1. Telencephalon - The most anterior, superior portion of the central nervous system. It includes the cerebrum and the basal nuclei.
   1. Cerebrum
      • Function: It is responsible for much of the conscious thought process, judgment, intelligence, memory storage and retrieval, attention, awareness, sensations, language, and voluntary and involuntary regulation of somatic motor patterns.
      • Communication: It consists of two hemispheres which communicate with one another through a band of 200-250 million neurons known as the corpus callosum.
      • Blood Flow: The internal carotid arteries enter the cerebral cortex and, once they penetrate the dura, divide into the anterior and middle cerebral arteries. The posterior cerebral arteries, arising from the basilar artery, also supply blood to the cerebral cortex.
      • Each hemisphere is subdivided into four lobes:
        1. Frontal lobe
           • Function: It is involved in voluntary motor function, cognitive function and expressive language.
             • The frontal lobe contains more dopamine receptors than any other cerebral lobe. Dopamine is associated with reward, attention, long-term memory, planning and drive.
             • Broca's area - helps mediate speech.
           • Dysfunction
             • Damage to Broca's area from stroke or other injury can cause patients to be unable to verbalize their thoughts.
             • Damage to the posterior frontal lobe, which is involved in voluntary movement, can cause weakness or paralysis.
             • Damage to the middle frontal lobe can impair movements of the eyes.
             • Damage to the anterior frontal cortex can cause: impaired concentration, reduced fluency of speech, apathy, inattentiveness, delayed response to questions, or socially inappropriate behavior.
             • A reduction of dopamine activity in the prefrontal cortex is related to poorer performance and inefficient functioning of memory tasks.
        2. Parietal lobe
           • Function: It is primarily involved with sensory functions such as: awareness of body parts, interpretation of touch, pressure, and pain, and recognition of object size, shape and texture.
             • It is involved in self-preservative behavior such as recall of pleasurable or unpleasurable and potentially dangerous events.
             • It modifies mood and emotional response to perceived events and responsively adjusts heart rate and respiration.
             • It contains the rhinencephalon (a.k.a., limbic lobe) which is involved in interpretation of smells.
           • Dysfunction
             • Optic Apraxia - the inability to voluntarily guide eye movements.
             • Optic Ataxia - incoordination of hand and eye movements.
             • Stimultanagnosia - the inability to perceive more than one object at one time, even if two objects are in the same place.
             • Gerstmann syndrome - the result of lesions on the dominant posterior parietal lobe near the temporal and parietal lobe border. The syndrome will cause: deficiency in the ability to write, difficulty in learning or comprehending math, the inability to distinguish the fingers on the hand, and left-right disorientation.
             • Patients with right parietal lesions act as if the objects in the neglected visual field do not exist and have difficulty making eye movements into that field.
             • Some individuals may become confused, delirious, or unable do ordinary tasks (e.g., dressing themselves).
        3. Temporal lobe
           • Function: It is primarily involved with hearing, speech, behavior and memory.
             • Wernicke's area - involved in language comprehension.
           • Dysfunction
             • Patients with damage to Wernicke's area or the left temporal lobe will be able to hear speech but not comprehend its meaning.
             • Blunt traumas to the skull can easily contuse or lacerate the temporal lobes by pressing the hard and irregular surface of the sphenoid wings toward the brain.
             • Right temporal lobe damage tends to cause impairment of memory for sounds and shapes.
             • Dominant left temporal lobe damage often causes personality changes including: humorlessness, extreme religiosity, and loss of libido.
        4. Occipital lobe
           • Function: It is primarily responsible for interpretation of visual stimuli
           • Dysfunction:
             • Damage to both of the occipital lobes will cause cortical blindness; although the eyes are still functioning, the lobes will be unable to process the information
             • Damage to the anterior frontal lobe can impair the ability to recognize familiar objects and faces.
   2. Basal Nuclei (Basal Ganglia)
      • Function: They are directly involved in regulation of involuntary motor functions and indirectly involved in voluntary movements. Several regions are involved in emotions, cognition, and learning.
        • Amygdalae - considered part of the basal nuclei. They are partially responsible for social behavior and expression of emotion.
          • When stimulated, they produce feelings of fear and apprehension.
          • They play a primary role in the formation and storage of memories associated with emotional events.
          • They initiate autonomic nervous system mediation of certain emotional responses, including the "fight or flight response" (i.e. immobility, tachycardia, increased respiration and cortisol release).
        • Hippocampal formations (a.k.a., hippocampi) - involved in navigation and the shifting of short-term memory to long-term memory.
      • Communication: Unlike most areas of the brain, the basal nuclei do not directly communicate with the spinal cord; they send signals to the brainstem via the thalami and then back to the frontal motor cortices. Communication with these regions helps to coordinate fluid involuntary and voluntary movements and may also contribute to conscious awareness of those movements.
      • The basal ganglia group includes the: lenticular nuclei, neostriata, striata and corpora striata, pallida and paleostriata. The substantiae nigrae and subthalamic nuclei are located in the mesencephalon, but contain tracts within the basal ganglia. All these structures work together to coordinate motor functions within the motor cortices, cerebellum and reticular formations.
      • Blood flow: Branches of the middle cerebral arteries, called lenticulostriate branches, primarily supply the basal nuclei. They are frequently sites of cerebral hemorrhage in individuals with uncontrolled hypertension.
      • Dysfunction
        • Damage or dysfunction of these regions can lead to tremor, twitches, rigid muscle tone, slowness of movement without paralysis, and behavioral disturbances.
        • Damage to the amygdalae can result in loss of the fear response, hypoemotionality, or the inability to discriminate differences in facial expressions.
        • Damage to the hippocampi can result in the inability to retain memories.
2. Diencephalon - lies on either side of the third ventricle above the brain stem. The diencephalon is the primary link between the nervous system and the endocrine system.
   1. Thalami - sensory processing and relay centers.
      • Function: They are involved in pain awareness and aspects of attention. They also help regulate wakefulness and consciousness as part of the Reticular Activating System (RAS).
      • Communication: It connects every sensory system to appropriate regions of the cerebral cortex.
      • Blood flow: Branches of the posterior cerebral artery supply the thalamus.
      • Dysfunction
        • Stroke affecting the thalamus can result in contralateral hemianaesthesia, painful anaesthesia, and mood swings; bilateral damage can result in oculomotor troubles, akinetic mutism (i.e., a patient neither moves nor speaks), or coma.
   2. Hypothalamus
      • Function: It is involved in emotion, autonomic functions, and the production of hormones. It plays a role in regulation of temperature, appetite, water balance (via antidiuretic hormone), and certain psychobiological activities (e.g., sleep, menstruation, anger).
      • Blood flow: A collection of branches from the internal carotid (called the hypophyseal arteries), the basilar artery, and the cerebral arterial circle supply this region.
      • Dysfunction
        • Behavioral disturbances include: depression, hyperactivity, abnormal responses to stress, or sleep disorders.
        • Damage to the hypothalamus is usually fatal because it affects the body's ability to maintain a consistent temperature and blood chemistry.
        • Bilateral lesions on the lateral ventromedial nucleus can cause complete cessation of food intake.
        • Bilateral lesions on the medial ventromedial nucleus can cause hyperphagia and obesity.
        • Kallmann syndrome - a deficiency of hypothalamic hormones that produces hypogonadism (lowered function of sexual hormones) and congenital anosmia (inability to smell).
        • A variety of other hormone, immune, or autonomic dysfunctions have been linked to hypothalamic dysfunction.
        • It may be involved in fibromyalgia.
   3. Reticular formation
      • Function: It influences the level of consciousness and the sleep/wake cycle. It is also important in the regulation of respiration rate, heart rate and gastrointestinal activity.
      • Blood flow: Both pontine and medullary regions are supplied by branches of the basilar artery. They are also supplied by the superior cerebellar arteries (pontine region) and the vertebral arteries (medullary region).
      • Dysfunction:
        • Lesions in this region can cause coma or death.
        • Abnormal activity in this region has been observed in patients with chronic fatigue syndrome.
3. Mesencephalon (a.k.a., midbrain)
   • Function: processes visual and auditory data, helps maintain consciousness, conveys motor information, and initiates signals for several visual and somatic reflexes (e.g., eye movement, arm swinging).
     • Composed of: Corpora quadrigemini, tegmentum, ventricular mesocoelia, and cerebral peduncles.
     • The red nuclei and substantiae nigrae are visible in a cross-sectional view of the mesencephalon.
     • Cranial nerves III and IV originate from this brain region.
   • Blood flow: The basilar artery , posterior cerebral arteries , and superior cerebellar artery supply this region.
   • Dysfunction:
     • Damage to the substantia nigra impairs dopamine secretion and causes overactivity of the cerebral nuclei. Symptoms of this dysfunction include increased muscle tone and difficulty starting voluntary movements without intense concentration.
     • The Edinger-Westphal nucleus in the midbrain receives light information from the retina via cranial nerve II and produces a pupillary motor response via cranial nerve III. Impaired pupillary light reflex suggests tentorial herniation of the midbrain, stroke, or impairment of these cranial nerves.
4. Metencephalon - a portion of the hindbrain consisting of the cerebellum and pons.
   1. Cerebellum
      • Function: coordinates the timing and intensity of complex motor patterns by integrating and directing information from numerous sensory and motor regions of the brain and spinal cord.
      • Blood flow: The superior cerebellar artery (SCA), anterior inferior cerebellar artery (AICA), and posterior inferior cerebellar artery (PICA) supply this region.
      • Communication: Feedback from somatic portions of the brain and spinal cord allows the cerebellum to continuously inform the rest of the brain about the body's position in space to coordinate appropriate movements.
      • Dysfunction: Cerebellar injury can disrupt feedback and lead to postural and fine motor abnormalities including: disturbances in gait, inability to perform rapid repetitive movements, and intention tremors.
   2. Pons
      • Function: It relays sensory information between the cerebellum and thalamus and serves as an involuntary somatic and visceral motor center.
        • Portions of the pons control the transition of inspiration to exhalation by promoting or inhibiting inspiratory depth
      • It includes the origins of cranial nerves V, VI, VII and VIII.
      • Blood Flow: The pontine arteries (branches of the basilar artery), the anterior inferior cerebellar arteries, and the superior cerebellar arteries supply this region.
      • Dysfunction:
        • Central pontine myelinosis - the destruction of the myelin sheath in the pons often caused by rapid correction of hyponatremia. It can cause problems with balance, walking, sense of touch, vision, swallowing, or speaking. It can also lead to death or "locked-in" syndrome.
        • Pontine lesions have been related to circadian rhythm sleep disorders.
        • Medial inferior pons damage can affect the emerging sixth cranial nerve and result in ipsilateral abduction paralysis.
        • Damage to the lateral inferior pons can affect fibers of the seventh cranial nerve and result in ipsilateral facial paralysis, tinnitus, deafness, or taste loss on the ipsilateral half of the anterior tongue.
        • Damage to the lateral middle pons can affect the fifth cranial nerve and cause ipsilateral jaw weakness or facial numbness.
5. Myelencephalon - a portion of the hindbrain consisting of the medulla oblongata and several cranial nerves.
   1. Medulla oblongata
      • Function
        • It relays signals between the brain and spinal cord and regulates essential autonomic motor functions including:
          1. Blood pressure and heart rate
          2. Respiration rhythmaticity
          3. Swallowing, gagging, vomiting and sneezing
          4. Defecation
        • It contains the origins of the cranial nerves IX, X, XI and XII.
      • Blood flow: The anterior spinal artery , posterior inferior cerebellar artery, and the vertebral artery supply this region.
      • Dysfunction
        • Damage to the medulla can result in vomiting, impaired respiratory rate, and impaired sensory/motor functions.
        • Damage to the medial medulla can affect cranial nerve XII and cause ipsilateral hemiparalysis of the tongue.
        • Damage to the lateral medulla can affect cranial nerve IX or X, the descending tract of cranial nerve V, or other structures resulting in: dysphagia, hoarseness, ipsilateral paralysis of vocal cords, ipsilateral loss of pharyngeal reflex, vertigo, nystagmus, ipsilateral facial analgesia, or taste loss on the ipsilateral half of the posterior tongue.

The human brain uses about 20% of cardiac output at rest. Cerebral blood flow normally is about 60mL per 100g of brain tissue per minute. Ischemia below 10mL per 100g per minute will result in tissue death. Excess flow will increase the likelihood of developing or rupturing cerebral aneurysms.

The Circle of Willis is a set of communicating arteries that provide redundant blood supply to the brain; it is most commonly found to be a complete circle (~34% of cases). Two pairs of arteries supply the entire circle and its branches:

1. Internal carotid arteries
   • Supply: The internal carotids supply the anterior two thirds of the cerebrum, including the basal ganglia.
   • Ischemia: Signs of internal carotid ischemia include:
     • Paralysis and sensory deficits of the contralateral face, arms and legs
     • Aphasia of the dominant hemisphere
     • Apraxia, agnosia, and unilateral neglect of the non-dominant hemisphere.
     • Bruits in the carotid artery.
   • Branches: The common carotid artery branches terminate beneath the base of the frontal lobe. The internal carotids supply the brain up to the level of the optic nerves, where they divide into three branches:
     1. Ophthalmic artery - supplies the eyes and other structures in and around the orbit
     2. Anterior cerebral artery (ACA)
        • Supply: Its branches supply the medial and superior parts of the frontal lobes and anterior parietal lobes. This region contains the primary motor and somatosensory cortices of the leg and foot.
        • Ischemia: Proximal ACA ischemia can produce contralateral somatosensory dysfunction and cognitive deficits (e.g., flat affect, loss of spontaneity). Urinary incontinence may occur because the patient is unaware of the bladder filling or because the patient no longer inhibits the behavior.
     3. Middle cerebral artery
        • Supply: Its branches supply the mesencephalon and lateral surfaces of the cerebral hemispheres.
        • Ischemia: Symptoms include contralateral sensory weakness, paralysis, or neglect. If the dominant hemisphere is affected (where Wernicke's and Broca's areas are found), aphasia may occur.
   • About 85% of aneurysms occur in the anterior part of the Circle of Willis. The most common locations for an aneurysm are at the bifurcation of the internal carotid artery and the anterior communicating artery or the bifurcation of the internal carotid artery and posterior communicating artery.
2. Vertebral arteries
   • Supply: Its branches supply the posterior third of the cerebrum, the midbrain, the hindbrain, and the spinal cord.
   • Ischemia: Symptoms often include deficits in balance, gait, swallowing, breathing, and speaking. Ischemia of specific branches often causes syndromes with characteristic neurological deficits.
     • Aneurysms and blockages in the basilar artery and its branches are relatively rare.
   • Branches: The vertebral arteries branch from the subclavian arteries and ascend through foramina in the cervical vertebrae. They merge within the cranium near the ventral medulla and pons to become the basilar artery. The branches of the basilar artery include:
     1. Anterior spinal arteries
        • Supply: lower brainstem and spinal cord.
        • Ischemia: Occlusion affects the ventral two thirds of the spinal cord and results in weakness and loss of sensation below the level of injury.
     2. Posterior inferior cerebellar arteries
        • Supply: inferior pons and brainstem.
        • Ischemia: Occlusion will cause nausea, vomiting, dysphagia, dysarthia, horizontal nystagmus, ipsilateral Horner's syndrome, and loss of sensation on the contralateral side of trunk and limbs.
     3. Anterior inferior cerebellar arteries
        • Supply: middle pons and cerebellum.
        • Ischemia: Occlusion produces lateral inferior pontine syndrome; its symptoms include vertigo, nausea, vomiting, tinnitus and nystagmus. Occlusion also results in ipsilateral ataxia, nystagmus, Horner syndrome, and impaired pain and temperature sensation the contralateral trunk, limbs, or face.
     4. Superior cerebellar arteries
        • Supply: superior pons and cerebellum.
        • Ischemia: It occurs rarely in these arteries; it will result in coma at onset and tetraplegia.
     5. Posterior cerebral arteries
        • Supply: posterior temporal lobe, occipital lobe, and portions of the midbrain and forebrain (e.g., the thalamus).
        • Ischemia: Occlusion can produce a large variety of visual, sensory, motor, and cognitive deficits. Patients may, for example:
          1. lose the ability to identify specific categories of visual objects (e.g., faces, colors)
          2. develop intention tremors (similar to those in Parkinson's disease)
          3. lose a large variety of sensory and motor functions
          4. be unable to create new long-term memories
          5. hallucinate

The Circle of Willis is completed by two communicating arteries; they anastamose to encircle the infundibulum of the pituitary gland. They can receive blood from either the carotids or the verterbrals which decrease the chance of ischemic events.

1. Anterior communicating artery
   • Communication: It connects the left and right anterior cerebral artery.
   • Aneurysms: (~32% reported cases) They can cause vision problems by compressing or damaging the optic nerve and/or the optic chiasm.
2. Posterior communicating arteries
   • Communication: It connects the internal carotids to the posterior cerebral arteries.
   • Aneurysm: Even unruptured, they can compress the oculomotor nerve and impair contralateral eye movement.

The ventricles are an interconnected set of fluid-filled structures found in the brain.

• Lateral ventricles - two large bilateral ventricles in the telencephalon which communicate with the central third ventricle through the intraventricular foramina (a.k.a., foramina of Monro).
• Third ventricle - located in the center of the diencephalon in between the thalami. It communicates with the fourth ventricle through the cerebral aqueduct.
• Fourth ventricle - located on the posterior surface of the pons and the superior half of the medulla.

Cerebrospinal fluid (CSF) circulates through the ventricles, the subarachnoid space, and around the spinal cord. It supplies nutrients and provides a protective cushion for the central nervous system.

• Several neuroendocrine structures of the brain lie adjacent to the ventricular system (e.g., the pituitary gland, hypothalamus, and pineal gland; these structures have an incomplete blood-brain barrier which allows them to respond directly to a variety of different chemical/hormone concentrations in blood and bodily fluid.
• CSF is produced by ependymal cells in the choroid plexus. The choroid plexus lines the majority of the lateral ventricle and the top portion of the third ventricle.
• Production and absorption of CSF is tightly regulated. Typically, 500 mL/day of CSF is produced, but the ventricular system can only hold 135-150mL of CSF at one time. Arachnoid granulations absorb the excess and drain it into the superior sagittal sinus.

The CSF in an adult cranium holds a resting intracranial pressure (ICP) of 7-15 mmHg. Values outside of this range can be attributed to changes in cranial volume; there are a variety of causes for such changes (e.g., bleeding, tumors, lack of drainage, etc.). Increased ICP can impair cerebral perfusion and damage the brain.

• A patient with increased ICP should maintain a forward-facing head position; turning the patient's head to one side can obstruct the internal jugular blood return and further increase intracranial blood volume.
• An intracranial hemorrhage will often cause a midline shift that compresses the ventricles and reduces the brain's available CSF capacity. The increased ICP can lead to a brain herniation.
  • In some cases, the brain stem is pushed out the foramen magnum; the damage or compression this causes can result in a decreased respiratory drive and fatality.
• A decompressive craniectomy is typically performed when the cranial volume exceeds cranial capacity enough to threaten brain herniation.

ICP is most commonly measured by inserting a device directly into the intraventricular space. This procedure is called a ventriculostomy.

• A hole is drilled in the top frontal nondominant part of the skull (typically the right side of the head).
• A catheter is advanced through the nondominant frontal lobe ending in the anterior horn of the lateral ventricle.

Advantages

• Accurate ICP measurements
• Provides access to CSF for drainage or sampling
• Provides access for instillation of contrast
• Allows reliable evaluation of intracranial compliances

Disadvantages

• Most invasive and difficult approach
• Requires frequent transducer recalibration
• Highest risk of infection and catheter occlusion by blood or tissue