Midbrain Anatomy and Function

Midbrain

• Part of the brainstem connecting the pons and cerebellum to the forebrain
• The shortest part of the brainstem
• Contains relay nuclei for processing auditory and visual information
• Houses the nuclei of oculomotor, trochlear, and trigeminal cranial nerves, controlling eye movement and facial sensation
• Provides a passageway for pathways between the cerebral cortex and spinal cord

Parts of the Midbrain

• Consists of the tectum and tegmentum

Tectum

• Responsible for auditory and visual processing
• Contains the superior and inferior colliculi nuclei

Tegmentum

• The larger part of the midbrain
• Responsible for coordinating movement, pain processing, and alertness

Corpora Quadrigemina

• Inferior colliculi are involved in the preliminary processing of auditory stimuli, working with the cerebellum
• Superior colliculi are involved in the preliminary processing of visual stimuli, working with the cerebellum

Substantia Nigra

• Contains nerve cells that make dopamine, a neurotransmitter
• Serves as a relay station for nerve signals controlling movement
• Parkinson's Disease: Degeneration occurs that lowers dopamine levels that reach the basal ganglia, resulting in tremors and rigid movements

Basal Ganglia

• A region at the base of the brain consisting of the caudate nucleus, putamen, and globus pallidus
• Responsible for involuntary movements, such as tremors

Cerebral Aqueduct

• The Aqueduct of Sylvius or the Sylvian Aqueduct are other names
• Location causes some to refer to it as the mesencephalic duct or the aqueductus mesencephalic
• Function allows cerebrospinal fluid to flow between the third and fourth ventricles
• Cerebrospinal fluid cushions the brain, protecting it from harm

Cerebral Peduncles

• Located on either side, on the front of the midbrain
• Connect the rest of the midbrain with the thalamic nuclei and the cerebrum

Lateralization of Hemispheres of the Brain

• Functional dominance of one hemisphere over the other
• One hemisphere is more or entirely responsible for controlling a function compared to the other

Split Brain

• Also known as callosal syndrome
• A type of disconnection syndrome where the corpus callosum, connecting the two hemispheres, is severed

Roger Sperry’s Split Brain Experiments (1959–1968)

• Sperry severed the corpus callosum in cats and monkeys to study each side of the brain
• If the hemispheres were not connected, they functioned independently, which he called a split-brain
• Split brains enabled animals to memorize double the information

Spinal Cord

• A complex cable of nerves
• Conducts impulses from peripheral nerves to the brain and vice versa, causing reflex movements

Count of Spinal Nerves from the Spinal Cords

• 8 pairs of cervical nerves
• 12 pairs of thoracic nerves
• 5 pairs of lumbar nerves
• 5 pairs of sacral nerves
• 1 pair of coccygeal nerves

Meninges

• Connective tissue membranes that protect the CNS, brain, and spinal cord
• Houses the main arteries and veins supplying the brain

Three Layers of the Meninges

• Dura mater: The outermost membrane
• Arachnoid mater: Lies under the dura mater
• Pia mater: A delicate layer of connective tissue that contours the brain, dipping into the sulci of the cerebral and cerebellar cortices

Peripheral Nervous System

• Connects the Central Nervous System to the organs, limbs, and skin
• Nerves connect all parts of the body with the brain and the spinal cord
• Division of the nervous system enables the CNS's control

Three Primary Tasks of the Peripheral Nervous System

• Transmits motor commands to all voluntary striated muscles
• Delivers sensory information about the external world and the body to the brain and spinal cord
• Regulates autonomic activities, including blood pressure and perspiration

Divisions of the Peripheral Nervous System

• Consists of the sensory division and motor division

Sensory Division

• Sensory receptors detect internal stimuli, such as increased blood pressure, or external stimuli, such as a raindrop landing on your arm
• Sensory information is carried into the brain and spinal cord through cranial and spinal nerves

Motor Division

• Once sensory information is integrated in the central nervous system, the central nervous system may elicit a motor response by activating muscles and glands through cranial and spinal nerves
• Stimulation of the effectors causes muscles to contract and glands to secrete

Cranial Nerves

• There are 12 pairs
• Connect the brain to different parts of the head, neck, and trunk

Motor Division

• Somatic Nervous System enables the body to move
• Information from the brain regarding movement travels down the spine into the peripheral nerves

Neurological Disorders

• Medically defined as disorders affecting the brain, nerves throughout the body, and the spinal cord

Acute Spinal Cord Injury

• Traumatic injury that bruises, partially tears, or completely tears the spinal cord
• A common cause of disability and death in children and adults

Causes of Acute Spinal Cord Injury

• Falls
• Motor vehicle accidents
• Sports or diving injuries
• Trampoline accidents
• Violence (gunshot/stab wounds)
• Infections
• Birth injuries in the neck area

Symptoms of Acute Spinal Cord Injury

• Muscle weakness
• Loss of voluntary muscle movement in the chest, arms, or legs
• Breathing problems
• Loss of feeling in the chest, arms, or legs
• Loss of bowel and bladder function

Alzheimer’s Disease

• A progressive, neurodegenerative disease that occurs when nerve cells die
• The disease causes impaired memory, thinking, and behavior, confusion, restlessness, personality changes, impaired judgment and communication, inability to follow directions, language deterioration, impaired thought processes involving visual and spatial awareness, and emotional apathy

Bell’s Palsy

• An unexplained episode of facial muscle weakness or paralysis that worsens over 48 hours
• Results from damage to the 7th cranial nerve
• Causes pain and discomfort on one side of the face or head
• Can strike anyone at any age, but occurs most often in pregnant women, and people who have diabetes, influenza, a cold, or another upper respiratory ailment

Headache

• Pain or discomfort in the head or face
• Varies greatly in location, intensity, and frequency
• Brain tissue doesn't have pain-sensitive nerve fibers, but other parts of the head can be responsible for a headache, including a network of nerves over the scalp, certain nerves in the face, mouth, and throat, muscles of the head, neck, and shoulders, and blood vessels along the surface and at the base of the brain

Migraine

• Headaches characterized by nausea, vomiting, lightheadedness, sensitivity to light (photophobia), and other visual symptoms, often with distinct phases

Tension Headaches

• Stress and tight muscles are often factors
• Slow onset, head usually hurts on both sides, pain is dull or feels like a band or vice around the head, mild to moderate pain affecting the back part of the head or neck
• Do not typically cause nausea, vomiting, or sensitivity to light (photophobia).

Cluster Headaches

• Cause severe pain on one side of the head behind one eye, redness and watering of the affected eye with a droopy lid and small pupil, swelling of the eyelid, runny nose or congestion, and swelling of the forehead

Hydrocephalus

• A condition caused by an abnormal accumulation of cerebrospinal fluid within the ventricles of the brain
• CSF surrounds the brain and spinal cord
• When the circulatory path of the CSF is blocked, fluid accumulates, enlarging the ventricles and increasing pressure inside the head

Types of Hydrocephalus

• Communicating hydrocephalus occurs when CSF flow is blocked after exiting the ventricles
• Non-communicating hydrocephalus (obstructive hydrocephalus) occurs when CSF flow is blocked along one or more narrow passages connecting the ventricles
• Normal pressure hydrocephalus (NPH) is a communicating form that most commonly affects the elderly
• Congenital hydrocephalus is congenital and may be caused by events or influences during fetal development, or genetic abnormalities
• Acquired hydrocephalus develops at the time of birth or afterward
• Symptoms include gait disturbance with difficulty walking, mild dementia, and impaired bladder control

Epilepsy

• A neurological condition involving the brain that makes people more susceptible to having recurrent seizures
• One of the most common disorders of the nervous system, affecting people of all ages, races, and ethnic backgrounds

Huntington Disease

• Inherited disorder causing nerve cells (neurons) in parts of the brain to gradually break down and die, attacking areas that control voluntary (intentional) movement, as well as other areas
• People develop uncontrollable dance-like movements (chorea) and abnormal body postures, as well as problems with behavior, emotion, thinking, and personality

Parkinson’s Disease

• Progressive disorder affecting the motor system, resulting in shaking, rigidity, slowness, and difficulty walking

Multiple Sclerosis (MS)

• A condition affecting the brain and spinal cord
• Causes a range of potential symptoms, including problems with vision, arm or leg movement, sensation, or balance
• AUTOIMMUNE: the immune system attacks part of the body as if it were a foreign substance; attacks the myelin sheath of CNS axons

Encephalitis

• A serious condition affecting the brain that requires prompt treatment to lower the risk of lasting complications or death
• Symptoms include headache, sensitivity to light, stiff neck, mental confusion, and seizures
• Timely diagnosis and treatment are key to recovery from encephalitis
• Viral infection or an abnormal immune system response are causes

Brain Damage

• Understanding a healthy brain and the development of new treatments are the goals

Causes of Brain Damage

• May include brain tumors, cerebrovascular disorders, strokes, closed-head injury, open-head injury, infections of the brain, neurotoxins, genetic factors, programmed cell death, and neuropsychological diseases

Brain Tumors

• A tumor or neoplasm is a mass of cells that grows independently of the rest of the body
• Originates in the brain itself, tissues close to it like meninges, cranial nerves, and pituitary or pineal gland
• Tumors begin when normal cells acquire errors (mutations) in their DNA

Types of Tumors

• Meningiomas arise from the meninges (most are noncancerous)
• Acoustic neuromas (schwannomas) are benign tumors that develop on the nerves that control balance and hearing
• Infiltrating tumors grow diffusely through surrounding tissue, making removal/destruction difficult, with the remaining tissue after surgery
• Metastatic tumors result from cancer elsewhere in the body that spreads (metastasizes) to the brain

Cerebrovascular Disorders

• Sudden-onset cerebrovascular disorders that cause brain damage
• Occur when the brain's blood supply is interrupted or reduced

Stroke

• Deprives the brain of oxygen and nutrients, leading to the death of brain cells

Cerebral Hemorrhage

• Occurs when a cerebral blood vessel ruptures and blood seeps into the surrounding neural tissue, damaging it
• An aneurysm, a pathological balloon-like dilation that forms in the wall of an artery, is a common cause

Cerebral Ischemia

• A condition in which a blockage in an artery restricts the delivery of oxygen rich blood to the brain
• Main causes include thrombosis with a thrombus being formed and blocking blood flow, embolism with an embolus being a traveling thrombus, or arteriosclerosis where walls of blood vessels thicken and the channels narrow

Closed-Head Injury

• Brain injuries produced by blows that do not penetrate the skull
• Such damage can result in contusions, which produce internal hemorrhaging thus resulting in a hematoma affecting the cerebral circulatory systems
• Contrecoup injuries occur when the blow causes the brain to strike the inside of the skull on the other side of the head

Concussion

• A traumatic brain injury affecting brain function and typically caused by a blow to the head
• Symptoms including a loss of consciousness lasting longer than 30 seconds or a headache that worsens over time

Open-Head Injury

• A head injury in which the dura mater and the outer layer of the meninges are breached
• Focal which means that they affect a specific area of brain tissue

Encephalitis

• Inflammation of the brain
• There are several causes, but the most common is a viral infection
• Mild symptoms include headache, fever, aches in muscles or joints, and fatigue or weakness
• Severe symptoms include confusion, agitation, hallucinations, seizures, loss of sensation or paralysis in certain areas of the face or body, muscle weakness, and problems with speech or hearing

Brain Infections

• Bacterial infections/cerebral abscesses are pockets of pus formed when bacteria infect the brain
• SYPHILIS: bacteria that are passed from infected to non-infected individuals through contact with genital sores

General Paresis

• A syndrome of mental illness and dementia resulting from a syphilitic infection
• Symptoms include fatigue, headaches, and insomnia
• Disease progression is related to and defined by mental deterioration and personality changes, loss of social inhibitions, gradual impairment of judgment, concentration and short-term memory, euphoria, mania, depression, or apathy.

Viral Infections

• Viruses are tinier, the largest of them are smaller than the smallest bacteria
• Viruses have a protein coat and a core of genetic material, either RNA or DNA

Rabies

• Classic example of a virus that has a particular affinity for the nervous system
• Its effects on the brain almost always lethal for example dogs, cats, raccoons, bats, and mice

Mumps and Herpes Viruses

• Common examples of viruses that can attack the nervous system but typically attack other tissues of the body
• Several viruses can lead to an infection of the brain, spinal cord, or surrounding area
• Possible causes include varicella zoster virus, cytomegalovirus, epstein-Barr virus, and influenza viruses

Neurotoxins

• The nervous system can be damaged by exposure to toxic chemicals entering general circulation from the gastrointestinal tract, the lungs, or the skin
• For example, heavy metals such as mercury and lead
• Some are endogenous (produced by the body)

Endogenous Neurotoxins

• Excessive release of glutamate is caused by aspartame (sugar-free foods), monosodium glutamate as well as Sucralose (Splenda), aluminum, and fluoride in drinking water

Drug-Induced Psychosis

• A chronic mental illness in the form of psychosis that is attributed to substance use
• It is a psychosis is a result from the effects of chemicals or drugs
• Sometimes, treatments for neurological or psychiatric disorders prove to be toxic

Symptoms of Drug-Induced Psychosis

• Difficulty concentrating
• Depressed mood, sleeping too much or not enough
• Anxiety
• Suspiciousness
• Involuntary smacking and sucking movements of the lips, thrusting and rolling of the tongue, and lateral jaw movements

Genetic Factors

• Chromosomes consist of 23 pairs of normal human cells in the body, however an accident during cell division can occur which may result in an abnormal amount of chromosomes

Abnormal Recessive Gene

• Passed from the the parents to the offspring- for example phenylketonuria (PKU)
• PKU leads to a toxic build-up of the amino acid phenylalanine that is toxic to the nervous system and causes intellectual disabilities if left untreated

Genetic Accident

• Causes an extra chromosome 21 created in the egg during ovulation which causes Down Syndrome

Programmed Cell Death

Apoptosis

• Came from Latin origin word meaning to fall off
• Genetic programs neurons and other types of cells have that allows them to destroy themselves
• Eliminates extra neurons in early development but also plays a role, in brain damage
• Eliminates unwanted cells during early development
• In adults, apoptosis removes cells that have been damaged beyond repair