Nervous System and Tissue

Action Potentials: Generation and Propagation

The action potential is a rapid electrical signal that travels along the membrane of a neuron, allowing communication within the nervous system.

• Generation: Action potentials are initiated when the membrane potential reaches a threshold, typically due to the opening of voltage-gated sodium channels. This causes a rapid influx of Na+ ions, depolarizing the membrane.

• Propagation: The depolarization spreads to adjacent regions, opening more sodium channels and moving the action potential along the axon. Repolarization follows as potassium channels open, allowing K+ to exit the cell.

• Key Steps:

  1. Resting state:

  2. Depolarization: influx

  3. Repolarization: efflux

  4. Hyperpolarization: temporary overshoot

• Example: Transmission of a pain signal from a finger to the spinal cord.

Central Nervous System – The Brain

Structural and Functional Divisions of the Nervous System

The nervous system is divided into structural and functional components for organization and specialization.

• Structural Divisions:

  • Central Nervous System (CNS): Brain and spinal cord

  • Peripheral Nervous System (PNS): Nerves and ganglia outside the CNS

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits sensory information to the CNS

  • Motor (Efferent) Division: Sends commands from CNS to effectors (muscles/glands)

  • Somatic Nervous System: Voluntary control of skeletal muscles

  • Autonomic Nervous System: Involuntary control (sympathetic and parasympathetic)

Major Regions of the Adult Brain

The adult brain is organized into distinct regions, each with specialized functions.

• Cerebrum: Largest part, responsible for higher functions

• Diencephalon: Includes thalamus, hypothalamus, epithalamus

• Brain Stem: Midbrain, pons, medulla oblongata

• Cerebellum: Coordinates movement and balance

• Example: The cerebrum is involved in reasoning, while the cerebellum manages coordination.

Major Lobes, Fissures, and Functional Areas of the Cerebral Cortex

The cerebral cortex is divided into lobes and functional areas, separated by fissures.

• Lobes: Frontal, parietal, temporal, occipital, insula

• Fissures: Longitudinal (separates hemispheres), lateral (separates temporal lobe), central (separates frontal and parietal lobes)

• Functional Areas:

  • Motor Areas: Control voluntary movement

  • Sensory Areas: Receive sensory input

  • Association Areas: Integrate information

Lateralization of Cortical Function

Lateralization refers to the specialization of functions in each hemisphere of the brain.

• Left Hemisphere: Language, logic, analytical tasks

• Right Hemisphere: Spatial abilities, creativity, emotion

• Example: Speech production is typically localized in the left hemisphere (Broca's area).

Primary Motor Cortex and Somatosensory Cortex: Homunculus

The primary motor cortex and somatosensory cortex are mapped to specific body regions, forming a "homunculus" or body map.

• Primary Motor Cortex: Located in the precentral gyrus; controls voluntary movements

• Somatosensory Cortex: Located in the postcentral gyrus; receives sensory input

• Homunculus: Visual representation of the body, showing areas with greater motor or sensory control (e.g., hands, face)

• Example: Fine motor control in fingers is represented by a large area in the motor homunculus.

Major Regions of the Brain Stem and Their Functions

The brain stem is essential for basic life functions and connects the brain to the spinal cord.

• Midbrain: Visual and auditory reflexes, motor control

• Pons: Relay between cerebrum and cerebellum, regulates breathing

• Medulla Oblongata: Controls heart rate, respiration, and other autonomic functions

• Example: The medulla oblongata regulates breathing and heart rate.

Structure and Function of the Cerebellum

The cerebellum is located at the back of the brain and is involved in coordination and balance.

• Structure: Two hemispheres, surface cortex of gray matter, internal white matter (arbor vitae)

• Function: Coordinates voluntary movements, maintains posture and balance, motor learning

• Example: The cerebellum helps you ride a bicycle by coordinating muscle activity.

Location, Subdivisions, and Functions of the Diencephalon

The diencephalon is located deep within the brain, between the brain stem and cerebrum.

• Subdivisions:

  • Thalamus: Relay station for sensory information

  • Hypothalamus: Controls autonomic functions, endocrine system, homeostasis

  • Epithalamus: Includes pineal gland, regulates sleep-wake cycles

• Example: The hypothalamus regulates body temperature and hunger.

Ventricles of the Brain: Names and Locations

The brain contains four ventricles, which are interconnected cavities filled with cerebrospinal fluid (CSF).

• Lateral Ventricles: Located in each cerebral hemisphere

• Third Ventricle: Located in the diencephalon

• Fourth Ventricle: Located between the brain stem and cerebellum

• Example: CSF flows from the lateral ventricles to the third and then fourth ventricle.

Formation and Circulation of Cerebrospinal Fluid (CSF)

Cerebrospinal fluid is produced by the choroid plexus and circulates through the ventricles and subarachnoid space.

• Formation: Choroid plexus filters blood plasma to produce CSF

• Circulatory Pathway: Lateral ventricles → third ventricle → fourth ventricle → subarachnoid space → absorbed by arachnoid villi into venous blood

• Example: CSF cushions the brain and removes waste products.

Protection of the CNS: Meninges, CSF, and Blood Brain Barrier

The CNS is protected by three main structures: meninges, cerebrospinal fluid, and the blood brain barrier.

• Meninges: Three connective tissue layers (dura mater, arachnoid mater, pia mater) surround the brain and spinal cord

• Cerebrospinal Fluid: Provides cushioning, buoyancy, and chemical stability

• Blood Brain Barrier: Selectively restricts passage of substances from blood to brain tissue, protecting neurons from toxins and pathogens

• Example: The blood brain barrier prevents most drugs from entering the brain.