# Central Nervous System

Psychology I · Behavioral Biology · https://tryals.app/learn/psychology-i/central-nervous-system

## The CNS and Its Armor

The **Central Nervous System (CNS)**, comprising the brain and spinal cord, is guarded by three **meninges**: the tough **dura mater**, web-like **arachnoid mater**, and delicate **pia mater**. 

The subarachnoid space holds **cerebrospinal fluid (CSF)**, which cushions the CNS and stabilizes its chemical environment. Meanwhile, the **blood-brain barrier** uses tight capillary junctions to filter blood contents, letting small lipid-soluble molecules (like alcohol) pass much easier than pathogens.

| Region | Grey Matter Layout | White Matter Layout |
| --- | --- | --- |
| **Brain** | Outer cortex | Inner core |
| **Spinal Cord** | H-shaped core | Outer tracts |

## Spinal Reflexes

The spinal cord runs fast, local **reflex arcs** to react to danger before the brain even gets involved. 

A withdrawal reflex routes a signal from a **sensory (afferent) neuron**, through a spinal **interneuron**, straight to a **motor (efferent) neuron** to pull away instantly.

> **Common pitfall:** Assuming a spinal reflex still involves the brain "just very quickly." It does not. The entire arc completes inside the spinal cord, making it faster than any voluntary response.

## Practice questions

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

### 1. Order the events of a withdrawal reflex, from stimulus to the moment you feel it.

**Answer:**

1. A pain receptor in the fingertip is stimulated by a hot surface.
2. A sensory (afferent) neuron carries the signal into the spinal cord.
3. An interneuron in the spinal cord relays it directly to a motor neuron.
4. A motor (efferent) neuron drives the muscle to pull the hand back.
5. Only afterward does the signal reach the brain and conscious awareness.

**Why:** The reflex completes entirely within the spinal cord - sensory in, interneuron across, motor out - and only afterward does the pain signal ascend to the brain. That is why you jerk your hand away before you feel the burn.

Page: https://tryals.app/practice/psychology-i/central-nervous-system/order-the-events-of-a-withdrawal-reflex-from-stimulus-to-the-moment

### 2. The blood-brain barrier admits small, lipid-soluble molecules but stops most others. Sort each item.

**Answer:**

- Crosses the barrier readily: Alcohol (small, lipid-soluble), Nicotine (small, lipid-soluble)
- Largely kept out: A large water-soluble drug molecule, Bloodborne bacteria, Most large protein-based medicines

**Why:** The barrier's tight junctions favour small, lipid-soluble molecules (alcohol, nicotine) and exclude large or water-soluble ones and pathogens. Applying the size/solubility criterion - not a value judgement - is the skill being tested.

Page: https://tryals.app/practice/psychology-i/central-nervous-system/the-blood-brain-barrier-admits-small-lipid-soluble-molecules-but

### 3. Select every statement that is TRUE about the CNS.

A. Cerebrospinal fluid is produced in the brain's ventricles.
B. The pia mater is the innermost meninx.
C. In the spinal cord, white matter lies outside and grey matter at the core.
D. A spinal reflex arc must route through the brain before the muscle responds.
E. The blood-brain barrier blocks all molecules from entering brain tissue.

**Answer:** A. Cerebrospinal fluid is produced in the brain's ventricles.; B. The pia mater is the innermost meninx.; C. In the spinal cord, white matter lies outside and grey matter at the core.

**Why:** CSF is made in the ventricles, the cord has white-outside/grey-core layout, and the pia is innermost - all true. The barrier is selective (lipid-soluble molecules pass), and a reflex arc need not reach the brain, so those two are false.

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

### 4. A withdrawal reflex has your hand off the hot pan before you consciously feel the heat. Why is the circuit wired through the spinal cord rather than up to the brain and back?

A. Because a trip to the brain and back costs time the tissue cannot spare.
B. Because the spinal cord processes pain and the brain does not, so there would be nothing for the signal to do up there.
C. Because the sensory neuron of a reflex arc has no connection to the brain at all, which is why the reaction is unconscious.
D. Because reflex circuits are built from a faster class of axon than any pathway ascending to the brain.

**Answer:** A. Because a trip to the brain and back costs time the tissue cannot spare.

**Why:** Everything here is distance: hand to cord to muscle is about a metre and a half, and routing through the brain adds most of the body. Roughly 20 ms against 250 ms is the difference between a reddened palm and a burn. You do feel it, a moment later, the sensory neuron branches, and the loop finishes before the brain is told.

Page: https://tryals.app/practice/psychology-i/central-nervous-system/a-withdrawal-reflex-has-your-hand-off-the-hot-pan-before-you

### 5. A patient whose spinal cord is completely severed above the knee still shows a knee-jerk reflex when the tendon is tapped, even though the brain can no longer feel or move the leg.

**Answer:** True

**Why:** True. The knee-jerk reflex arc is contained in the spinal cord, so it can still fire when tapped even though the severed cord blocks sensation and voluntary movement from reaching or leaving the brain. This is direct evidence that spinal reflexes do not require the brain.

Page: https://tryals.app/practice/psychology-i/central-nervous-system/a-patient-whose-spinal-cord-is-completely-severed-above-the-knee

### 6. A patient with a complete spinal transection at the neck still shows a knee-jerk reflex when tapped, despite feeling nothing below the injury. What does the surviving reflex reveal?

A. That reflexes are stored in the muscle rather than the nervous system
B. That the transection must be incomplete, since a reflex requires brain involvement
C. That the brain is sending signals through an alternative pathway around the injury
D. That the circuit is complete within the spinal cord

**Answer:** D. That the circuit is complete within the spinal cord

**Why:** The arc is entirely below the injury: sensory neuron in, motor neuron out, no brain required. Feeling the tap needs an ascending pathway that has been cut, so the leg moves and the patient never knows. The spinal cord is a processor in its own right, not a cable to the brain.

Page: https://tryals.app/practice/psychology-i/central-nervous-system/a-patient-with-a-complete-spinal-transection-at-the-neck-still-shows