19.11
体感系统传递来自皮肤、粘膜、四肢和关节的感觉信息。体感被称为触觉。典型的体感通路包括三种长神经元:初级、次级和三级。初级神经元在脊髓附近有一组称为背根神经节的神经元。神经节的感觉神经元支配皮肤上被称为皮肤小体的指定区域。
在皮肤中,被称为机械感受器的特殊结构将机械压力或变形转化为神经信号。在无毛皮肤…
在躯体感觉过程中 触觉
皮肤内的感觉神经元首先
通过机械感受器检测机械扰动
如果刺激是轻微的振动
迈斯纳小体回应
随着振动不断增加
梅克尔盘反映 因为它们会慢慢适应
鲁菲尼末梢对更深
持续的刺激也适应 反映较慢
如果是重复性的
则由帕西尼氏小体监测到
检测后 感觉被转换成电信号
并向脊髓传播
特别是背根神经节
产生自身体同一区域
称为皮节 构成单个背根
继续到脑干
特别是髓质 感觉信息交叉
意味着它在被送到丘脑之前
越过另一侧
从这里 消息最终被送到
躯体感觉皮层
这个皮质区域包含身体的地图
称为脑中小人
由于交叉 身体左侧
表示右侧皮质 反之亦然
手指占据大的皮层区域
因为它们具有高密度的机械感受器
具有小的感受野
提供了更好的刺激辨别
相反 背部受体较少
感受野较大
因此皮质表现较小
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Q1: What are the four main types of mechanoreceptors in hairless skin?
The four main mechanoreceptors are Meissner corpuscles, Merkel disks, Ruffini endings, and Pacinian corpuscles. Meissner corpuscles and Pacinian corpuscles are rapidly adapting and detect dynamic stimuli like vibrations. Merkel disks and Ruffini endings are slow adapting and respond to sustained touch and pressure.
Q2: How does the somatosensory pathway transmit information from skin to brain?
Mechanoreceptors in skin detect stimuli and send signals through primary neurons to the dorsal root ganglion. Secondary neurons in the medulla decussate, crossing to the opposite side before reaching the thalamus. Tertiary neurons then carry the signal to the somatosensory cortex, where the body is mapped as a homunculus.
Q3: Why do fingers have larger cortical representation than the back?
Fingers have a high density of mechanoreceptors with small receptive fields, enabling better stimulus discrimination. This dense receptor distribution requires more cortical area for processing fine sensory details. In contrast, the back has fewer receptors and larger receptive fields, resulting in smaller cortical representation.
Q4: What is decussation and why does it occur in the somatosensory pathway?
Decussation is the crossing of sensory information from one side of the body to the opposite side of the brain. This occurs in the medulla when secondary neurons cross over before sending signals to the thalamus. As a result, the left side of the body is processed in the right cortex and vice versa.
Q5: What is a dermatome and how does it relate to the dorsal root ganglion?
A dermatome is a designated area of skin innervated by sensory neurons from a single dorsal root ganglion. Primary neurons with cell bodies in the dorsal root ganglia extend axons to specific dermatomes. Each dermatome represents a distinct region where sensory information is initially detected before traveling to the spinal cord.
Q6: How can the somatosensory cortex reorganize after injury or specialized use?
The somatosensory cortex undergoes cortical reorganization when body regions do not function as intended. Braille readers develop larger finger representations in the cortex due to intensive tactile training. In forearm amputees, the cortical region previously connected to the amputated arm can be remapped to adjacent regions, sometimes causing phantom limb sensations.
Q7: How do slow-adapting and rapidly-adapting mechanoreceptors differ in function?
Slow-adapting mechanoreceptors like Merkel disks and Ruffini endings continue responding to prolonged contact, detecting sustained touch and pressure. Rapidly-adapting mechanoreceptors like Meissner corpuscles and Pacinian corpuscles respond to dynamic stimuli such as vibrations and changing pressure. This functional difference allows the sensory system to detect both constant and changing tactile information.