Fundamentals of the Nervous System & Nervous Tissue
Organization, neurons and glia, membrane potentials, action potentials and synapses
By the end of this unit you can…
- ✓Describe the functions and divisions of the nervous system
- ✓Compare the six types of neuroglia and the structure of a neuron
- ✓Classify neurons structurally and functionally
- ✓Explain the resting membrane potential, graded potentials and the action potential
- ✓Describe conduction velocity, myelination and saltatory conduction
- ✓Explain chemical synapses, EPSPs, IPSPs, summation and the major neurotransmitters
Key terms
Practice →1Functions & divisions
The nervous system is the body's master control and communication system. It has three overlapping functions:
- Sensory input — sensory receptors monitor changes (stimuli) inside and outside the body.
- Integration — the system processes and interprets sensory input and decides what to do.
- Motor output — activates effectors (muscles and glands) to cause a response.
| Division | Parts | Role |
|---|---|---|
| Central nervous system (CNS) | Brain and spinal cord | Integration and command center |
| Peripheral nervous system (PNS) | Cranial and spinal nerves and ganglia | Communication lines between CNS and body |
| PNS — Sensory (afferent) division | Somatic and visceral sensory fibers | Carries impulses toward the CNS |
| PNS — Motor (efferent) division | Somatic and autonomic | Carries impulses from the CNS to effectors |
| Somatic nervous system | Somatic motor nerves | Voluntary control of skeletal muscle |
| Autonomic nervous system (ANS) | Sympathetic & parasympathetic divisions | Involuntary control of cardiac muscle, smooth muscle and glands |
2Neuroglia
Neuroglia (glial cells) outnumber neurons. They support, insulate and protect neurons — and, unlike most neurons, they can divide (most brain tumours, gliomas, arise from glia).
| Glial cell | Location | Function |
|---|---|---|
| Astrocytes | CNS | Most abundant; anchor neurons to capillaries; help form the blood–brain barrier; regulate the chemical environment (recapture K⁺ and neurotransmitters) |
| Microglial cells | CNS | Defensive phagocytes — monitor neurons and engulf microbes and debris (CNS immune cells) |
| Ependymal cells | CNS | Line the brain ventricles and central canal; cilia help circulate cerebrospinal fluid |
| Oligodendrocytes | CNS | Wrap processes around axons to form myelin sheaths in the CNS (one cell myelinates many axons) |
| Satellite cells | PNS | Surround neuron cell bodies in ganglia (like astrocytes) |
| Schwann cells | PNS | Form myelin sheaths in the PNS (one cell per segment of one axon); vital to regeneration of damaged peripheral fibers |
3Neurons
Neurons (nerve cells) are the structural units of the nervous system. Special traits: extreme longevity (can last a lifetime), they are amitotic (lose the ability to divide) and they have an exceptionally high metabolic rate (need constant oxygen and glucose).
- Cell body (soma)
- Biosynthetic center: nucleus, rough ER clusters called chromatophilic (Nissl) bodies, neurofilaments. Clusters of cell bodies are nuclei (CNS) or ganglia (PNS).
- Dendrites
- Short, branching receptive regions; convey graded potentials toward the cell body. Dendritic spines are synapse sites.
- Axon hillock
- Cone-shaped region where the axon leaves the cell body — the trigger zone where action potentials are generated.
- Axon
- One per neuron; the conducting region that generates and transmits action potentials away from the cell body. Branches end in axon terminals (terminal boutons) that release neurotransmitter. Bundles of axons are tracts (CNS) or nerves (PNS).
- Myelin sheath
- Whitish, fatty segmented wrapping that insulates the axon and increases conduction speed. Gaps between segments are nodes of Ranvier (myelin sheath gaps).
White matter = dense collections of myelinated fibers; gray matter = mostly cell bodies and unmyelinated fibers.
| Structural class | Description | Example |
|---|---|---|
| Multipolar | 3+ processes (1 axon, many dendrites) — most common (>99%) | Motor neurons, interneurons |
| Bipolar | 2 processes (1 axon, 1 dendrite) — rare | Retina, olfactory mucosa |
| Unipolar (pseudounipolar) | 1 short process that divides into peripheral and central branches | Sensory neurons in dorsal root ganglia |
| Functional class | Direction | Note |
|---|---|---|
| Sensory (afferent) | Receptors → CNS | Almost all unipolar |
| Motor (efferent) | CNS → effectors | Multipolar |
| Interneurons (association) | Between motor and sensory neurons | ~99% of neurons; almost all multipolar, in the CNS |
4Membrane potentials
Neurons are highly excitable: they respond to stimuli by changing the voltage across their membrane. Voltage changes are produced by ions moving through channels:
- Leakage (nongated) channels
- Always open.
- Chemically (ligand-) gated channels
- Open when a neurotransmitter binds (on dendrites and cell bodies).
- Voltage-gated channels
- Open and close in response to changes in membrane potential (on axons).
- Mechanically gated channels
- Open in response to stretch, touch or pressure (sensory receptors).
At rest a neuron is polarized with a resting membrane potential of about −70 mV (inside negative). It's caused by differences in ion concentrations (Na⁺ high outside; K⁺ high inside, along with negatively charged proteins) and differential permeability — the membrane has many more K⁺ leakage channels than Na⁺, so K⁺ diffuses out, leaving the inside negative. The Na⁺–K⁺ pump (3 Na⁺ out, 2 K⁺ in) maintains the gradients.
- Depolarization
- Inside becomes less negative (moves toward 0 and above) — increases the chance of firing.
- Hyperpolarization
- Inside becomes more negative than resting — decreases the chance of firing.
- Graded potentials
- Short-lived, localized changes in membrane potential (on dendrites and the cell body); magnitude varies with stimulus strength; decay with distance. Called receptor potentials, generator potentials or postsynaptic potentials.
5The action potential
An action potential (AP) — a nerve impulse — is a brief reversal of membrane potential (~100 mV total change) that does not decay as it travels along the axon. Only axons (muscle fibers too) generate APs.
- 1Resting state — all voltage-gated Na⁺ and K⁺ channels closed (−70 mV).
- 2Depolarization — a graded potential at the axon hillock reaches threshold (≈ −55 mV); voltage-gated Na⁺ channels open, Na⁺ rushes in, and the inside becomes positive (peak ≈ +30 mV). Positive feedback: depolarization opens more Na⁺ channels.
- 3Repolarization — Na⁺ channels inactivate; slower voltage-gated K⁺ channels open, K⁺ rushes out, and the membrane returns toward negative.
- 4Hyperpolarization — some K⁺ channels stay open a bit too long, so the inside briefly becomes more negative than rest (undershoot). The Na⁺–K⁺ pump later restores ion distributions.
The AP is all-or-none: once threshold is reached, the AP is the same size every time. The intensity of a stimulus is coded by the frequency of APs, not their size.
- Absolute refractory period
- Na⁺ channels are open or inactivated — no stimulus, however strong, can trigger another AP. Ensures one-way propagation and that each AP is separate.
- Relative refractory period
- Most Na⁺ channels have reset; K⁺ channels still open. Only an exceptionally strong stimulus can trigger an AP.
Conduction velocity increases with (1) larger axon diameter (less resistance) and (2) myelination. In myelinated axons, APs are generated only at the nodes of Ranvier and appear to jump from node to node — saltatory conduction — about 30× faster than continuous conduction in unmyelinated axons.
6Synapses & neurotransmitters
A synapse is a junction that transfers information from one neuron to another (or to an effector). The presynaptic neuron sends; the postsynaptic neuron receives. Most synapses are axodendritic or axosomatic.
Electrical synapses (gap junctions) are rare, very fast and allow synchronized activity. Chemical synapses are the most common: they release a neurotransmitter across a fluid-filled synaptic cleft.
- 1An action potential arrives at the axon terminal.
- 2Voltage-gated Ca²⁺ channels open and Ca²⁺ enters the terminal.
- 3Ca²⁺ causes synaptic vesicles to release neurotransmitter by exocytosis.
- 4Neurotransmitter diffuses across the cleft and binds receptors on the postsynaptic membrane.
- 5Binding opens ion channels, creating a graded potential (EPSP or IPSP).
- 6Neurotransmitter effects are terminated by reuptake (into the presynaptic terminal or astrocytes), enzymatic degradation (e.g. acetylcholinesterase), or diffusion away.
| Postsynaptic potential | Ions | Effect |
|---|---|---|
| EPSP (excitatory) | Opens channels letting Na⁺ in (and K⁺ out) — net depolarization | Brings the membrane closer to threshold |
| IPSP (inhibitory) | Opens K⁺ or Cl⁻ channels — hyperpolarization | Moves the membrane farther from threshold |
A single EPSP can't trigger an AP — postsynaptic potentials must summate at the axon hillock: temporal summation (one presynaptic neuron fires rapidly) or spatial summation (many presynaptic neurons fire at the same time). EPSPs and IPSPs cancel each other out.
| Neurotransmitter | Notes |
|---|---|
| Acetylcholine (ACh) | Neuromuscular junctions, ANS, brain; degraded by acetylcholinesterase |
| Norepinephrine (NE) | “Feel-good”; main sympathetic neurotransmitter; a catecholamine (biogenic amine) |
| Dopamine | Reward, movement; deficient in Parkinson's disease |
| Serotonin | Sleep, mood, appetite; SSRIs (antidepressants) block its reuptake |
| GABA | Main inhibitory neurotransmitter of the brain |
| Glutamate | Main excitatory neurotransmitter of the brain; excess causes excitotoxicity in stroke |
| Endorphins | Natural opiates — reduce pain perception |
| Nitric oxide (NO) | A gas — diffuses through membranes; vasodilation, learning |