Cells of the Nervous System
High-Yield Summary
- Neurons transmit signals; glial cells support the environment neurons need. Signal flow: dendrites → soma → axon hillock (fire threshold) → axon → myelin speeds transmission → nerve terminal → neurotransmitter release.
- Myelin: oligodendrocytes (CNS, myelinate multiple axons) vs. Schwann cells (PNS, one axon each). Nodes of Ranvier enable saltatory conduction (signal jumps node to node).
- PNS: bundled axons = nerve (sensory + motor); cell bodies cluster = ganglia. CNS: bundled axons = tract (one info type only); cell bodies cluster = nuclei.
- 5 glial cell types: astrocytes (blood-brain barrier, nutrients), ependymal cells (CSF production), microglia (CNS immune defense/phagocytosis), oligodendrocytes + Schwann cells (myelin).
Key Terms
- Axon hillock
- Threshold gatekeeper between soma and axon; initiates an action potential if incoming signal is strong enough.
- Nodes of Ranvier
- Exposed axon segments between myelin where ion channels regenerate the signal (saltatory conduction).
- Nerve terminal
- Axon terminal / synaptic bouton; releases neurotransmitters into the synaptic cleft.
- Astrocyte
- Star-shaped glial cell; helps form the blood-brain barrier, supplies nutrients, regulates extracellular ions/neurotransmitters.
- Microglia
- CNS's resident immune/phagocytic cells; clear debris and pathogens.
Signal Flow Through a Neuron
- 1Dendrites receive the signal (usually neurotransmitters from other neurons).
- 2Soma processes incoming information.
- 3Axon hillock decides whether to fire (threshold reached → action potential).
- 4Axon carries the signal; myelin sheath (oligodendrocytes/Schwann cells) speeds it via saltatory conduction at the nodes of Ranvier.
- 5Nerve terminal converts the electrical signal to a chemical one, releasing neurotransmitters into the synaptic cleft.
PNS vs. CNS Organizational Terms
| Peripheral (PNS) | Central (CNS) |
|---|---|
| Nerve (bundled axons; sensory + motor) | Tract (bundled axons; sensory OR motor, not both) |
| Ganglia (clustered cell bodies) | Nuclei (clustered cell bodies) |
Oligodendrocytes vs. Schwann Cells
| Oligodendrocytes | Schwann Cells |
|---|---|
| Central nervous system | Peripheral nervous system |
| Myelinates multiple axons at once | Myelinates one axon per cell |
Common MCAT Trap
- Nerve (PNS) can carry BOTH sensory and motor info; tract (CNS) carries only ONE type — a frequently reversed detail.
- Ganglia = PNS cell body clusters; nuclei = CNS cell body clusters — easy to swap with the axon-bundle terms (nerve/tract).
- Oligodendrocytes reach multiple axons; Schwann cells wrap only one — opposite of what "multiple support cells for one system" intuition might suggest.
Quick Recall
What structure decides whether an action potential fires?
Which glial cell produces cerebrospinal fluid?
What is the key functional difference between a nerve and a tract?
Which glial cells act as the CNS's immune defense?