Organization and Function of the Nervous System: Study Notes

Organization of the Nervous System

Main Divisions of the Nervous System

The nervous system is a complex network responsible for coordinating body activities. It is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. Acts as the command center, processing and interpreting sensory information and issuing instructions.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS, such as nerves and ganglia. It is further divided into sensory and motor divisions.

Sensory and Motor Divisions

• Sensory Division (Afferent): Carries information from sensory receptors to the CNS. For example, sensory receptors in the skin detect heat and transmit this information to the CNS via sensory neurons.

• Motor Division (Efferent): Transmits action potentials from the CNS to effector organs (muscles and glands). It is further divided into:

  • Somatic Nervous System: Controls voluntary movements by transmitting signals to skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary functions by transmitting signals to cardiac muscle, smooth muscle, and glands. It has two subdivisions:

    • Sympathetic Division: Prepares the body for 'fight or flight' responses.

    • Parasympathetic Division: Promotes 'rest and digest' activities, such as relaxation and energy storage.

  • Enteric Nervous System: Found within the digestive tract, sometimes called the 'second brain,' and manages digestive functions independently of the CNS.

Neurons and Glial Cells

Neuron Structure and Types

Neurons are the primary cells of the nervous system, specialized for receiving, processing, and transmitting information.

• Cell Body (Soma): Contains the nucleus and organelles; controls the neuron's activities.

• Dendrites: Branch-like extensions that receive signals from other neurons.

• Axon: A single, long process that transmits signals away from the cell body to other neurons, muscles, or glands.

Types of Neurons

• Multipolar Neurons: Many dendrites and one axon; common in neurons controlling skeletal muscles.

• Bipolar Neurons: One dendrite and one axon; specialized for senses such as vision and smell.

• Pseudo-Unipolar Neurons: A single process splits into two branches; one receives sensory input, the other delivers signals to the CNS.

Types of Glial Cells

• Astrocytes: Provide structural support, regulate the chemical environment, influence neuronal firing, and form the blood-brain barrier (BBB).

• Ependymal Cells: Line the ventricles of the brain and produce cerebrospinal fluid (CSF).

• Microglia: Act as macrophages in the CNS, removing bacteria, dead cells, and debris.

• Oligodendrocytes: Provide myelin sheaths to axons in the CNS.

• Schwann Cells: Provide myelin sheaths to axons in the PNS; each Schwann cell myelinates one axon segment.

• Satellite Cells: Support and protect neuron cell bodies in ganglia.

Myelin Sheath and Nerve Tissue Organization

Myelin and Signal Transmission

The myelin sheath is a fatty covering that insulates axons, speeding up the transmission of electrical signals. Gaps in the myelin sheath, called nodes of Ranvier, allow ions to move and enable rapid signal conduction (saltatory conduction). Unmyelinated axons conduct signals more slowly.

• Gray Matter: Contains neuron cell bodies and unmyelinated axons; found in the center of the spinal cord (H- or butterfly-shaped region).

• White Matter: Contains myelinated fibers; found in the outer layer of the spinal cord.

Action Potentials and Synapses

Electrical Signaling in Neurons

Action potentials are electrical signals sent by neurons. The speed of conduction depends on axon diameter and myelination.

• Continuous Conduction: Occurs in unmyelinated axons; slower, step-by-step transmission.

• Saltatory Conduction: Occurs in myelinated axons; faster and more efficient due to jumping of action potentials between nodes of Ranvier.

• Synaptic Cleft: The small gap between two neurons where neurotransmitters transmit signals.

• Neurotransmitters: Chemicals that transmit signals across the synaptic cleft.

Neural Pathways

• Converging Pathway: Many neurons send signals to one neuron.

• Diverging Pathway: One neuron sends signals to many neurons.

Spinal Cord and Reflexes

Structure and Function

The spinal cord extends from the foramen magnum to the cauda equina, facilitating communication between the brain and body. It contains 31 pairs of spinal nerves. The spinal cord is organized into gray matter (inner, H-shaped region) and white matter (outer layer).

• Reflex Arc: The pathway of a reflex, including the receptor, sensory neuron, interneuron, motor neuron, and effector.

Types of Reflexes

• Knee Jerk Reflex (Patellar Reflex): A stretch reflex.

• Withdrawal Reflex (Flexor Reflex): Removes a limb from a painful stimulus.

Brain Regions and Functions

Major Brain Regions

The brain is divided into four major regions, each with specialized functions:

• Brainstem: Includes the medulla oblongata, pons, and midbrain; controls essential functions such as heart rate, breathing, swallowing, and reflexes.

• Cerebellum: Fine-tunes movements for smoothness, balance, and precision.

• Diencephalon: Includes the thalamus, epithalamus, and hypothalamus.

  • Thalamus: Relays sensory information to the cerebral cortex.

  • Epithalamus: Involved in emotional and visceral responses; contains the pineal gland, which secretes melatonin.

  • Hypothalamus: Maintains homeostasis by regulating body temperature, hunger, thirst, and hormone release.

• Cerebrum: The largest part of the brain, divided into two hemispheres and four main lobes, each responsible for different functions.

Lobes of the Cerebrum

• Frontal Lobe: Involved in voluntary movement, problem-solving, and decision-making.

• Parietal Lobe: Evaluates sensory input.

• Temporal Lobe: Involved in auditory processing, memory, and spatial orientation.

• Occipital Lobe: Processes visual information.

• Insula: Processes taste, visceral sensations, self-awareness, and emotions.

Sensory and Motor Functions

Sensory Pathways

Sensory functions involve ascending tracts that carry signals from the spinal cord to the brain.

• Spino-Thalamic Tract: Carries pain and temperature information.

• Dorsal Column: Carries fine touch and proprioception information.

• Spino-Cerebellar Tracts: Carry proprioceptive information to the cerebellum for balance and coordination.

Motor Pathways

Motor functions involve descending tracts that carry signals from the brain to the spinal cord or brainstem to control movement.

• Pyramidal Tracts (Direct): Control precise voluntary movements of skeletal muscles.

• Indirect Pathways: Involved in more complex motor control.

Motor Areas of the Cerebral Cortex

• Primary Motor Cortex: Controls voluntary movements.

• Pre-Motor Area: Plans and organizes movements.

• Prefrontal Area: Provides motivation and foresight for planning movements.

Hemispheric Lateralization, Language, and Brainwaves

The brain's hemispheres are specialized for different functions. The left hemisphere is typically dominant for language, while the right handles spatial tasks. Language processing involves Broca's area (speech production) and Wernicke's area (language comprehension). Brainwaves, recorded by EEG, include alpha (awake, relaxed), beta (alert), theta (light sleep), and delta (deep sleep) waves.

Memory and the Limbic System

Memory is categorized into working memory (seconds to minutes), short-term memory (minutes to days), and long-term memory (days to years). The hippocampus is crucial for converting short-term to long-term memory. The limbic system is involved in emotion, motivation, and memory.

• Declarative Memory: Memory for facts and events.

• Procedural Memory: Memory for skills and habits.

Meninges, Ventricles, and Cerebrospinal Fluid (CSF)

The brain and spinal cord are protected by three layers of meninges: dura mater, arachnoid mater, and pia mater. The CNS contains fluid-filled cavities called ventricles, which produce cerebrospinal fluid (CSF). CSF cushions and protects the brain and spinal cord.

• CSF Flow: CSF is produced by the choroid plexus, flows through the ventricles, enters the subarachnoid space, and is reabsorbed into the bloodstream.

Cranial Nerves

There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions. They carry information to and from the brain for various head and neck functions.

Cranial Nerve	Function
I. Olfactory	Sensory: Smell
II. Optic	Sensory: Vision
III. Oculomotor	Motor: Eye movement, pupil constriction
IV. Trochlear	Motor: Eye movement
V. Trigeminal	Mixed: Sensation from face, chewing muscles
VI. Abducens	Motor: Eye movement
VII. Facial	Mixed: Facial expression, taste
VIII. Vestibulocochlear	Sensory: Hearing, balance
IX. Glossopharyngeal	Mixed: Taste, swallowing
X. Vagus	Mixed: Autonomic control of viscera
XI. Accessory	Motor: Neck and shoulder muscles
XII. Hypoglossal	Motor: Tongue movement

Example: The facial nerve (VII) controls muscles of facial expression and conveys taste sensations from the anterior two-thirds of the tongue.