CNS pt 1

cerebrospinal fluid

fills the ventricles and flows through the central canal of the spinal cord. Made from blood but not by filtering it like a kidney would. Instead, it’s produced by active secretion. Specific ions are transported into the ventricles, and water follows by osmosis. This creates the clear fluid that cushions the brain, removes waste, and helps regulate the chemical environment. Eventually, CSF is reabsorbed back into the blood, completing a full circulation loop.

Choroid plexus

specialized structure found in brain ventricles, made up of epithelial cells wrapped around a core of blood capillaries and connective tissue. Its main function is to produce CSF which fills the ventricles and flows through the central canal of the spinal cord. Also, forms part of the blood-brain barrier, having tight junctions between cells that prevent unwanted substances in the blood from leaking into the CNS. 

CSF- active secretion

Specific ions are transported into the ventricles, and water follows by osmosis. This creates the clear fluid that cushions the brain, removes waste, and helps regulate the chemical environment. 

Path of CSF circulation in the Brain:

1) CSF is produced by the choroid plexuses and begins its journey in the lateral ventricles, one in each cerebral hemisphere.

2) From there, it flows through the interventricular foramina into the third ventricle, located in the diencephalon.

3) Next, CSF passes through the cerebral aqueduct, a narrow channel in the midbrain, and enters the fourth ventricle

4) From the fourth ventricle, CSF has two main pathways: it can move down into the central canal of the spinal cord, or it can exit into the subarachnoid space surrounding the brain and spinal cord.

5) Eventually, CSF is absorbed into the bloodstream. This occurs through arachnoid villi and arachnoid granulations, which project into the dural sinuses.

6) From there, it’s reabsorbed back into the blood and lymphatic system.

Gray matter (cell bodies and dendrites) forms…

the cortex and deep nuclei

white matter (myelinated axons) is…

deep forming tracts

The adult brain has ___ neurons.

100 billion

the Brain weighs about…

1.5 kg (3 to 3.5 pounds).

the Brain receives __ of the total blood flow to the body per

minute

15%

research has shown that neurogenesis…

does occur in specific regions, even in adults. One of these is the subventricular zone of the lateral ventricles, where neural stem cells generate new cells that migrate to the striatum. These new cells may help regulate motor control and cognitive functions.

Another key site is the subgranular zone of the hippocampus, a region critical for learning and memory. Here, new interneurons are formed, which may support memory formation and cognitive flexibility.

So while neurogenesis is limited, it plays a powerful role inbrain plasticityand gives us hope for developing treatments for injury and neurodegenerative disease.

cerebrum

the largest and most complex part of the brain. It’s derived from the telencephalon during development and makes up about 80% of the brain’s mass. The cerebrum is where higher mental functions happen—things like reasoning, problem solving, memory, and voluntary movement. Structurally, it’s divided into a right and left hemisphere, and these two halves communicate via a thick bundle of axons called the corpus callosum.

corpus callosum

a large bundle of nerve fibers that connects the left and right cerebral hemispheres, allowing them to communicate. Its primary function is to transmit motor, sensory, and cognitive information between the two halves of the brain, which is essential for coordinating complex behaviors, integrating sensory input, and supporting higher cognitive functions like language and memory.  

Contralateral Control in the Brain

Contralateral control refers to the arrangement where one side of the brain controls the opposite side of the body. In other words, the left hemisphere of the brain controls the right side of the body, and vice versa. 

cerebellum control:

ipsilateral describes how the cerebellum controls movements on the same side of the body; for example, the right cerebellar hemisphere controls the right side of the body.

cerebral cortex

the outer layer of the cerebrum. It’s only about 2 to 4 millimeters thick and is made up of gray matter, which contains neuron cell bodies. Beneath this is white matter, where myelinated axons run. The surface of the cortex is highly folded to increase surface area. The raised folds are called gyri, and the grooves between them are called sulci. Together, these folds and grooves are referred to as convolutions.

Each hemisphere is divided by deep sulci or fissures into 5 lobes - Frontal, Parietal, Temporal, Occipital, Insula

fissures

deep sulci, the largest, deepest grooves or divisions on the surface of the brain that divide the cerebral hemispheres and separate major lobes

convolutions

both sulci and gyri, the folds and ridges that cover the surface of the cerebral cortex, the outer layer of the brain

sulci

grooves or depressions on the surface of the brain, between gyri

gyri

a ridge or raised folds on the surface of the brain's cerebral cortex, which is typically surrounded by grooves called sulci

white matter

the brain's "superhighway" of nerve fibers (axons) covered in a fatty substance called myelin, which gives it its white appearance. Its primary function is to transmit electrical signals between different areas of the brain and the rest of the central nervous system

gray matter

the "thinking part" of the central nervous system, composed of neuronal cell bodies, which processes information, controls motor movements, and receives sensory input

Each cerebral hemisphere is divided into five lobes:

• The frontal lobe is involved in motor control and executive functions.

• The parietal lobe processes sensory input.

• The temporal lobe handles auditory information and memory.

• The occipital lobe is responsible for vision.

• And the insula, which is not visible on the surface, plays a role in taste, visceral sensation, and integrating emotional responses.

The insula

hidden beneath the other lobes, is involved in visceral sensory integration and emotional memory. It receives information related to taste, smell, pain, and internal organ function—and helps mediate autonomic responses like heart rate and digestion.

The occipital lobe is responsible…

for vision, is the brain’s primary visual center, important for interpreting what we see and coordinating eye movement.

The temporal lobe…

handles auditory processing and memory, and plays a role in interpreting both sound and visual information.

The frontal lobe is involved…

motor control and executive functions

The parietal lobe..

processes sensory input.

The precentral gyrus

in the frontal lobe, contains the primary motor cortex

primary motor cortex

located in the pre central gyrus, contains upper motor neurons responsible for voluntary muscle movement throughout the body

somatosensory cortex

located in the parietal lobe/ the postcentral gyrus, processes somesthetic sensation—input from skin, joints, muscles, and tendons, like touch, pressure, and proprioception

The auditory cortex

in the temporal lobe processes sound

the visual cortex

in the occipital lobe helps interpret visual information.

central sulcus

separates the frontal and parietal lobes

upper motor neurons

responsible for voluntary muscle movement throughout the body, located in the primary motor cortex

sensory and motor homunculus

maps of the body in the brain that represent the proportional amount of brain tissue dedicated to sensory processing and motor control for different body parts. The sensory homunculus is in the postcentral gyrus of the parietal lobe, mapping sensations, while the motor homunculus is in the precentral gyrus of the frontal lobe, mapping motor commands.

Body regions that require fine motor control or detailed sensory input, like the hands, lips, and face, take up disproportionately large areas of cortex in both the precentral (motor) and postcentral (sensory) gyri.

the motor homunculus

is in the precentral gyrus of the frontal lobe, mapping motor commands.

The sensory homunculus

is in the postcentral gyrus of the parietal lobe, mapping sensations

mirror neurons

cells found in the frontal and parietal lobes. These neurons fire when we perform a goal-directed action, but also when we observe someone else doing the same thing. This allows us to understand and imitate others’ behavior. Mirror neurons are connected via the insula and cingulate gyrus to emotional centers of the brain, linking what we see to how we feel. They’re thought to play a role in developing social skills and language, and disruptions in this system (ex. having fewer or a genetic difference) have been implicated in autism spectrum disorders.

CT scanning

uses X-rays to create detailed images of soft tissue structures, helping to detect tumors, injuries, or bleeding.

PET scanning

positron emission tomography, involves injecting radioactive tracer molecules into the blood, which emit gamma rays in metabolically active tissues. This is useful for cancer staging and monitoring response to cancer treatment, but also for studying brain activity, including how metabolism changes in depression and other psychiatric conditions.

MRI

magnetic resonance imaging, which uses powerful magnets to align protons in the body’s tissues. Because different tissues contain different amounts of water, they show up with distinct contrast based on proton alignment. This makes MRI ideal for seeing the difference between gray matter, white matter, and cerebrospinal fluid. Using contrast agents can further enhance these images.

fMRI

functional MRI, which builds on traditional MRI to show brain activity in real time. It does this by detecting changes in blood flow—specifically, how active neurons cause nearby vessels to dilate via glutamate release. Active areas receive more oxygen-rich blood, and this change is detected as a BOLD signal—short for blood oxygenation level dependent contrast. fMRI is used extensively in neuroscience research, clinical diagnosis, and planning for brain surgery.

magnetoencephalography (MEG)

measures the magnetic fields generated by postsynaptic currents in the brain. It uses highly sensitive detectors called SQUIDs—that stands for superconducting quantum interference devices. MEG is particularly useful for surgical planning, like locating seizure foci in epilepsy or identifying regions to avoid during tumor removal. Compared to EEG, MEG provides higher spatial accuracy, especially for detecting activity deep within the brain.

Delta waves

are the slowest and are normal during deep sleep, but if seen in awake adults, they may suggest brain damage, occur at lowest frequency.

Theta waves

show up during sleep but can also appear when awake under stress or during memory tasks, especially in the occipital and temporal lobes

Beta waves

are faster, seen during mental activity and visual stimulation, most prominent in the frontal lobe, occur at highest frequency

Alpha waves

occur in a relaxed, awake state—strongest in the parietal and occipital lobes

electroencephalography (EEG)

uses electrodes on the scalp to detect electrical activity from neurons in the cerebral cortex. There are four classic wave patterns:

• Alpha waves occur in a relaxed, awake state—strongest in the parietal and occipital lobes.

• Beta waves are faster, seen during mental activity and visual stimulation, most prominent in the frontal lobe, occur at highest frequency.

• Theta waves show up during sleep but can also appear when awake under stress or during memory tasks, especially in the occipital and temporal lobes.

• Delta waves are the slowest and are normal during deep sleep, but if seen in awake adults, they may suggest brain damage, occur at lowest frequency.

Sleep

a fundamental biological process that appears to be genetically influenced, though environmental factors like light exposure and stress also play a major role. Several neurotransmitters help regulate sleep and wakefulness. For example, histamine and other excitatory neurotransmitters promote wakefulness, while adenosine and GABA promote sleep. Adenosine builds up during the day and helps create sleep pressure, and GABA quiets neural activity.

Sleep occurs in two major forms:

REM and non-REM

EEG- REM sleep

rapid eye movement sleep, is when most dreaming occurs. During REM, EEG readings show theta waves, and the brain becomes highly active even as muscles remain paralyzed.

EEG- Non-REM sleep

also called resting sleep and is divided into four stages based on EEG wave patterns. Stages 3 and 4 are known as slow-wave sleep and are marked by delta waves. This deep sleep is crucial for physical recovery, memory consolidation, and immune function

Non-REM sleep- Stages 3 and 4

known as slow-wave sleep and are marked by delta waves

Sleep pattern:

1) When we first fall asleep, we enter non-REM sleep and progress through its four stages, moving from light to deep sleep.

2) After that, we reverse direction—ascending back through the stages until we enter REM sleep, or rapid eye movement sleep.

3) This sleep cycle repeats every 90 minutes, and most people experience about five full cycles per night.

4) Interestingly, if someone wakes up naturally without an alarm, it usually happens during REM sleep, when brain activity is high and arousal is easier.

5) Slow-wave sleep—the deepest stage of non-REM—is most prominent in the first half of the night, while REM sleep dominates the second half.

Slow-wave sleep

the deepest stage of non-REM—is most prominent in the first half of the night, while REM sleep dominates the second half.

if someone wakes up naturally without an alarm, it usually happens during…

REM sleep, when brain activity is high and arousal is easier.

This sleep cycle repeats…

every 90 minutes, and most people experience about five full cycles per night.

When we first fall asleep, we enter…

we enter non-REM sleep and progress through its four stages, moving from light to deep sleep.

After that, we reverse direction—ascending back through the stages until we enter REM sleep, or rapid eye movement sleep.

During REM sleep:

some brain areas are more active than when we’re awake, especially the limbic system, which controls emotion. Breathing and heart rate become irregular, and this is the stage where dreaming is most vivid. REM sleep is also important for consolidating non-declarative memories, such as skills and habits—meaning that even while asleep, our brains are hard at work making connections

During Non-REM sleep:

As we drift into sleep, neurons reduce their firing rates, leading to decreased blood flow and energy use. Breathing and heart rate become very regular, which helps create a stable environment for recovery. This phase may allow cells to repair metabolic damage, especially from oxidative stress caused by free radicals. It also supports neuroplasticity, the brain’s ability to reorganize and strengthen connections—and is particularly important for consolidating spatial and declarative memories, like facts and locations.

Other benefits of sleep:

research shows that waiting about three hours after learning before sleeping improves how well declarative memories are stored. And importantly, sleep may play a role in preventing neurodegenerative disease. During deep sleep, the glymphatic system—the brain’s waste clearance pathway—removes abnormal proteins like those seen in Alzheimer’s disease. Unfortunately, this system becomes less efficient with age, which may help explain cognitive decline in older adults.

basal nuclei

also known as the basal ganglia, are clusters of gray matter deep within the cerebral white matter. Their primary role is to help regulate voluntary motor control, acting as a sort of "filter" that helps initiate and fine-tune purposeful movement while suppressing unwanted motion.

corpus striatum

The most prominent structure in the basal nuclei, which includes two main components: the caudate nucleus and the lentiform nucleus

lentiform nucleus

part of the corpus striatum, subdivided into the putamen and the globus pallidus

subthalamic nucleus

part of the diencephalon and the substantia nigra of the midbrain, which provides dopaminergic input to the corpus striatum. This dopaminergic connection is vital for normal function—when neurons in the substantia nigra degenerate, as in Parkinson's disease, dopamine levels drop, leading to symptoms like tremors and muscle rigidity.

Parkinson's disease

a chronic, progressive neurological disorder that affects movement, balance, and coordination. It is caused by the gradual loss of dopamine-producing neurons in a region of the brain called the substantia nigra. 

Huntington’s disease

due to a genetic mutation that causes excessive glutamate and destruction of GABAergic neurons in the caudate and putamen, leading to involuntary, jerky movements called chorea.

Motor circuit:

It begins when neurons from the motor cortex in the frontal lobe send excitatory glutamate to the putamen. The putamen, in turn, sends inhibitory GABA signals to other basal nuclei regions. One important output is from the globus pallidus, which sends inhibitory signals to the thalamus. The thalamus then sends excitatory signals back to the motor cortex.

This loop helps to facilitate appropriate voluntary movements while inhibiting conflicting or unnecessary movements. When this circuit is disrupted—either by loss of dopamine, genetic damage, or neurodegeneration—movement becomes uncoordinated or excessive.

Motor circuit: motor cortex

in the frontal lobe send excitatory glutamate to the putamen.

Motor circuit: thalamus

sends excitatory signals back to the motor cortex.

Motor circuit: globus pallidus

sends inhibitory signals to the thalamus.

Motor circuit: putamen

receives excitatory glutamate from motor cortex in frontal lobe, the putamen, in turn, sends inhibitory GABA signals to other basal nuclei regions.

Cerebral lateralization:

Each side of the brain tends to control the opposite side of the body—a concept called contralateral control. So, the right precentral gyrus directs movements on the left side of the body, and vice versa, due to the crossing of motor fibers. Likewise, somatosensory signals from one side of the body are processed on the opposite side of the postcentral gyrus.

epilepsy- corpus callosum

In rare cases of severe epilepsy, this connection may be surgically severed to reduce seizure activity.

The right hemisphere is dominant for:

visuospatial skills, like recognizing faces, reading maps, or understanding part-whole relationships. It’s also involved in music composition and pattern recognition.

The left hemisphere is dominates in:

Language, analytical ability, speech, writing, calculations, understand music, describe visual appearance

Broca’s area

located in the left inferior frontal gyrus in the frontal lobe, is responsible for the motor aspects of speech.

Broca’s aphasia

damage to Broca’s area, as called nonfluent aphasia. Patients with this condition understand language but struggle to produce speech fluently. Their speech is slow and poorly articulated, although other motor actions like moving the lips or tongue remain unaffected.

Wernicke’s area

located in the superior temporal gyrus, in the temporal and parietal lobes, and is crucial for language comprehension.

Wernicke’s aphasia

Damage to Wernicke’s area, where speech becomes a meaningless jumble—often called word salad. These individuals speak fluently but without meaningful content and have impaired understanding.

When we speak, information about word meaning starts in…

Wernicke’s area and travels to Broca’s area via the arcuate fasciculus. Broca’s area then activates the motor cortex to produce speech.