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Brain and Cranial Nerves

The brain and spinal cord are your central nervous system, helping you understand complex stuff. The brain takes in sensory info, figures it out, and then tells your body what to do.

Organization of the Brain

Your brain has four main parts:

• Cerebrum:- The biggest part, responsible for your higher-level thinking, memory, personality, and smarts.
• Cerebellum:- This part smoothly coordinates your voluntary movements, working with your cerebrum and body signals.
• Diencephalon (deep):- Acts like a control tower, linking your brain to your hormone system and managing automatic body functions to keep things stable.
• Brainstem:- Carries messages to and from the cerebrum and cerebellum. It also handles basic, essential body functions you don't even think about, like breathing.

Gray vs White Matter

Your brain and spinal cord have two main types of tissue:

• Gray matter:- This is where neuron cell bodies and their branch-like dendrites are found. It's the main area for processing information.
  • In the cerebrum, the outer gray layer is the cerebral cortex, and deeper clumps of cell bodies are called nuclei.
• White matter:- Consists of bundles of myelinated axons, which are like insulated cables that quickly send nerve signals.
  • Outside the brain and spinal cord (in the peripheral nervous system), these axon bundles are called nerves, and cell body clusters are called ganglia.
• The cerebral cortex has ridges called gyri and shallow grooves called sulci. These folds increase the brain's surface area, meaning more room for neurons.

Meninges

3 Layers (from outside in):

• Dura mater:- The tough outer layer, actually two layers fused together, except where they split to form channels for blood (dural venous sinuses).
• Arachnoid mater:- The middle layer, which looks a bit like a spiderweb. It has threads that go through a space (subarachnoid space) filled with cerebrospinal fluid (CSF).
• Pia mater:- A very thin layer that sticks right to the brain's surface.

Potential spaces:

These spaces aren't usually there but can fill with blood from an injury, like in a subdural hematoma:

• Epidural space:- Between the dura mater and the skull/spine bones.
• Subdural space:- Between the dura mater and the arachnoid mater.

Cranial Dural Septa

The dura mater also forms internal dividers in your skull that separate and support parts of the brain:

• Falx cerebri:- Sits between your two brain halves (cerebral hemispheres) and houses major blood sinuses.
• Tentorium cerebelli:- Divides the back part of your brain (occipital lobes) from the cerebellum below.
• Falx cerebelli:- Separates the two halves of your cerebellum.

Protection & Support: Ventricles

Ventricles are connected, CSF-filled spaces inside the brain, lined by special cells:

• Right and left lateral ventricles:- One in each cerebral hemisphere, separated by a thin membrane called the septum pellucidum.
• Third ventricle:- Located in the center, surrounded by the diencephalon. It connects to the lateral ventricles via interventricular foramina and to the fourth ventricle by the cerebral aqueduct.
• Fourth ventricle:- Between the pons and cerebellum, connecting to the central canal of the spinal cord.

Protection & Support: Cerebrospinal Fluid

Cerebrospinal fluid (CSF) flows through the ventricles, spinal cord, and around the brain and spinal cord. It mainly:

• Buoyancy:- Makes your brain “float,” supporting about $95\%$ of its weight.
• Protection:- Acts as a liquid cushion, protecting your brain from bumps.
• Environmental stability:- Delivers nutrients and chemical messages, and removes waste, keeping your brain tissue healthy.

Protection & Support: CSF Circulation

CSF is made from blood plasma by special cells in the ventricles called choroid plexuses. Here's its path:

• From the lateral ventricles to the third ventricle through interventricular foramina.
• Down the cerebral aqueduct to the fourth ventricle.
• Then into the space around the brain (subarachnoid space) or down the central canal of the spinal cord.
• Finally, it circulates back to blood vessels in the dural venous sinuses.

Protection & Support: CSF Recovery

CSF is reabsorbed into your blood through arachnoid villi, which are tiny extensions of the arachnoid mater that poke into the dural venous sinuses (blood-filled spaces formed by the dura mater).

• About $500\text{ mL}$ of CSF is made daily, but only around $100-160\text{ mL}$ is present at any time.

Protection & Support: Blood-Brain Barrier

The blood-brain barrier (BBB) acts like a bouncer around brain capillaries, strictly controlling what leaves your bloodstream and enters your brain's fluid.

Key Features:

• The lining cells of brain capillaries (endothelial cells) are super tightly connected, preventing most things from passing through.
• These capillaries also have a thicker basal membrane.
• Brain capillaries are further wrapped by perivascular feet from astrocytes, which help decide what chemicals get into the brain.

Notes on BBB:

• It's not perfect; fat-soluble molecules can sneak through.
• Some areas, like the choroid plexuses, hypothalamus, and pineal gland, don't have this barrier.

Cerebrum

The cerebrum is your brain's command center for conscious thought and all your complex mental tasks like intelligence, reasoning, memory, personality, and deciding what to do. It also controls voluntary movement and how you perceive senses.

• The outer gray matter, the cerebral cortex, handles most of this processing.
• Your left and right brain halves are separated by the longitudinal fissure.
• Deeper white matter, like the corpus callosum, connects different brain areas.
• Different parts of the cerebrum often work together.
• Each brain half handles sensory input and motor commands for the opposite side of your body.
• Some functions are mainly in one half, a concept called cerebral lateralization. For example, speech is usually on the left side.

Lobes of the Cerebrum

Each half of your brain has five distinct lobes:

Frontal Lobe:

• The central sulcus is its back border.
• The lateral sulcus (or fissure) is its side border.
• Contains the primary motor cortex (precentral gyrus), which controls voluntary movements.

Parietal Lobe:

• The central sulcus is its front border.
• The lateral sulcus/fissure is its side border.
• Holds the primary somatosensory cortex (postcentral gyrus), which processes touch, pressure, pain, and temperature.

Temporal Lobe:

• Separated from the frontal and parietal lobes by the lateral sulcus.
• Mainly processes hearing and smell.

Occipital Lobe:

• The very back part of the cerebrum.
• Separated from the cerebellum by the transverse fissure.
• Handles all visual information.

Insula:

• Hidden deep inside the lateral fissure.
• Involved in taste and forming memories.

Motor Areas of the Cerebrum

The primary (somatic) motor cortex is in the precentral gyrus, at the front of your frontal lobe.

• Nerve fibers here cross over (decussate) so that the left motor cortex controls muscles on the right side of your body, and vice versa.
• A motor homunculus is like a map showing which parts of this cortex control different body parts, with more important areas (like hands) taking up more space.

Other Motor Areas:

• The motor speech area, or Broca’s area, controls the muscles you use to speak. It's usually in the lower left frontal lobe.
• The frontal eye field helps control eye movements for reading, located in the front of your frontal lobe.
• The premotor cortex coordinates skilled movements and works with the primary motor cortex. It's just in front of the precentral gyrus.

Sensory Areas of the Cerebrum

The primary somatosensory cortex is in the postcentral gyrus of your parietal lobe, just behind the central sulcus.

• Neurons here get sensory info like touch, pressure, pain, and temperature from your skin.
• A sensory homunculus maps out how different body parts are represented in this cortex (also with crossover of signals).
• Bigger somatosensory association areas nearby help you understand and combine different sensory inputs.

Additional Sensory Areas:

• Primary visual cortex and its association areas are in your occipital lobes.
• Primary auditory cortex and its association areas are in your temporal lobes.
• Primary olfactory cortex (smell) is also in your temporal lobes.
• Primary gustatory cortex (taste) is in the insula.

Functional Brain Regions

Functional regions combine info from many association areas:

• Prefrontal cortex:- Involved in complex thinking, judgment, personality, planning, and decision-making. This area doesn't fully develop until your twenties.
• Wernicke’s area:- Essential for understanding spoken and written language, usually in the left hemisphere.

Central White Matter of the Cerebrum

Gray matter areas in the cerebrum are heavily connected by bundles of insulated nerve fibers called tracts:

• Projection tracts:- Connect the cerebral cortex to the brainstem and spinal cord.
• Association tracts:- Connect different areas within the same cerebral hemisphere.
• Commissural tracts:- Connect the left and right cerebral hemispheres. The most important one is the corpus callosum.

Cerebral Lateralization

Your left and right brain halves have some specialized jobs:

• Language centers like Broca's and Wernicke's areas are usually in the left hemisphere.
• This specialization often links to handedness:
  • About $90\%$ of people are right-handed. For most of