BRAINCAKEMyelinated Neurones
In vertebrates, many neurones are myelinated: their axons (and the dendrons of sensory neurones) are wrapped in a myelin sheath.
In the peripheral nervous system the sheath is made by Schwann cells. Each Schwann cell wraps itself around the axon many times, so the sheath is made of many layers of Schwann cell membrane. Because membranes are mostly phospholipid, myelin is rich in lipid and is a good electrical insulator. Ions cannot flow across the axon membrane where it is covered.
Between neighbouring Schwann cells there are small gaps, about 1 µm wide, where the axon membrane is exposed. These are the nodes of Ranvier, found roughly every 1 mm along the axon. The membrane at the nodes contains a high density of voltage-gated sodium ion channels, so ions can move across the membrane only at these points.
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Key terms in this lesson
- myelin sheath
- An insulating layer of Schwann cell membrane wrapped around an axon that speeds up nerve impulse conduction.
More in Mammalian Nervous System
- Nervous System
- Structure and Function of Neurones
- Nerve Impulses
- Speed of Nerve Impulses
- Investigating Nerve Impulses
- Resting Potential
- Action Potential
- Depolarisation
All 38 lessons in Mammalian Nervous System · All Edexcel A-level Biology topics