CENTRAL NERVOUS SYSTEM

Lecture Overview

• Part 1: Integration of information & the spinal cord (spinothalamic, corticospinal, dorsal column tracts)

• Part 2: Brain: perfusion & protective features (skull, meninges, blood-brain barrier, CSF)

• Part 3: Brain regions: Brainstem, cerebellum, limbic system, cerebral cortex

Integration in the CNS

• Central nervous system (CNS) receives, processes sensory information

• Prepares & sends motor responses

• Integration occurs in both spinal cord & brain

Spinal Cord Integration

• Dorsal root: sensory neurons enter cord

• Ventral root: motor neurons exit cord

• Interneurons facilitate integration in gray matter

• Organized into ascending (sensory) and descending (motor) tracts

Brain Anatomy

• Thalamus: sensory & motor relay center

• Regions: Brainstem, cerebellum, limbic system, cerebral cortex

Brain Protection

• Structures: Skull, meninges, cerebrospinal fluid (CSF), blood-brain barrier

• Intracranial pressure increases when volume in the skull increases

Blood Supply to the Brain

• Arterial supply: Aorta, carotid arteries, Circle of Willis

• Venous drainage forms protect brain's vascular system

Protective Features

• Meninges: Dura mater, arachnoid, pia mater

• CSF: Acts as a cushioning agent

Brain Functions

• Brainstem: Controls breathing & heartbeat

• Cerebellum: Coordinates body movement, postural adjustment

• Limbic System: Motivation, learning, emotion regulation

• Cerebral Cortex: Higher functions (reasoning, speech, perception)

Hemispheric Functionality

• Dominant hemisphere typically contains language & speech areas

• Lateralization observed in brain functionality

Association Areas

• Integration of sensory information, complex behaviors, memory, and emotional responses.

Lecture Overview

• Part 1: Integration of information & the spinal cord (spinothalamic, corticospinal, dorsal column tracts)

  • Focus on how sensory signals ascend to the brain and motor commands descend from the brain, and the role of interneurons in processing.

• Part 2: Brain: perfusion & protective features (skull, meninges, blood-brain barrier, CSF)

  • Detailed exploration of the physical and physiological barriers safeguarding the brain.

• Part 3: Brain regions: Brainstem, cerebellum, limbic system, cerebral cortex

  • Examination of the structural components and their specialized functions.

Integration in the CNS

• Central nervous system (CNS) receives, processes, and interprets sensory information from both the external and internal environments.

• Prepares and sends appropriate motor responses to muscles and glands.

• Integration is a complex process involving interneurons within both the spinal cord and various brain regions, facilitating communication between sensory and motor pathways.

Spinal Cord Integration

• Dorsal root: Formed by sensory (afferent) neurons entering the posterior aspect of the spinal cord, carrying information from peripheral receptors to the CNS. The cell bodies of these neurons are located in the dorsal root ganglia.

• Ventral root: Formed by motor (efferent) neurons exiting the anterior aspect of the spinal cord, transmitting commands from the CNS to effector organs (muscles and glands).

• Interneurons: Facilitate integration within the gray matter of the spinal cord, often forming reflex arcs that allow for rapid, involuntary responses without direct brain involvement.

• Gray matter: Composed of neuron cell bodies, dendrites, and unmyelinated axons, serving as the primary site for synaptic contact and information processing within the cord.

• Organized into ascending (sensory) tracts that transmit information to the brain and descending (motor) tracts that carry signals from the brain to the peripheral nervous system.

Brain Anatomy

• Thalamus: A large, bilateral egg-shaped nuclei forming part of the diencephalon, serving as a crucial sensory and motor relay center.

  • It processes and relays all sensory information (except olfaction) to the cerebral cortex and plays a role in regulating consciousness, sleep, and alertness.

• Regions: Comprises the Brainstem, Cerebellum, Limbic System, and Cerebral Cortex, each specialized for distinct functions.

Brain Protection

• Structures: The brain is protected by several layers:

  • Skull: The rigid bony casing that provides the primary physical defense against external trauma.

  • Meninges: Three connective tissue membranes surrounding the brain and spinal cord, providing physical protection and containing CSF.

  • Cerebrospinal Fluid (CSF): A clear fluid that bathes the brain and spinal cord.

  • Blood-Brain Barrier (BBB): A physiological barrier that regulates the passage of substances from the bloodstream into the brain tissue.

• Intracranial pressure (ICP): Increases significantly when there is an increase in volume within the rigid confines of the skull (e.g., due to hemorrhage, tumor, or edema). Elevated ICP can compromise blood flow to the brain and lead to neurological damage.

• Meninges:

  • Dura mater: The outermost, thickest, and toughest meningeal layer. It consists of two fibrous layers that are fused together, forming dural venous sinuses where they separate.

  • Arachnoid mater: The middle delicate, web-like layer. It loosely covers the brain, and beneath it is the subarachnoid space, which contains CSF and major blood vessels.

  • Pia mater: The innermost, thinnest, and highly vascularized layer that closely adheres to the brain surface, following its every convolution.

• CSF: Produced by the choroid plexuses within the brain's ventricles.

  • Functions: Acts as a cushioning agent, providing buoyancy to the brain (reducing its effective weight by about $97\%$) and protecting it from shock and trauma. It also helps in chemical stability and waste removal.

  • Circulates through the ventricles and subarachnoid space and is reabsorbed into the venous blood via arachnoid villi.

• Blood-Brain Barrier (BBB): Formed by specialized endothelial cells lining brain capillaries, which have tight junctions that largely prevent the movement of most substances.

  • Composition: Primarily endothelial cells with continuous tight junctions, thick basal lamina, and astrocytes (star-shaped glial cells) that surround the capillaries.

  • Function: Selectively regulates the passage of substances from the blood into the brain, protecting it from circulating toxins and pathogens while allowing essential nutrients to pass.

Blood Supply to the Brain

• Arterial supply: Primarily delivered by the internal carotid arteries (anterior circulation) and vertebral arteries (posterior circulation), which unite to form the unique Circle of Willis at the base of the brain.

  • The Circle of Willis is an arterial anastomosis that provides collateral circulation, ensuring continuous blood flow to the brain even if one of the major arteries is partially occluded.

• Venous drainage: The venous blood from the brain drains into a system of dural venous sinuses (e.g., superior sagittal sinus, transverse sinuses, sigmoid sinuses), which ultimately empty into the internal jugular veins, protecting the brain's vascular system by maintaining appropriate pressure.

Brain Functions

• Brainstem: Located between the cerebrum and spinal cord, comprising the midbrain, pons, and medulla oblongata.

  • Controls crucial involuntary functions such as breathing rate, heart rate, blood pressure, and sleep-wake cycles (reticular activating system).

  • Also serves as the origin for many cranial nerves and contains centers for reflex actions like vomiting, coughing, and swallowing.

• Cerebellum: Located inferior to the occipital and temporal lobes, posterior to the brainstem.

  • Coordinates body movement, maintains posture, and ensures balance.

  • Plays a vital role in motor learning, fine-tuning motor commands, and integrating sensory input to optimize motor control.

• Limbic System: A group of structures forming a ring around the diencephalon, including the hippocampus, amygdala, and parts of the hypothalamus.

  • Crucial for motivation, learning, memory formation (especially conversion of short-term to long-term memory via the hippocampus), and emotion regulation (e.g., fear, aggression via the amygdala).

• Cerebral Cortex: The outermost layer of the cerebrum, composed of gray matter.

  • Responsible for higher functions such as conscious thought, reasoning, voluntary movement, perception of sensory information, language (speech and comprehension), and abstract thinking.

  • Divided into four main lobes: frontal, parietal, temporal, and occipital, each with specialized functions.

Hemispheric Functionality

• The brain is divided into left and right hemispheres, each often specialized for different functions.

• The dominant hemisphere (typically the left for most people) usually contains the language (Broca's and Wernicke's areas) and speech processing centers, as well as logical and analytical abilities.

• Lateralization is observed in various brain functionalities, with the non-dominant hemisphere often excelling in spatial reasoning, creativity, musical abilities, and facial recognition.

Association Areas

• These are regions of the cerebral cortex that are not directly involved in primary motor or sensory functions but integrate diverse information.

• They are crucial for complex cognitive processes such as integrating sensory information from multiple modalities, abstract thinking, planning complex behaviors, memory retrieval, and generating appropriate emotional responses to various stimuli.