Anatomy & Physiology · Chapter 13

Anatomy of the Nervous System

Inside your skull sits the most complex object we know of. It is about three pounds of soft tissue that lets you read this line, remember when you were small, and pull your hand off a hot stove before you even feel the burn. This chapter is a guided tour of that brain. We will walk over the wrinkled top of the cerebrum. We will drop down through its inner regions to the brainstem, which keeps you alive, and look at the cerebrospinal fluid that cushions it all. We will peel back the three meninges, the wraps that guard it. We will follow the spinal cord down its bony tunnel. Last, we will count off the twelve cranial nerves that leave the brain directly. By the end, you will be able to picture a whole brain and name what you see.

By the end of this chapter you can

Find the four lobes of the cerebrum. Name the skull bone over each one, and match each lobe to its main jobs.
Find the primary motor and primary somatosensory cortex. Find the central sulcus, the groove that splits them.
Split the brain into its four main regions: cerebrum, diencephalon, brainstem, and cerebellum. Say what each one does.
Trace how cerebrospinal fluid flows through the four ventricles and out into the subarachnoid space. Explain its three jobs.
Name the three meninges in order. Describe the spaces between them, and link each one to an emergency.
Read a cross-section of the spinal cord, with its gray horns, white columns, and dorsal and ventral roots. Explain its one-way traffic rule.
List the twelve cranial nerves in order. Sort each one as sensory, motor, or mixed, and give its main job.

Section 1The cerebral lobes

The four lobes of the brain — **Reference.** The four lobes of the cerebral cortex — OpenStax, CC BY 4.0 (via Wikimedia Commons)

A wrinkled sheet of gray matter

Picture a brain. You likely see a pale, deeply folded dome. That is the cerebrum, the biggest part of the brain and the place where thinking happens. Its outer layer, only a few millimeters thick, is the cerebral cortex. It is a sheet of gray matter, which is packed with billions of nerve cell bodies.

Under the cortex lies white matter. It is made of axons, the long nerve fibers, wrapped in a fatty coat called myelin, and these fibers wire one part of the brain to another. The wrinkles are not just for looks, though. Each ridge is a gyrus, and each groove is a sulcus. A deep groove is a fissure. Folding lets a huge sheet of cortex fit inside the skull, like a crumpled towel in a cup. About two-thirds of it is hidden down in the grooves. More cortex means more brain power. That is why smart animals tend to have very folded brains.

Four lobes, four skull bones

Each half of the brain is called a hemisphere, and the cortex of each half has four lobes. They are easy to name, because each lobe gets its name from the skull bone right on top of it. Two big grooves mark the borders. The central sulcus runs down across the top of the brain and splits the frontal lobe in front from the parietal lobe behind it. The deep lateral sulcus (Sylvian fissure) runs along the side and marks off the temporal lobe below. At the very back sits the occipital lobe. One more area, the insula, is tucked deep inside the lateral sulcus, and it is often called a hidden fifth lobe.

The two halves are not exact twins. They show lateralization, which means they split up the work. One side tends to lead on certain tasks. Language is usually on the left, while space and music skills are often on the right. The two halves talk all the time through a thick bridge of white matter called the corpus callosum. It carries hundreds of millions of axons from one side to the other.

Who does what

Each lobe has its own special jobs. A few areas are worth learning well, because damage to them causes classic, testable problems. Just in front of the central sulcus is a strip called the precentral gyrus. It is the primary motor cortex, which sends the orders for movements you choose to make. Just behind the central sulcus is the postcentral gyrus. It is the primary somatosensory cortex, which takes in touch, pressure, and temperature from the whole body. Each strip holds a map of the body that is upside down and out of shape, with huge areas for the hands and face.

Lobe	Sits under	Main jobs
Frontal	Frontal bone	Movements you choose (primary motor cortex), planning, judgment, personality, and making speech (Broca’s area)
Parietal	Parietal bone	Touch, pressure, pain, and temperature (primary somatosensory cortex); sense of space
Temporal	Temporal bone	Hearing, smell, memory, and understanding language (Wernicke’s area)
Occipital	Occipital bone	Vision; holds the primary visual cortex

The central sulcus splits moving from feeling. Just in FRONT of it is the motor strip (precentral gyrus). Just BEHIND it is the sensory strip (postcentral gyrus). Here is an easy way to remember. M for Motor comes before S for Sensory in the alphabet, and the motor strip comes first, in front. Damage in front of the groove weakens movement. Damage behind it dulls feeling.

Tap through the lobes and watch each one light up below.

Interactive The wrinkled cortex, split into lobes. Each lobe is named for the skull bone over it. Tap a lobe to light it up. Open full ↑

Key points

The cerebral cortex is a folded sheet of gray matter. Its gyri (ridges) and sulci (grooves) fit more surface into the skull.
There are four lobes: frontal, parietal, temporal, occipital. Each is named for the skull bone on top of it.
The central sulcus splits the motor cortex (precentral, in front) from the somatosensory cortex (postcentral, behind).
The corpus callosum links the two halves. The halves split up the work, which is called lateralization.

Section 2The major brain regions

The four main brain regions, seen in a cut down the middle — **Reference.** The four major brain regions in midsagittal section John A Beal, PhD Dep't. of Cellular Biology Anatomy, Louisiana State Uni, CC BY 2.5 (via Wikimedia Commons)

Slice it down the middle

The lobes are only the outside story. To see how the whole brain is built, picture cutting it right down the middle. This cut is called a midsagittal section. On the cut face you will see four main regions, which stack from the thinking parts on top to the life-support parts below. They are the cerebrum, the diencephalon, the brainstem, and the cerebellum.

Here is a rough rule. The higher a part sits, the more “human” and chosen its job. The lower it sits, the older and more automatic its job. You can lose a lot of cerebrum and still live. But a tiny stroke in the brainstem can kill you.

The diencephalon: the brain’s relay and thermostat

The diencephalon is buried under the cerebrum. It wraps around the third ventricle, one of the brain’s fluid-filled spaces. It is small but very important, and it has two stars. The first is the thalamus, the brain’s main train station. Almost every sense signal heading to the cortex, such as sight, sound, touch, and taste, stops here first. The thalamus sorts each signal and sends it on. Smell is the one sense that skips it.

Below and in front sits the hypothalamus. It is only about the size of an almond, yet it runs a huge share of homeostasis, the body’s way of keeping things steady inside. It controls body temperature, hunger, thirst, and the sleep–wake cycle. It also runs the pituitary gland that hangs below it, so it steers that gland’s hormones. The hypothalamus is where the nervous system and the hormone (endocrine) system shake hands.

The brainstem and the cerebellum

The brainstem links the brain to the spinal cord, and inch for inch, it is the most vital tissue you have. From top to bottom, it has three parts: the midbrain, the pons, and the medulla oblongata. The medulla holds the reflex centers that set your heart rate, blood pressure, and breathing. You never think about these jobs, but you cannot live without them. All twelve cranial nerves, except the first two, attach to the brainstem.

Behind the brainstem, tucked under the occipital lobe, is the cerebellum. Its name means “little brain,” and it is tightly folded. It fine-tunes movement, keeps your balance, and makes your motions smooth. It does not start a movement. Instead, it edits one, like an editor fixing a draft. That is why damage to it makes a person clumsy and unsteady, but not paralyzed.

Region	Key parts	Main job
Cerebrum	Two hemispheres, cortex, corpus callosum	Thinking, feeling, movements you choose, memory, language
Diencephalon	Thalamus, hypothalamus, epithalamus	Relays sense signals (thalamus); keeps the body steady and runs hormones (hypothalamus)
Brainstem	Midbrain, pons, medulla oblongata	Life-saving reflexes (heartbeat, breathing), hub for cranial nerves, highway between brain and cord
Cerebellum	Two hemispheres, folia (folds), vermis (middle strip)	Teamwork of muscles, balance, smooth and exact movement

The brainstem is life support. The medulla oblongata holds the centers for the heart, blood vessels, and breathing. That is why brain death is defined as the loss of brainstem function. It is also why a small brainstem stroke can stop breathing, while a much bigger stroke in the cortex does not.

Cut the brain down the middle and tap each region below.

Interactive Cut the brain down the middle to see its four regions, from the thinking cerebrum to the life-support brainstem. Tap each one. Open full ↑

Key points

There are four main regions: cerebrum, diencephalon, brainstem, cerebellum. Higher parts do chosen tasks. Lower parts do automatic ones.
The thalamus passes almost all sense signals on to the cortex. The hypothalamus keeps the body steady and runs the pituitary.
The brainstem (midbrain, pons, medulla) controls life-saving reflexes. Most cranial nerves attach to it.
The cerebellum smooths movement and keeps balance. It edits motion. It does not start it.

Section 3Ventricles & cerebrospinal fluid

The ventricles and the flow of brain fluid — **Reference.** The four ventricles and the flow of cerebrospinal fluid — OpenStax, CC BY 4.0 (via Wikimedia Commons)

A brain that floats

The brain is not solid all the way through. In its core is a set of linked hollow rooms called the ventricles, and a clear liquid fills them and also bathes the outside of the brain. This liquid is cerebrospinal fluid, or CSF. There are four ventricles. Two big, C-shaped lateral ventricles sit one in each half of the brain. A thin, slit-like third ventricle sits in the middle of the diencephalon. A diamond-shaped fourth ventricle sits between the brainstem and the cerebellum.

CSF is made by the choroid plexus. This is a lacy web of tiny blood vessels (capillaries) that lines the ventricles. It filters plasma, the liquid part of blood, and makes about 500 mL of CSF a day. Yet the total amount stays near 150 mL. That is because the fluid is drained and replaced several times each day.

Following the flow

CSF is not a still pond. Instead, it flows on a one-way path, from the top of the ventricles down and out around the brain and cord. Trace it once and you will remember it. Fluid made in the lateral ventricles passes through small holes called the interventricular foramina and enters the third ventricle. Then it flows down a narrow channel, the cerebral aqueduct, into the fourth ventricle.

From there, it leaves through three small openings into the subarachnoid space, which surrounds the whole brain and spinal cord. This gives the whole central nervous system a liquid cushion. Last, the fluid goes back into the blood through arachnoid granulations, little bumps shaped like cauliflower that poke into the large veins that drain the brain. That closes the loop.

Step	Structure	What happens
1	Choroid plexus (lateral ventricles)	CSF is made from filtered blood plasma
2	Interventricular foramina	Fluid passes into the third ventricle
3	Cerebral aqueduct	Fluid flows down to the fourth ventricle
4	Openings → subarachnoid space	Fluid bathes the brain and spinal cord
5	Arachnoid granulations	Fluid goes back into the blood in the veins

Why the fluid matters

CSF does three jobs. The first is buoyancy, which means it helps the brain float. The brain weighs about 1,400 g, but floating in CSF it presses down with a weight of only about 50 g. Without this, the brain would crush its own base. The second job is protection. The liquid soaks up shocks, so the brain bobs gently instead of slamming into the skull during a bump. The third job is chemical stability. CSF brings food in and washes waste out, and it also helps keep the exact mix of salts (ions) that nerve cells need to fire.

When the drain clogs, pressure climbs. Sometimes CSF cannot flow or drain. The aqueduct might be blocked, for example. Then fluid backs up and the ventricles swell. This is called hydrocephalus. In babies, the skull bones have not yet joined, so the head gets bigger. In adults, the rising pressure squeezes the brain. Doctors treat it with a shunt, a tube that drains the extra fluid away.

Watch the fluid flow and tap each chamber below.

Interactive The brain floats in cerebrospinal fluid, which is made in and flows through four hollow ventricles. Watch the fluid move and tap each part. Open full ↑

Key points

There are four ventricles: two lateral, the third, and the fourth. They are hollow rooms filled with CSF.
The choroid plexus makes CSF. It flows lateral → third → fourth ventricle → subarachnoid space → blood in the veins.
CSF gives the brain and cord buoyancy, shock protection, and chemical stability.
A blocked flow causes hydrocephalus, with swollen ventricles and rising pressure.

Section 4The meninges

The three layers that wrap the brain and spinal cord — **Reference.** The three meninges wrapping the brain and spinal cord https://www.scientificanimations.com, CC BY-SA 4.0 (via Wikimedia Commons)

Three coats around the cord

The brain and spinal cord are too precious to rest right against bone. Instead, they are wrapped in three thin sheets of tissue called the meninges. From the outside in, they are the dura mater, the arachnoid mater, and the pia mater. Learn them in that order, because the spaces between them are where the big medical emergencies happen. The Latin names tell you what they are like. Dura mater means “tough mother.” Pia mater means “tender mother.”

From tough to tender

The dura mater is the outer layer, and it is thick, strong, and like leather. Around the brain it has two layers. Where the two layers pull apart, they form the dural venous sinuses, channels that drain blood from the brain. The dura also folds inward. One fold, the falx cerebri, dips down between the two halves of the brain and holds it steady.

Under the dura is the arachnoid mater. It is a thin layer that looks like a web, and its name comes from the Greek word for spider. Fine threads called arachnoid trabeculae cross the gap beneath it. That gap is the subarachnoid space, and it is filled with CSF. Last comes the pia mater, a very thin film that is rich in blood vessels. It clings to every ridge and groove of the brain, like plastic wrap pressed into every fold.

Layer	What it is like	Space / feature
Dura mater	Tough outer coat, like leather	Forms dural venous sinuses; the epidural space lies outside it
Arachnoid mater	Thin middle layer, like a web	The subdural space is above it; the subarachnoid space (with CSF) is below it
Pia mater	Thin inner film, full of blood vessels	Sticks right onto the brain and cord

The spaces tell the story of a bleed. An epidural hematoma is a pool of blood outside the dura. It usually comes from a torn artery (the middle meningeal artery). It builds up fast and looks lens-shaped on a CT scan. A subdural hematoma is a pool of blood under the dura. It usually comes from torn veins (bridging veins). It builds up slowly and looks crescent-shaped, like a thin moon. When the meninges get infected, it is called meningitis. The swollen subarachnoid space causes the classic stiff neck, fever, and headache.

Peel back the layers one at a time below.

Interactive Three layers wrap the brain and cord, from a tough outer layer to a soft inner one, with fluid-filled spaces between. Tap each layer. Open full ↑

Key points

The three meninges, from outside in, are the dura mater (tough), arachnoid mater (like a web), and pia mater (thin and soft).
The subarachnoid space, under the arachnoid, holds flowing CSF.
The pia mater follows every fold of the brain. The dura forms blood channels (sinuses) and steadying folds like the falx cerebri.
The spaces name the bleeds. An epidural bleed is from an artery and lens-shaped. A subdural bleed is from veins and crescent-shaped. Infection of the meninges is meningitis.

Section 5The spinal cord

**Reference.** A slice across the spinal cord, showing gray and white matter — OpenStax, CC BY 4.0 (via Wikimedia Commons)

A cable and a switchboard

The spinal cord is the body’s main message cable. It starts at the bottom of the brainstem and runs down through a tunnel in the backbone (the vertebral column). It ends near the first or second lumbar bone of the lower back (L1–L2). Below that, it splits into a spray of nerve roots called the cauda equina, a name that means “horse’s tail.”

The cord does two things at once. First, it carries messages up and down between the body and the brain. Second, it works as its own local switchboard that can handle reflexes without waiting for the brain.

Reading a slice

Cut the cord across, and you will see a neat layout. In the center is a region of gray matter shaped like a butterfly or the letter H. It holds nerve cell bodies, and around it is white matter, made of coated nerve fibers in bundles. This is the reverse of the brain, where gray matter is on the outside.

The gray matter has wings called horns. The dorsal (posterior) horns, toward the back, handle incoming sensory signals. The ventral (anterior) horns, toward the front, send out motor orders to muscles. Small lateral horns on the sides, in the chest and upper belly part of the cord, hold nerve cells of the automatic (autonomic) system. A tiny central canal, filled with CSF, runs down the middle.

One-way traffic at the roots

Each spinal nerve splits into two roots where it joins the cord. These roots follow a strict one-way traffic rule. The dorsal root carries sensory signals into the cord. It has a clear bulge called the dorsal root ganglion, which is where the sensory nerve cell bodies sit. The ventral root carries motor signals out to muscles and glands.

Just past the cord, the two roots join into one mixed spinal nerve. There are 31 pairs of these nerves. The white matter is sorted into columns. The columns hold bundles of fibers called tracts. Some tracts go up and carry feeling, while others go down and carry movement orders. They are the express lanes that run the length of the cord.

Structure	Type	Function
Dorsal (posterior) horn	Gray matter	Takes in sensory signals
Ventral (anterior) horn	Gray matter	Sends motor orders to muscles
Dorsal root (with ganglion)	Sensory	Carries feeling into the cord
Ventral root	Motor	Carries orders out of the cord
White columns / tracts	White matter	Highways going up (sensory) and down (motor)

Dorsal in, ventral out. Feeling comes in through the back (dorsal root). Motor orders go out through the front (ventral root). This one-way street is called the Bell–Magendie rule. Also, the cord itself ends near L1–L2. So a lumbar puncture (spinal tap) is done lower down, near L3–L4. There the needle only meets the loose roots of the cauda equina. It does not risk hitting the cord.

Slice the cord and tap each part below.

Interactive A slice shows a gray butterfly of cell bodies inside white tracts. Feeling enters at the back and motor orders leave at the front. Tap each part. Open full ↑

Key points

The cord runs to about L1–L2. Below that, it goes on as the nerve roots of the cauda equina. 31 pairs of spinal nerves branch off.
In a slice, gray matter (horns) sits in the center, with white matter (tracts) around it. The brain is the other way around.
Dorsal horns and roots = sensory in. Ventral horns and roots = motor out. This is the Bell–Magendie rule.
The dorsal root ganglion holds sensory cell bodies. A lumbar puncture is done below the end of the cord to be safe.

Section 6The cranial nerves

The twelve cranial nerves on the underside of the brain — **Reference.** The twelve pairs of cranial nerves on the underside of the brain — OpenStax, CC BY 4.0 (via Wikimedia Commons)

Twelve nerves straight from the brain

Most nerves branch off the spinal cord. But twelve special pairs come straight out of the brain. These are the cranial nerves, and they are numbered with Roman numerals, I through XII. The numbers go from front to back, in the order the nerves appear on the underside of the brain. The first two, the olfactory and optic nerves, grow out of the cerebrum and diencephalon. The other ten sprout from the brainstem.

Together, these nerves handle the special senses of the head. They move the eyes, face, and tongue. And one “wandering” nerve reaches all the way down into the chest and belly.

Sensory, motor, or both

Each cranial nerve is sorted by the kind of message it carries. Some are only sensory, which means they bring information in. Some are only motor, which means they send orders out. Some are mixed, doing both. An old memory trick shows the pattern: “Some Say Marry Money, But My Brother Says Big Brains Matter Most.” The first letter of each word is S, S, M, M, B, M, B, S, B, B, M, M. Each letter tells you if nerves I through XII are Sensory, Motor, or Both. To remember the names, students have long used “On Old Olympus’ Towering Tops, A Finn And German Viewed Some Hops.”

#	Name	Type	Main function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Most eye movements; makes the pupil smaller; lifts the eyelid
IV	Trochlear	Motor	Moves the eye (superior oblique muscle)
V	Trigeminal	Both	Feeling in the face; chewing muscles
VI	Abducens	Motor	Moves the eye out to the side (lateral rectus muscle)
VII	Facial	Both	Facial expressions; taste (front of tongue); tears and spit (saliva)
VIII	Vestibulocochlear	Sensory	Hearing and balance
IX	Glossopharyngeal	Both	Taste (back of tongue); swallowing; saliva
X	Vagus	Both	Calming (parasympathetic) control of the heart, lungs, and gut
XI	Accessory	Motor	Moves the neck and shoulders (sternocleidomastoid and trapezius muscles)
XII	Hypoglossal	Motor	Moves the tongue

The wandering nerve

One cranial nerve needs a special note. The vagus nerve (X) is the odd one out. Every other cranial nerve stays in the head and neck, but the vagus travels down into the chest and belly. Its name even means “wanderer.” It is the main highway of the parasympathetic system, the calming side of your automatic nerves. It slows the heart, drives digestion, and checks on the inner organs. When you hear about the “rest-and-digest” response, that is the vagus at work.

Only the vagus leaves the head. Cranial nerves serve the head and neck, with one exception. CN X, the vagus, wanders all the way to the gut. It carries most of the body’s parasympathetic (calming) signals. Think of it as the one cranial nerve that will not stay home.

Count off each nerve and tap it below.

Interactive Twelve pairs of nerves come straight out of the brain, numbered I to XII from front to back. Some are sensory, some motor, some both. Tap each one. Open full ↑

Key points

There are twelve pairs of cranial nerves, numbered I–XII from front to back. All but the first two come from the brainstem.
Each one is sensory, motor, or mixed. The trick “Some Say Marry Money…” gives the pattern.
Several move the eye (III, IV, VI). Others carry special senses: I smell, II vision, VIII hearing and balance.
The vagus (X) is the only cranial nerve that leaves the head and neck. It carries most calming (parasympathetic) signals to the organs.

The chapter in one breath

The cerebrum is a folded sheet of gray matter with four lobes: frontal, parietal, temporal, occipital. Each lies under the skull bone with the same name.
The central sulcus splits the motor cortex (front) from the somatosensory cortex (back). The corpus callosum links the two halves.
Four brain regions stack from thinking to survival: cerebrum → diencephalon → brainstem → cerebellum. The medulla runs the life-saving reflexes.
The choroid plexus makes cerebrospinal fluid. It flows through four ventricles into the subarachnoid space. It helps the brain float, guards it, and keeps its chemistry steady.
Three meninges (dura, arachnoid, pia) wrap the brain and cord. The spaces between them name the epidural, subdural, and subarachnoid bleeds.
The spinal cord has gray horns in the center and white tracts around them. Feeling enters by the dorsal root. Motor orders leave by the ventral root.
Twelve cranial nerves (I–XII) serve the head and neck. Each is sensory, motor, or mixed. Only the vagus reaches the chest and belly.

ReferenceWords worth knowing

Here is a quick list of the key words in this chapter. Skim it now, or come back when a word trips you up.

Arachnoid mater: The middle layer of the meninges, thin and like a web. Under it, the subarachnoid space holds flowing cerebrospinal fluid.
Brainstem: The stalk that links the brain to the spinal cord. It is made of the midbrain, pons, and medulla oblongata, and holds life-saving reflex centers and most cranial nerves.
Central sulcus: The groove that splits the frontal lobe (with its motor cortex) from the parietal lobe (with its somatosensory cortex).
Cerebellum: The “little brain” behind the brainstem. It keeps your balance and smooths the movements you choose.
Cerebral cortex: The thin outer sheet of gray matter on the cerebrum. It handles thinking, feeling, and the movements you choose.
Cerebrospinal fluid (CSF): The clear liquid made by the choroid plexus. It fills the ventricles and subarachnoid space, where it cushions and feeds the brain and cord.
Choroid plexus: The web of tiny blood vessels lining the ventricles. It filters blood plasma to make cerebrospinal fluid.
Corpus callosum: The thick band of white matter that carries nerve fibers between the two halves of the brain.
Cranial nerves: The twelve pairs of nerves (I–XII) that come straight out of the brain. They serve the head and neck, and through the vagus, the inner organs.
Diencephalon: The central brain region that holds the thalamus and hypothalamus. It wraps around the third ventricle.
Dorsal root ganglion: The bulge on a spinal nerve’s dorsal root. It holds the cell bodies of sensory nerve cells.
Dura mater: The tough outer layer of the meninges, like leather. It forms the dural venous sinuses and folds like the falx cerebri.
Gyrus: A raised ridge on the surface of the cerebral cortex. The plural is gyri.
Hypothalamus: A part of the diencephalon that controls body heat, hunger, thirst, sleep, and hormone release. It links the nervous and hormone systems.
Medulla oblongata: The lowest part of the brainstem. It holds the reflex centers for heart rate, blood pressure, and breathing.
Meninges: The three sheets of tissue that wrap the brain and spinal cord: dura, arachnoid, and pia mater.
Pia mater: The thin inner layer of the meninges, rich in blood vessels. It clings right onto the brain and cord.
Sulcus: A shallow groove between the ridges of the cerebral cortex. The plural is sulci. A deep one is called a fissure.
Thalamus: The paired relay station in the diencephalon. It sorts almost all sense signals and sends them on to the cortex.
Vagus nerve: Cranial nerve X. It is the only cranial nerve that goes past the head and neck, carrying most calming (parasympathetic) signals to the heart, lungs, and gut.
Ventricles: The four linked, fluid-filled rooms inside the brain: two lateral, the third, and the fourth. CSF is made in them and flows through them.

Check yourself

1. Why is the cerebral cortex so deeply folded into gyri and sulci? What does this do?

Folding fits much more cortex into the fixed space of the skull. About two-thirds of the cortex is hidden in the grooves. More cortex means more nerve cells and more brain power.

2. A patient has weak movement on one side of the body. Which gyrus and which lobe would you suspect? How does the central sulcus help you find it?

The precentral gyrus (primary motor cortex) of the frontal lobe. The motor strip lies just in front of the central sulcus. The sensory strip lies just behind it. So movement problems point to the front of the groove, and feeling problems point behind it.

3. Name the four lobes of the cerebrum. Give one main job of each.

Frontal (movement, judgment, making speech), parietal (touch and other body feelings), temporal (hearing, memory, understanding language), occipital (vision). Each lobe is named for the skull bone right on top of it.

4. Which brain part passes almost all sense signals on to the cortex? Which nearby part keeps the body steady and controls the pituitary gland?

The thalamus relays sense signals. The hypothalamus keeps the body steady and controls the pituitary. Both sit in the diencephalon. Smell is the one sense that skips the thalamus.

5. Why can a small stroke in the medulla oblongata kill, when a much bigger stroke in the cerebrum may not?

The medulla holds the reflex centers for heart rate, blood pressure, and breathing. Losing them stops the automatic jobs that keep you alive. Big areas of cerebrum can be damaged without stopping life support.

6. Trace a drop of CSF from where it is made to where it goes back into the blood. Name the four ventricles and the key passages in order.

Choroid plexus of the lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → arachnoid granulations into the blood in the veins. It is a one-way loop. The fluid is drained and replaced several times a day.

7. List the three jobs of cerebrospinal fluid. Use numbers to explain how it helps the brain float.

Buoyancy, protection (soaking up shocks), and chemical stability. The brain weighs about 1,400 g. Floating in CSF, it presses down with a weight of only about 50 g. This keeps it from crushing itself.

8. Name the three meninges from outside in. Which space between them holds flowing CSF?

Dura mater, arachnoid mater, pia mater. The subarachnoid space (under the arachnoid) holds the CSF. The dura is tough and on the outside. The pia is a thin film that hugs the brain.

9. On a CT scan, one bleed looks lens-shaped and one looks crescent-shaped. Which space and which kind of blood vessel causes each?

Lens-shaped = epidural hematoma (an artery, often the middle meningeal artery). Crescent-shaped = subdural hematoma (veins, from torn bridging veins). The epidural bleed builds up fast under artery pressure. The subdural bleed builds up more slowly.

10. In a slice of the spinal cord, which root carries feeling into the cord, and which carries motor orders out? Where do the sensory cell bodies sit?

The dorsal (posterior) root carries feeling in. The ventral (anterior) root carries motor orders out. Sensory cell bodies sit in the dorsal root ganglion. This one-way setup is the Bell–Magendie rule: dorsal in, ventral out.

11. The spinal cord ends near the L1–L2 bone. Why does this matter for a lumbar puncture?

Because the cord ends high, the needle can go in lower (near L3–L4) to take CSF without piercing the cord. Below L1–L2, the canal holds only the loose, floating roots of the cauda equina. The needle can safely push them aside.

12. Which cranial nerve is the only one that goes past the head and neck? What is its main job, and what does its name mean?

The vagus nerve (CN X). Its name means “wanderer.” It carries most of the body’s calming (parasympathetic) signals to the heart, lungs, and gut. Every other cranial nerve stays in the head and neck.

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