# The Nervous System

Psychology I · Psychophysiology · https://tryals.app/learn/psychology-i/the-nervous-system

## The Body's Wiring Diagram

The **nervous system** is the body's rapid electrochemical communication network, split anatomically into two parts. The **central nervous system (CNS)** is the brain and spinal cord, the processing core where information is integrated. The **peripheral nervous system (PNS)** is every nerve outside that core, carrying signals to and from the body.

The PNS then splits functionally. The **somatic nervous system** carries sensory input and commands to skeletal muscles for voluntary movement. The **autonomic nervous system (ANS)** governs involuntary machinery like heart rate, digestion, and breathing.

| Branch | Nickname | Action |
| --- | --- | --- |
| Sympathetic | Fight or flight | Mobilizes energy |
| Parasympathetic | Rest and digest | Conserves energy |

## Autonomic Balance

The sympathetic and parasympathetic branches are **antagonistic**: the sympathetic branch mobilizes energy during threat, and the parasympathetic branch returns the body to a calm baseline afterward. 

**Common pitfall:** assuming the two branches take turns being on and off. Both are active at all times; arousal is a shift in balance, not a single switch.

$$\text{arousal} = \text{sympathetic} - \text{parasympathetic tone}$$

Health depends on the smooth back-and-forth between these two constant forces.

## Practice questions

7 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. The sympathetic branch speeds the heart and slows digestion; the parasympathetic does the reverse. Why would evolution couple those two effects together in each branch rather than controlling each organ independently?

A. Because digestion physically cannot occur while the heart rate is elevated.
B. Because the two branches serve coherent whole-body situations.
C. Because independent control would require twice as many neurons, which the body cannot afford.
D. Because the heart and digestive tract share a nerve supply and cannot be controlled separately.

**Answer:** B. Because the two branches serve coherent whole-body situations.

**Why:** The situations an animal faces are whole-body situations: escaping a predator needs a raised heart rate, dilated airways, blood at the muscles, and digestion and repair all deferred, so wiring them to one switch reconfigures everything on a single signal. The design predicts its own failure mode: deferring immunity and repair is reasonable for two minutes and ruinous for months.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/the-sympathetic-branch-speeds-the-heart-and-slows-digestion-the

### 2. A perfectly regular heartbeat sounds healthy and is a warning sign. Why does clinical medicine treat **variability** in heart rate as the good news?

A. Because irregular rhythms are simply more efficient at pumping blood
B. Because the beat-to-beat variation IS the two branches adjusting
C. Because a regular rhythm indicates the heart muscle is stiffening
D. Because variability shows the heart is working harder, which strengthens it

**Answer:** B. Because the beat-to-beat variation IS the two branches adjusting

**Why:** The wobble is the regulation showing through: sympathetic and parasympathetic trimming the rate breath by breath. Lose it and you are seeing a heart no longer being steered, which is why reduced heart rate variability predicts poor cardiac outcomes and tracks chronic stress.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/a-perfectly-regular-heartbeat-sounds-healthy-and-is-a-warning-sign

### 3. The sympathetic and parasympathetic branches both innervate the heart continuously, pulling in opposite directions. Why is a permanent tug-of-war a better design than simply switching one branch on when it is needed?

A. Because the sympathetic branch is too powerful to be switched on directly and must be restrained at all times.
B. Because continuous opposing signals prevent the heart muscle from becoming fatigued.
C. Because two branches provide redundancy, so the heart keeps beating if one fails.
D. Because a balance point can be adjusted instantly either way.

**Answer:** D. Because a balance point can be adjusted instantly either way.

**Why:** Holding 70 against an intrinsic pacemaker near 100 means the vagus is applying a constant brake, and a brake can be released instantly, the rate rises before any sympathetic signal is sent. It is why heart rate variability is medically informative: the beat-to-beat wobble *is* the two branches adjusting against each other, and a steady heart has lost the balance.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/the-sympathetic-and-parasympathetic-branches-both-innervate-the-heart

### 4. Select every statement that is TRUE about the nervous system.

A. The autonomic nervous system is under conscious voluntary control.
B. Resting heart rate reflects a balance the two branches hold continuously.
C. The somatic branch controls voluntary skeletal movement.
D. Everything outside the brain and spinal cord belongs to the peripheral system.
E. Sympathetic and parasympathetic branches act on the same organs in opposite directions.

**Answer:** A. The autonomic nervous system is under conscious voluntary control.; D. Everything outside the brain and spinal cord belongs to the peripheral system.; E. Sympathetic and parasympathetic branches act on the same organs in opposite directions.

**Why:** The balance principle and the two branches' jobs are true. The somatic system runs voluntary muscle (not the involuntary heart), and the CNS is the brain and cord itself - the PNS is what lies outside - so those two are false.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/select-every-statement-that-is-true-about-the-nervous-system

### 5. Order the arc of a sympathetic stress response and its recovery.

**Answer:**

1. A sudden threat is perceived by the brain.
2. The sympathetic branch activates, mobilizing energy.
3. Heart rate rises, pupils dilate, and digestion halts.
4. The threat passes.
5. The parasympathetic branch restores the calm baseline.

**Why:** Threat drives sympathetic activation and its bodily changes; once the danger passes, the parasympathetic branch actively restores calm. Recovery is not passive - it is the opposing branch taking over.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/order-the-arc-of-a-sympathetic-stress-response-and-its-recovery

### 6. A car pulls out in front of you and you brake hard. Order what your nervous system does, from the first signal to the return to baseline.

**Answer:**

1. Visual information reaches the brain and the threat is registered.
2. Sympathetic outflow surges: heart rate up, pupils wide, blood to the muscles.
3. The adrenal medulla adds adrenaline to the bloodstream, sustaining the response.
4. The danger passes and sympathetic outflow ceases.
5. Parasympathetic activity restores the resting balance over the next several minutes.

**Why:** The last two steps are where the interesting asymmetry lies. Sympathetic nerve traffic stops the moment the threat does, but the adrenaline already circulating has no off-switch and must be metabolised - which is why you are still trembling a minute after the car has gone.

That is the practical difference between the two delivery routes. Direct sympathetic innervation is fast and terminates cleanly; hormonal delivery is slower to start and much slower to end, and it is what turns a half-second event into several minutes of physiology.

Recovery is also not passive. Returning to baseline is active parasympathetic work, and how quickly someone accomplishes it is a meaningful individual difference - people whose recovery is slow spend more of their day in a mobilised state, which is the physiological route by which repeated acute stress becomes chronic.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/a-car-pulls-out-in-front-of-you-and-you-brake-hard-order-what-your

### 7. Sort each bodily change by the autonomic branch driving it.

**Answer:**

- Sympathetic (fight-or-flight): Pupils dilate as headlights swerve toward you, Airways widen and heart pounds before a sprint
- Parasympathetic (rest-and-digest): Salivating and digesting during a calm dinner, Heart rate settling as you drift toward sleep

**Why:** Dilated pupils and a pounding heart mobilize the body (sympathetic); active digestion and a slowing heart conserve it (parasympathetic). Digestion is the tell: it runs under parasympathetic control and halts under sympathetic.

Page: https://tryals.app/practice/psychology-i/the-nervous-system/sort-each-bodily-change-by-the-autonomic-branch-driving-it