioception. Fast and precise.
What does the anterolateral system carry?
Carries pain, temperature, and crude touch. Broader range but less detail.
Pathway of medial branch in dorsal column system?
Large myelinated fibers enter dorsal roots, medial branch ascends in dorsal column on same side to brain.
Pathway of lateral branch in dorsal column system?
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.
Role of spinocerebellar fibers?
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.
Spatial orientation in the dorsal column–medial lemniscal system
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.
Where somatosensory signals are processed in the brain
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.
Parts of the somatosensory cortex
It has two parts: Somatosensory Area I (S1) and Somatosensory Area II (S2).
Somatosensory Area I (S1)
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.
Somatosensory Area II (S2)
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.
Sensory homunculus representation
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.
Number of layers in the cerebral cortex
There are six layers (I–VI) from the surface inward. Each has distinct functions in processing sensory signals.
First layer activated by incoming sensory signals
Layer IV is activated first. Signals then spread to upper and lower layers.
Function of Layers I and II
Receive input from lower brain areas to regulate overall cortical activity and prepare the cortex for action.
Function of Layers II and III
Send signals to the opposite brain hemisphere through the corpus callosum for inter-hemispheric communication.
Function of Layers V and VI
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.
Columnar organization of neurons in somatosensory cortex
Neurons are arranged in vertical columns spanning all six layers. Each column handles a specific sensory modality (e.g., pressure, stretch, touch).
Column diameter and neuron count
Each column is about 0.3–0.5 mm wide and contains ~10,000 neurons.
Function of Layer IV columns
Columns at Layer IV (where input first enters) work almost independently.
Function of upper layers of columns
Neurons work together to analyze and interpret sensory information more integratively.
Columns in area 3A
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.
Columns further back in the cortex
Respond more to skin signals. Slow-adapting touch receptors are processed centrally, deep pressure more posteriorly.
Directional columns in the posterior cortex
About 6% of columns respond only to stimuli moving in a specific direction across the skin. These help detect motion and refine touch perception.
Effect of damage to Somatosensory Area I
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.
Somatosensory association areas
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.
Inputs to somatosensory association areas
They receive signals from: 1) Somatosensory Area I, 2) Ventrobasal nuclei of the thalamus, 3) Other thalamic regions, 4) Visual cortex, 5) Auditory cortex.
Amorphosynthesis from association area damage
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.
Dorsal column–medial lemniscal signal transmission
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.
Two-point discrimination test
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.
Role of dorsal column in two-point discrimination
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
Somatosensory cortex response to two-point stimulation
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.
Function of lateral inhibition
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.
Purpose of lateral inhibition in sensory processing
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.
Dorsal column and rapidly changing signals
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.
Types of vibratory signals and their receptors
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.
Clinical test for dorsal column function
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
Why must sensory systems handle a wide range of intensities?
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.
How do receptors like Pacinian corpuscles adapt to stimulus intensity?
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.
How does the auditory system detect sound intensity?
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.
What does the range of sensory reception enable in real life?
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.
What is the Weber-Fechner Principle?
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.
When does the Weber-Fechner principle work best?
It applies well to high-intensity sensations like vision, hearing, and touch but is less accurate for other types of sensory input.
What is the Power Law of sensory intensity?
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.
What are the two types of position sense (proprioception)?
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.
What receptors contribute to proprioception?
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.
What is the role of muscle spindles in proprioception?
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.
What receptors are important at the ends of joint motion?
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.
What receptors detect fast movement?
Pacinian corpuscles and muscle spindles detect fast movement, making them key for sensing the rate of motion (kinesthesia).
How is position sense processed in the brain?
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.
What sensations are transmitted by the anterolateral pathway?
The anterolateral pathway transmits pain, temperature, crude touch, pressure, tickle, itch, and sexual sensations. These are less precise but still essential for sensory perception.
Where does the anterolateral pathway begin and where does it go?
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.
Where do tactile and pain signals go in the anterolateral pathway?
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.
What are key differences between the dorsal column and anterolateral pathways?
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.
What does damage to the somatosensory cortex cause?
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.
What is the role of corticofugal signals?
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.
What are dermatomes?
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.
