Chapter 11: Nervous System and Nervous Tissue

Action Potentials: Generation and Propagation

The action potential is a rapid electrical signal that travels along the membrane of a neuron, enabling 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 triggers adjacent voltage-gated channels, allowing the action potential to move 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 the skin to the spinal cord.

Chapter 12a: 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 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)

• Example: Reflex arc involving sensory input and motor output.

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 processes sensory information and initiates voluntary movement.

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

• Example: The precentral gyrus (primary motor cortex) initiates movement.

Lateralization of Cortical Function

Lateralization refers to the specialization of functions in the left and right hemispheres of the brain.

• Left Hemisphere: Language, logic, analytical tasks

• Right Hemisphere: Spatial abilities, creativity, intuition

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

Primary Motor Cortex and Somatosensory Cortex: Homunculus

The primary motor and somatosensory cortices 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: Diagram showing the proportion of cortex dedicated to each body part

• Example: Hands and face have large cortical representation due to fine motor and sensory skills.

Three 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: Relays information, regulates breathing

• Medulla Oblongata: Controls heart rate, respiration, reflexes

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

Structure and Function of the Cerebellum

The cerebellum is involved in coordination, balance, and fine motor control.

• Structure: Two hemispheres, connected by the vermis; contains folia (folds)

• Function: Coordinates voluntary movements, maintains posture and equilibrium

• Example: The cerebellum helps maintain balance while walking.

Location, Subdivisions, and Functions of the Diencephalon

The diencephalon is located deep within the brain and consists of several important structures.

• Location: Between the brain stem and cerebrum

• Subdivisions:

  • Thalamus: Relay station for sensory information

  • Hypothalamus: Regulates homeostasis, endocrine functions

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

• Example: The hypothalamus controls 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 fourth ventricles.

Formation and Circulation of Cerebrospinal Fluid (CSF)

Cerebrospinal fluid is produced and circulated to protect and nourish the brain.

• Formation: Produced by the choroid plexus in the ventricles

• Circulatory Pathway: Flows from 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 central nervous system is protected by several mechanisms.

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

• Cerebrospinal Fluid: Provides cushioning and nutrient transport

• Blood Brain Barrier: Selectively restricts passage of substances from blood to brain tissue

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