4.4 The Nervous System
Key Takeaways
- The central nervous system (CNS) is the brain and spinal cord; the peripheral nervous system (PNS) is all nerves outside it
- A neuron has dendrites (receive), a cell body, and an axon (transmit); the myelin sheath speeds transmission
- A neuron's resting membrane potential is about −70 mV; reaching the threshold of about −55 mV triggers an all-or-none action potential
- The sympathetic division drives “fight or flight”; the parasympathetic division drives “rest and digest”
- The four cerebral lobes are frontal (reasoning), parietal (sensory), temporal (hearing/memory), and occipital (vision)
The nervous system is the body's rapid communication and control network. It senses the environment, processes information, and triggers responses — all in milliseconds. On the TEAS you will be asked to split the system into its anatomical divisions, label the parts of a neuron, explain how a nerve impulse travels, locate functions to brain regions, and distinguish the two branches of the autonomic system. The unifying idea is that neurons communicate using a combination of electrical signals (along the axon) and chemical signals (across the gap between neurons).
Two Main Divisions: CNS and PNS
Every part of the nervous system belongs to one of two divisions.
| Division | Components | Role |
|---|---|---|
| Central nervous system (CNS) | Brain and spinal cord | Integration, processing, decision-making |
| Peripheral nervous system (PNS) | All nerves and ganglia outside the CNS | Carries information to and from the CNS |
The PNS is further split into a somatic division (voluntary control of skeletal muscle) and an autonomic division (involuntary control of organs and glands).
The Neuron: The Functional Unit
A neuron is a specialized cell built to carry signals. Each part has one job, and the TEAS frequently asks you to match a part to its function.
| Part | Function |
|---|---|
| Dendrites | Branch-like extensions that receive incoming signals |
| Cell body (soma) | Contains the nucleus; the metabolic center |
| Axon | Long fiber that transmits the signal away from the cell body |
| Myelin sheath | Fatty insulation that speeds signal transmission |
| Synaptic terminals | Release neurotransmitters to the next cell |
The simplest way to remember the flow is “dendrites receive, axons transmit.” Signals always travel one direction within a neuron: dendrite → cell body → axon → synaptic terminal.
The Action Potential
A nerve signal is an action potential, a brief electrical event that sweeps down the axon. Understanding it requires a few standard numbers.
- Resting potential: At rest, the inside of the neuron is about −70 mV relative to the outside (more sodium outside, more potassium inside).
- Reaching threshold: A stimulus must push the membrane to about −55 mV, the threshold, to fire.
- Depolarization: Sodium (Na⁺) channels open and sodium rushes in, making the inside positive.
- Repolarization: Potassium (K⁺) rushes out, returning the inside to negative.
- Synaptic transmission: At the axon's end, the signal triggers release of neurotransmitters across the synapse to the next neuron.
A crucial test concept is that the action potential is all-or-none: once threshold is reached, a full-strength impulse fires; a stronger stimulus does not produce a bigger impulse, only more frequent ones. Below threshold, nothing fires.
Major Neurotransmitters
Neurotransmitters are the chemical messengers released at the synapse. A few appear repeatedly on the exam:
- Acetylcholine — muscle activation and memory
- Dopamine — reward, movement, and mood
- Serotonin — mood, sleep, and appetite
- GABA — the main inhibitory (calming) transmitter
- Glutamate — the main excitatory transmitter, involved in learning
Brain Regions and the Four Lobes
The brain has three broad regions plus four cortical lobes you must locate.
| Region | Main Functions |
|---|---|
| Cerebrum | Thinking, memory, voluntary movement, the senses |
| Cerebellum | Balance, coordination, fine motor control |
| Brainstem | Vital automatic functions: breathing, heart rate |
The cerebrum's surface is divided into four lobes:
| Lobe | Location | Function |
|---|---|---|
| Frontal | Front | Reasoning, planning, speech, voluntary movement |
| Parietal | Top-back | Touch and other sensory processing |
| Temporal | Sides | Hearing, memory, language comprehension |
| Occipital | Back | Vision |
A reliable mnemonic for the back lobe: the occipital lobe is at the back of your head, the opposite end from your eyes, yet it is where vision is processed.
The Reflex Arc
A reflex is an automatic, protective response that bypasses the brain to save time. The pathway is the reflex arc:
- Stimulus (e.g., a hot stove) → 2. Receptor detects it → 3. Sensory neuron carries the signal to the spinal cord → 4. Integration center (spinal cord) → 5. Motor neuron carries the command out → 6. Effector (a muscle) → 7. Response (you pull your hand away).
Because the signal is processed in the spinal cord, you withdraw your hand before you consciously feel the pain.
The Autonomic Nervous System
The autonomic division controls involuntary functions through two opposing branches that the TEAS contrasts directly.
| Branch | State | Effects |
|---|---|---|
| Sympathetic | “Fight or flight” | ↑ heart rate, ↑ breathing, dilates pupils, slows digestion |
| Parasympathetic | “Rest and digest” | ↓ heart rate, ↑ digestion, constricts pupils |
Think of the sympathetic system as the accelerator that prepares you for danger and the parasympathetic system as the brake that conserves energy and runs routine maintenance like digestion.
Worked Example: You step on a sharp object and instantly lift your foot, only feeling pain a moment later. Trace the pathway. The sharp object is the stimulus; pain receptors in the skin detect it; a sensory neuron carries the signal to the spinal cord (the integration center); an interneuron routes it to a motor neuron, which signals the leg muscle (the effector) to contract and lift the foot. The reflex is completed in the spinal cord, so you react before the pain signal even reaches your conscious brain.
Match each neuron part to its function.
Match each item on the left with the correct item on the right
A neuron's resting membrane potential is about −70 mV. What must happen for an action potential to fire?
Which division of the nervous system would increase heart rate and dilate the pupils during a frightening event?
Which lobe of the cerebrum is primarily responsible for processing vision?