Chapter 3: Muscle & Nervous Tissue

The last two basic tissue types are both about signal and movement. Muscle is built to contract; neurons are built to conduct. Both have distinctive, high-yield microscopic signatures.

Three types of muscle

• Skeletal muscle: striated, long cylindrical fibers, multiple peripheral nuclei per fiber (a true syncytium formed by fusion of myoblasts). Under voluntary control, organized into fascicles.
• Cardiac muscle: also striated, but the fibers are shorter and branch. Typically one or two central nuclei per cell. The defining feature is the intercalated disc — a junction packed with gap junctions (electrical coupling) and desmosomes (mechanical coupling) that lets the heart contract as a functional unit.
• Smooth muscle: non-striated, spindle-shaped cells with a single central nucleus. Found in the walls of hollow viscera and blood vessels, under involuntary (autonomic) control.

A fast way to keep them straight: striated + many nuclei on the edge = skeletal; striated + branching with a central nucleus = cardiac; no striations at all = smooth.

Nervous tissue

• Neurons have a cell body (soma) with a large, pale, euchromatic nucleus and a prominent nucleolus — reflecting how metabolically active they are. Nissl bodies (clumps of rough ER and free ribosomes) fill the cytoplasm and extend partway into dendrites, but stop at the axon hillock — a useful landmark for telling axon from dendrite on a slide.
• Dendrites receive input and are typically short and branching; the axon is a single, long process that conducts the action potential away from the soma.
• Glial cells support neurons: astrocytes (blood-brain barrier, structural support), oligodendrocytes and Schwann cells (myelination in the CNS and PNS respectively), microglia (CNS resident immune cells), and ependymal cells (line the ventricles, produce CSF).

Key teaching point

For muscle, the fastest discriminators are striations (yes/no) and nuclear number/position. For nervous tissue, Nissl body distribution and the single-axon-versus-many-dendrites rule will answer most identification questions without needing to recognize the surrounding organ.