Muscle and nervous tissue
Introduzido| English | Significado |
|---|---|
| skeletal muscle | skeletal muscle |
| cardiac muscle | cardiac muscle |
| intercalated disc | intercalated disc |
| smooth muscle/smuːð ˈmʌsl/ | smooth muscle |
| sarcomere/ˈsɑːkəmɪə/ | A repeating contractile unit of striated muscle between two neighbouring Z lines. |
| nervous tissue | nervous tissue |
| glia | glia |
| demyelinating neuropathy | demyelinating neuropathy |
Machinery you can see
- Movement and control are tissues with visible machinery. The bands of a muscle fibre are the contraction mechanism; the shape of a neuron is its wiring.
- Here, histology is physiology you can see.
Three kinds of muscle
- Skeletal muscle 骨骼肌 is striated and voluntary, with many nuclei at the edge of long fibres. Cardiac muscle 心肌 is striated and involuntary, with branched cells joined by intercalated discs 闰盘.
- Smooth muscle 平滑肌 has no stripes and is involuntary: spindle-shaped cells, each with one central nucleus.
Match each muscle type to its fingerprint.
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Stripes, nucleus position and cell shape name the muscle in one look.
The sarcomere
- A sarcomere 肌节 runs from one Z-line to the next. The dark A-band is the full length of the myosin filaments; it does not change in contraction.
- The light I-band is actin only, and the H-zone is myosin only. As actin slides inwards, the I-band and H-zone narrow and the Z-lines move closer; no filament shortens.
Which band keeps the same width when a sarcomere contracts?
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The A-band is the length of the myosin filaments, which never changes; the I-band and H-zone narrow as actin slides in.
Measuring the slide
- In a resting sarcomere of 2.5 µm with an A-band of 1.6 µm, the two half I-bands together are 2.5 − 1.6 = 0.9 µm.
- Only the I-band and H-zone change as the sarcomere shortens: the A-band stays 1.6 µm whatever the length.
A sarcomere with a 1.6 µm A-band contracts to 2.0 µm. What is the total width of its I-band (both halves), in µm?
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2.0 − 1.6 = 0.4 µm, down from 0.9 µm at rest: the I-band takes all the shortening.
Nervous tissue
- Nervous tissue 神经组织 is neurons (cell body, dendrites, axon) plus glia 胶质细胞.
- Astrocytes support and buffer; oligodendrocytes (brain and cord) and Schwann cells (nerves) make myelin; microglia clear debris.
Which cells make myelin in the brain and spinal cord?
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Oligodendrocytes myelinate in the central nervous system; Schwann cells do it in peripheral nerves.
Worked example: naming the muscle
- Nuclei at the edge, long parallel fibres and stripes: skeletal. Central nuclei, branching and intercalated discs: cardiac.
- No stripes, spindle cells, one central nucleus each: smooth. Three fingerprints, no special stain needed.
A striated muscle with branching cells and central nuclei is skeletal muscle.
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Branching, central nuclei and intercalated discs mark cardiac muscle; skeletal fibres are unbranched with nuclei at the edge.
Worked example: onion bulbs
- (Fictional nerve biopsy) A child with slowly worsening weakness. Thinly myelinated axons are wrapped in rings of Schwann-cell processes: "onion bulbs".
- They are the record of repeated loss and repair of myelin, so the slide names a demyelinating neuropathy 脱髓鞘性神经病. Glia are nervous tissue too.
What do "onion bulbs" in a nerve biopsy show?
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Each cycle of demyelination and remyelination adds a ring of Schwann-cell processes around the axon.
Three common slips
- "The A-band shortens." It is myosin's fixed length; only the I-band and H-zone narrow.
- Intercalated discs for stripes. Discs are junctions between cells, seen as dark lines across the fibre.
- Glia as packing. They make myelin, buffer the fluid around neurons and form scars.
Recap
- Skeletal (edge nuclei, stripes), cardiac (branches, discs), smooth (no stripes, spindles).
- Sarcomere: the A-band stays the same; the I-band and H-zone narrow; filaments never shorten.
- Glia are working nervous tissue: myelin disease is glial disease.