Chapter 12 pt. 3

Structures in the Brain

Introduction to Deeper Structures

• The discussion transitions from superficial to deeper brain structures.

• Focus on the basal ganglia, which include large areas of gray matter.

  • Gray matter consists primarily of neuronal cell bodies, unmyelinated axons, and dendrites.

Basal Ganglia (Basal Nuclei)

• Comprised of structures within the corpus striatum:

  • Caudate nucleus

  • Lentiform nucleus

  • Contains:

    • Putamen

    • Globus pallidus

• Functions of Basal Ganglia:

  • Regulates slow, sustained lower movements.

  • Example: Arm swinging while walking or hiking helps maintain balance.

  • Inhibits unwanted movements (motor control).

  • E.g., precise finger movements when pressing the down arrow key on a keyboard.

  • Implications of dysfunction:

  • Disorders such as Parkinson's disease are characterized by tremors and unsteady movements due to basal ganglia dysfunction.

Diencephalon

• Encloses the third ventricle and comprises:

  • Thalamus

  • Hypothalamus

  • Epithalamus

Thalamus

• Often referred to as the "gateway to the cortex" or a relay station.

• Structure:

  • Composed of clusters of nuclei on each side of the brain, forming the sides of the third ventricle.

• Functions:

  • Integration of sensory information (excluding smell).

  • All afferent sensory information first reaches the thalamus for preliminary processing.

  • Connects sensory input (like touch and taste) to appropriate areas of the cerebral cortex.

Hypothalamus

• Located beneath the thalamus and made up of several nuclei.

• Connection to the pituitary gland via the infundibulum.

• Major functions include:

  • Regulating autonomic nervous system activity.

  • Control of emotional responses and behaviors.

  • Regulation of body temperature, food intake, water balance/thirst.

  • Regulation of sleep-wake cycles (circadian rhythms)—implications for jet lag.

  • Control of endocrine functions via neuroendocrine feedback mechanisms, generally using negative feedback.

Epithalamus

• Situated above the thalamus.

• Main structure: Pineal gland

  • Secretes melatonin, involved in sleep-wake cycles (higher levels during night).

• Contains the choroid plexus, responsible for producing cerebrospinal fluid.

Brain Stem

• Constituted by three sections:

  • Midbrain

  • Pons

  • Medulla Oblongata

Midbrain

• Situated between the diencephalon and pons.

• Contains the cerebral aqueduct, linking the third and fourth ventricles.

• Involved in automatic behaviors and survival instincts.

  • Contains axon tracks facilitating communication between the brain and spinal cord.

  • Connects cerebellum via cerebellar peduncles (superior, middle, inferior).

Pons

• An area for passing information rather than processing data.

• Supports cranial nerves linked to peripheral nervous system.

Medulla Oblongata

• Controls homeostasis.

  • Cardiovascular center: regulates heart rate.

  • Respiratory center: manages breathing rate.

  • Contains centers responsible for vital functions: vomiting, hiccuping, swallowing, coughing, sneezing.

Cerebellum

• Located posteriorly under the cerebrum, characterized by:

  • Surface cortex (gray matter) with folds called folia.

  • Internal white matter known as arbor vitae (tree of life).

• Functions:

  • Receives sensory input from muscles, tendons, joints, and visual information to integrate body positioning.

  • Compares intended movements with actual movements and makes necessary corrections to coordinate motion.

• Contains Purkinje cells crucial for processing movement corrections, found at the junction of two cellular layers:

  • Granular layer (inner)

  • Purkinje layer (outer, directly interfaces with granule layer).

Functional Systems of the Brain

Limbic System

• Positioned around the brainstem and plays a crucial role in emotional processing.

  • Comprised of gray matter, including the hypothalamus and olfactory bulbs.

• Functions:

  • Enhances memory through emotionally significant events related to survival.

  • Connects emotions to sensory processing (e.g., smells associated with emotional memories).

  • Damage can lead to significant memory impairment and altered behavioral responses.

Reticular Formation

• A network of gray matter within the brainstem (medulla, pons, midbrain).

• Functions:

  • Processes sensory, integrative, and motor functions.

  • Contains the Reticular Activating System (RAS):

  • Alerts the cerebrum about significant stimuli while filtering out unimportant background noise.

  • Maintains consciousness and regulates wakefulness.

  • Effects of substances:

  • Alcohol and certain drugs depress RAS, leading to drowsiness.

  • LSD disrupts sensory filtering, overwhelming the brain with excessive sensory input.

Protective Mechanisms for the Brain

Meninges

• Three protective layers envelop the brain and spinal cord:

  • Dura mater (outermost)

  • Comprised of two layers:

    • Periosteal layer (attached to skull)

    • Meningeal layer

  • Arachnoid mater (middle layer)

  • Loose connective tissue, contains arachnoid villi for CSF reabsorption.

  • Pia mater (innermost)

  • Closely adheres to brain contours, highly delicate.

Cerebrospinal Fluid (CSF)

• Functions include:

  • Mechanical cushioning, chemical stability.

  • Circulates nutrients; regulates extracellular environment.

  • Found in ventricles, the subarachnoid space, and central canal of the spinal cord.

• Composition: similar to plasma but devoid of red blood cells, contains nutrients, cations, anions, and proteins.

• Produced in choroid plexus and reabsorbed via arachnoid villi into venous blood.

Blood Brain Barrier (BBB)

• A selective barrier preventing harmful substances in blood from entering the brain.

• Mechanisms of protection include:

  • Tight junctions between capillary endothelial cells.

  • Thick connective tissue and astrocytic processes surrounding capillaries.

• Permeability characteristics:

  • Lipid-soluble molecules (e.g., alcohol, glucose) can cross, whereas charged molecules (e.g., proteins) generally cannot.

• Areas without a strict BBB:

  • Choroid plexuses, medulla's vomiting center, and hypothalamus must interact with blood for regulatory functions.

Homeostatic Imbalances

Types of Traumatic Brain Injuries (TBIs)

• Concussions: Result from significant impact causing brain to collide against the skull.

  • Symptoms may evolve over time; recurrent concussions increase the risk of additional injuries.

• Contusions: Small areas of bleeding within the brain due to injury.

• Lacerations: Severe tearing of brain tissue, typically from penetrating wounds.

Hemorrhages

• Can occur in various spaces: above or below dura mater, or within subarachnoid space.

• May cause increased intracranial pressure and require intervention like drainage or surgical procedures.

Cerebrovascular Accidents (CVAs) or Strokes

• Leading causes of death in the U.S.; categorized by:

  • Ischemic: Reduced blood flow causing tissue death (infarct).

  • Hemorrhagic: Bleeding leading to tissue deprivation downstream.

• Risk factors: hypertension, high cholesterol, diabetes, smoking, obesity, and alcohol abuse.

  • Transient ischemic attacks (TIAs) are brief episodes indicating potential increased stroke risk.

Degenerative Brain Diseases

• Alzheimer's Disease: Affects memory; confirmed post-mortem.

  • Patients face secondary mortality risks associated with cognitive decline.

• Parkinson's Disease: Associated with basal ganglia dysfunction; characterized by motor control issues (shaking and rigidity).

  • Progressive disease; potential treatments aim at dopamine restoration.

• Cerebral Palsy: Caused by asphyxia during development; affects motor function without cognitive impairment, leading to misperceptions of intelligence.

Conclusion

• Summary of the intricate structures, systems, and pathologies within the brain emphasizes their vital roles in maintaining human function and the consequences of their dysfunctionality.

  • Continual examination of brain mechanisms is essential for understanding both normal physiology and pathological states.