The Brain: Structure, Function, and Major Pathways (Chapter 14 Study Notes)

Brain Structure and Major Divisions

Cerebrum

The cerebrum is the largest part of the brain and is responsible for higher cognitive functions such as reasoning, planning, and voluntary movement. It is divided into two cerebral hemispheres, each containing folds called gyri and grooves called sulci.

• Gyri: Elevated ridges on the brain's surface.

• Sulci: Shallow grooves separating gyri.

• Longitudinal fissure: Deep groove separating right and left hemispheres.

Major Lobes and Functions:

• Frontal lobe: Voluntary motor functions, judgment, planning, memory, taste.

• Parietal lobe: Sensory input processing.

• Occipital lobe: Visual processing center.

• Temporal lobe: Hearing, smell, learning, memory, visual recognition.

Brain Stem

The brain stem connects the cerebrum with the spinal cord and is essential for vital functions such as breathing and heart rate. It consists of the midbrain, pons, and medulla oblongata.

• Midbrain: Visual and auditory reflexes, dopamine production.

• Pons: Respiratory control, bridge for neural pathways.

• Medulla oblongata: Cardiac, respiratory, and vasomotor centers; decussation of motor tracts.

Cerebellum

The cerebellum is located posterior to the brain stem and is responsible for balance, equilibrium, and coordination of voluntary movements. It compares intended movements with actual movements and makes adjustments as needed.

Diencephalon

The diencephalon is situated deep within the cerebrum and includes the thalamus, hypothalamus, and epithalamus. It is a major center for homeostasis and sensory/motor integration.

• Thalamus: Relay station for sensory and motor signals to the cortex.

• Hypothalamus: Regulates appetite, thirst, body temperature, autonomic nervous system, and hormone production.

• Epithalamus: Contains the pineal gland, which produces melatonin for sleep-wake cycles.

Meninges and Protective Coverings

Meningeal Layers

The meninges are three connective tissue membranes that surround the brain and spinal cord, providing protection and support.

• Dura mater: Tough, outermost layer; periosteal layer fused to skull, meningeal layer extends into fissures.

• Arachnoid mater: Middle, web-like layer; closely adheres to dura.

• Pia mater: Thin, innermost layer; follows contours of the brain into sulci.

The subarachnoid space between the arachnoid and pia mater contains cerebrospinal fluid (CSF).

Hematomas

• Epidural hematoma: Blood collects between dura and skull, often due to trauma; increases intracranial pressure.

• Subdural hematoma: Blood collects between dura and arachnoid; also increases pressure and can cause brain damage.

Cerebrospinal Fluid (CSF)

Functions of CSF

Cerebrospinal fluid is a clear, colorless liquid that surrounds the brain and spinal cord, providing several essential functions:

• Maintains a stable environment for neural tissue

• Provides nutrition and removes waste

• Protects the brain by cushioning against trauma

• Regulates intracranial pressure

Composition of CSF

• 99% water

• Ions: Na+, Cl-, Mg++

• Much less protein than blood

• Glucose (about 70% of blood level)

• Very few cells

CSF Circulation Pathway

CSF is produced by the choroid plexus in the lateral ventricles and flows through the ventricular system:

1. Lateral ventricles

2. Interventricular foramen

3. Third ventricle

4. Cerebral aqueduct

5. Fourth ventricle

6. Subarachnoid space (or central canal)

7. Arachnoid villi

8. Superior sagittal sinus (venous return)

Clinical Note: Hydrocephalus

Hydrocephalus occurs when more CSF is produced than reabsorbed, leading to water accumulation in the brain. This condition can cause increased intracranial pressure and requires medical intervention.

Functional Organization of the Cerebrum

Gray and White Matter

• Gray matter (cortex): Contains neuron cell bodies; site of major brain functions.

• White matter (tracts): Contains myelinated axons; connects different brain regions.

Types of White Matter Tracts

• Commissural tracts: Connect right and left hemispheres (e.g., corpus callosum).

• Association tracts: Connect gyri and lobes within the same hemisphere.

• Projection tracts: Connect cortex with lower brain regions and spinal cord.

Functional Areas of the Cortex

• Motor cortex (precentral gyrus, frontal lobe): Initiates voluntary movements.

• Sensory cortex (postcentral gyrus, parietal lobe): Receives sensory input.

• Broca's area (frontal lobe): Motor speech production.

• Wernicke's area (parietal lobe): Language comprehension and formulation.

• Visual center (occipital lobe): Processes visual information.

• Auditory center (temporal lobe): Processes auditory information.

• Gustatory center (frontal lobe/insula): Processes taste.

Hemispheric Specialization

• Right hemisphere: Controls left side of body; associated with music, artistic skills, spatial relationships, and insight.

• Left hemisphere: Controls right side of body; associated with speech, numerical and reasoning skills.

Basal Nuclei

Structure and Function

The basal nuclei are masses of gray matter deep within the white matter of the cerebrum. They are involved in the control of subconscious motor impulses, muscle tone, and coordination of movement.

• Receive and send signals to the substantia nigra (midbrain) and cerebral cortex.

• Work with the cerebellum and precentral gyrus to refine movements.

• Dopamine from the substantia nigra inhibits excessive movement; loss of these cells leads to Parkinson's disease.

Diencephalon: Thalamus, Hypothalamus, and Epithalamus

Thalamus

• Relay station for almost all sensory and motor information to the cortex.

• Sorts, edits, and relays impulses by function.

• Mediates sensation, motor activities, arousal, learning, and memory.

Hypothalamus

• Regulates appetite, thirst, body temperature, and autonomic nervous system.

• Controls the pituitary gland and hormone production.

• Involved in emotions, memory, and circadian rhythms (with pineal gland).

Epithalamus

• Contains the pineal gland, which produces melatonin to regulate sleep-wake cycles.

• Works with the hypothalamus to control biorhythms.

Limbic System

The limbic system includes parts of the cerebral hemispheres and diencephalon. It is essential for emotions, memory, and motivation.

Brain Stem: Mesencephalon, Pons, and Medulla Oblongata

Mesencephalon (Midbrain)

• Corpora quadrigemina: Superior colliculi (visual reflexes), inferior colliculi (auditory reflexes).

• Tegmentum: Substantia nigra (dopamine), red nucleus (motor coordination).

• Reticular formation: Controls alertness, sleep, and pain modulation.

• Cerebral peduncles: Descending motor tracts.

Pons

• Respiratory center.

• Bridge for neural pathways between brain regions and cerebellum.

Medulla Oblongata

• Contains vital centers for respiration, cardiac function, and vasomotor control.

• Non-vital centers for vomiting, sneezing, and coughing.

• Site of decussation (crossing) of motor tracts.

Cerebellum

The cerebellum coordinates voluntary movements, balance, and posture. It compares intended movements with actual performance and makes necessary adjustments. It works ipsilaterally (right side controls right body, left controls left).

Cranial Nerves

Overview

There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions. They emerge from the brain and brain stem and innervate structures primarily in the head and neck.

Number	Name	Type	Main Function(s)
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil constriction
IV	Trochlear	Motor	Eye movement (superior oblique muscle)
V	Trigeminal	Both	Sensory to face, motor to mastication
VI	Abducens	Motor	Eye movement (lateral rectus muscle)
VII	Facial	Both	Taste (anterior 2/3 tongue), facial expression, tear/saliva secretion
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Both	Taste (posterior 1/3 tongue), swallowing, salivation
X	Vagus	Both	Viscera control, taste, swallowing, parasympathetic output
XI	Accessory	Motor	Neck and shoulder movement, swallowing
XII	Hypoglossal	Motor	Tongue movement

Summary Table: Major Brain Regions and Functions

Region	Main Functions
Cerebrum	Higher thinking, voluntary movement, sensory perception
Brain Stem	Vital functions, cranial nerve origin, pathway for impulses
Cerebellum	Balance, coordination, motor learning
Diencephalon	Homeostasis, sensory/motor integration, hormone regulation