Central Nervous System Overview

Overview of the Central Nervous System (CNS)

The central nervous system in humans is a result of cephalization.
- Cephalization involves the concentration of neurons at the anterior end of the body, forming the brain.
- The CNS is attached to the spinal cord, with a higher density of neurons in the head.

The brain exhibits various folds and grooves:
- Gyri: The ridges of the brain.
- Sulci: The grooves between the gyri.
- Fissure: A deep groove that separates the two cerebral hemispheres (e.g., the longitudinal fissure).
The folds increase the brain's surface area allowing for more neurons in the limited space of the skull.

Composed of both white matter and gray matter:
- Gray matter: Unmyelinated, mainly composed of neuronal cell bodies (somas).
- White matter: Myelinated axons that appear white due to myelin, forming tracts that transmit signals within the CNS.
Arrangement within the hemispheres:
- Gray matter is in the deepest portion, surrounded by white matter, with additional gray matter on the outer surface.

The cortex consists of two hemispheres with functional differentiation:
- Left Hemisphere: Dominantly associated with language in about 90% of individuals.
- Right Hemisphere: More involved in visual-spatial tasks.
Both hemispheres communicate through white matter pathways, sharing sensory and motor information, maintaining contralateral control.
- Sensory information from the left body is processed by the right brain and vice versa.

The brain is divided into four major regions:
1. Cerebral Hemispheres
2. Diencephalon
3. Brain Stem
4. Cerebellum

Four main lobes within the cerebral hemispheres:
- Frontal Lobe: Involved in executive functions, planning, decision-making; contains the prefrontal cortex (not fully developed until mid-20s).
- Parietal Lobe: Integrates sensory information.
- Occipital Lobe: Primarily responsible for vision, contains the primary visual cortex.
- Temporal Lobe: Associated with memory and auditory processing; contains Wernicke's area (speech comprehension) and Broca's area (speech production).
Insular Cortex: Involved in self-awareness and consciousness, constituting about 40% of the brain's mass despite being thin.

Specific areas controlling voluntary functions:
- Primary Motor Cortex: Located in the precentral gyrus (just in front of the central sulcus); responsible for planning and initiating movements.
- Primary Somatosensory Cortex: Located in the postcentral gyrus (just behind the central sulcus); responsible for processing sensory information from the body.
The central sulcus separates the frontal and parietal lobes and is critical for understanding the structure and function that surround it.

Damage to specific areas of the brain leads to different types of aphasia:
- Wernicke's Aphasia: Difficulty understanding spoken language; speech may be fluent but nonsensical.
- Broca's Aphasia: Difficulty in speech production; comprehension is typically retained, but speech is laborious and fragmented.
- Importance of the central sulcus is highlighted, as it separates key functional areas such as the motor cortex and sensory cortex.

A group of nuclei involved in the control of movement:
- Comprises three main nuclei: caudate nucleus, putamen, and globus pallidus.
- Functions include influencing slow or stereotyped movements.
- Plays a role in cognition and emotional responses, filtering inappropriate muscle movements.
Huntington’s Disease: A genetic disorder affecting the basal nuclei, leading to uncontrolled movements and cognitive decline.

Divided into three main parts:
1. Thalamus: Relay station for sensory input; directs sensory data to appropriate cortical areas.
2. Hypothalamus: Regulates homeostasis, including temperature, hunger, thirst, and links with the pituitary gland.
3. Epithalamus: Houses the pineal gland, which secretes melatonin, crucial for sleep regulation and circadian rhythms.

Sleep is an active process; changes in brain activity can be measured through an EEG:
- Different sleep stages exhibit various brain waves:
- Awake: Low amplitude, high frequency waves.
- REM Sleep: Similar to awake pattern but indicates dreaming.
- Stage 1: Light sleep, transition from wakefulness.
- Stages 2-4: Deepening sleep with larger amplitude and slower frequency waves.
Circular pattern of sleep cycles with increasing REM duration toward morning, essential for cognitive functions.

Structures include the amygdala, hippocampus, and connections to olfactory processing (rhinencephalon).
The amygdala is central to emotional responses.
The hippocampus plays a significant role in memory formation and recall, influenced by emotional context.

Learning: The acquisition of new information.
Memory: Retention and recall of that information; physically changes neuronal connections.
Neurons that fire together wire together (long-term potentiation): Enhances synaptic strength with repeated activation.
Procedural Memory: Involves skills and actions (relies on the basal nuclei).
Declarative Memory: Knowledge of facts (relies on the hippocampus).

Composed of:
1. Midbrain: Involved in eye movement tracking, response to auditory stimuli.
2. Pons: Relays signals between higher brain centers and the cerebellum; regulates breathing cycles.
3. Medulla Oblongata: Controls vital autonomic functions (heart rate, blood pressure, respiration).

Begins below the medulla at the foramen magnum and ends at lumbar vertebrae.
Contains both sensory and motor pathways:
- Dorsal Horn: Receives sensory input; interneuron somas reside here.
- Ventral Horn: Contains motor neuron somas sending signals out.
Reflex arcs provide automatic responses to stimuli, bypassing conscious brain involvement for rapid reactions.

Meninges: Layers protecting the brain and spinal cord:
- Dura Mater: Tough outer layer.
- Arachnoid Mater: Web-like middle layer where cerebrospinal fluid (CSF) is found.
- Pia Mater: Delicate inner layer hugging the brain and spinal cord.
Cerebrospinal Fluid (CSF): Cushions the brain, circulates nutrients, and removes waste; produced by choroid plexus.
The Blood-Brain Barrier: Specialized capillary structure regulating substance exchange to protect the brain from pathogens.

Types include concussions, contusions, diffuse axonal injuries, and chronic traumatic encephalopathy (CTE).
Symptoms of TBIs can be immediate (e.g., confusion, memory loss) or long-term.
Neurons in the CNS have limited regenerative capacity; key exceptions exist in specific brain regions.

The brain receives 15% of blood supply despite being only 2% of body weight; critical for delivering oxygen and nutrients.
Ischemic Stroke: Occurs from blocked vessels leading to neuron death due to lack of oxygen, causing possible long-term deficits.
The Circle of Willis: Allows for collateral blood flow, protecting against ischemia.