Functions and Structure of the Nervous System

Overall Functions of the Nervous System

• Transmits signals throughout the body
• Integrates sensory input and motor outputs
• Regulates internal conditions (homeostasis)
• Facilitates reactions to environmental changes (response)
• Enables thought processes, memory, and learning (cognition)

Structure and Function of Nervous Tissue

• Composed of neurons and neuroglia (glial cells)
• Neurons transmit electrical signals
• Neuroglia support and protect neurons

Organs of the Nervous System

• Brain
• Spinal cord
• Nerves
• Ganglia

Divisions of the Nervous System

• Central Nervous System (CNS): Includes the brain and spinal cord; processes information and coordinates activity
  • Processes information
  • Coordinates activity
• Peripheral Nervous System (PNS): Connects the CNS to limbs and organs; facilitates communication
  • Afferent fibers: Carry sensory information to the CNS
  • Efferent fibers: Carry information from the CNS
  • Somatic Nervous System: Controls voluntary movements via skeletal muscles
  • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion)
    • Sympathetic Nervous System: Prepares the body for "fight or flight" responses
    • Parasympathetic Nervous System: Promotes "rest and digest" activities

Cells of the Nervous System

• Neurons: Transmit signals
• Neuroglia: Support, protect, and nourish neurons

Neuroglial Cell Functions

• Astrocytes: Maintain blood-brain barrier, provide nutrients, and repair tissue
• Microglial: Act as immune defense in the CNS
• Oligodendrocytes: Form myelin sheath in CNS
• Ependymal: Line ventricles and produce cerebrospinal fluid (CSF)
• Schwann cells: Form myelin sheath in PNS

Neuron Structure

• Cell body: Contains the nucleus and organelles
• Dendrites: Receive signals from other neurons
• Axons: Transmit impulses away from the cell body to other cells
• Schwann cells: Wrap around axons in PNS
• Myelin sheath: Insulates axons to speed up signal transmission
• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid conduction of nerve impulses

Nerves

• Nerves: Bundles of axons (nerve fibers) wrapped in connective tissue, transmitting signals between the CNS and PNS
  • Fascicles: Groups of nerve fibers bundled together, each surrounded by a perineurium.
  • Epineurium: The outermost layer of connective tissue enclosing the entire nerve, providing protection and support.
  • Perineurium: A protective sheath surrounding each fascicle; it helps maintain the internal environment of the nerve.
  • Endoneurium: The delicate connective tissue surrounding each individual nerve fiber, providing nourishment and support.

White vs. Gray Matter

• White matter: Composed of myelinated axons
• Gray matter: Contains neuronal cell bodies and unmyelinated fibers
• Tracts: Bundles of axons in the CNS that carry specific types of information

Sensory Receptors

• Structures that detect stimuli and convert them into neural signals

Sensory Neurons

• Neurons that carry information from sensory receptors to the CNS

Motor Neurons

• Neurons that transmit signals from the CNS to muscles or glands

Spinal Cord Parts

• Dorsal Root: Sensory information to the spinal cord
• Ventral Root: Motor commands away from the spinal cord
• Dorsal Root Ganglion: Cluster of sensory neuron cell bodies located just outside the spinal cord
• Synapse: The meeting point between two neurons via neurotransmitters
• Interneuron: Neuron that connects sensory and motor neurons within the CNS, allowing for processing and reflex actions
• Motor Neuron: Transmits signals to effector organs (muscles and glands)

Main Parts of the CNS

• Brain
• Spinal cord

Brain Regions

• Cerebrum
• Cerebellum
• Brainstem (midbrain, pons, medulla oblongata)
• Diencephalon (thalamus, hypothalamus, epithalamus)

Cerebral Cortex

• Outer layer of the cerebrum
• Composed of gray matter
• Responsible for higher brain functions (thought, perception, voluntary movement)

Basal Nuclei

• Located deep within cerebral hemispheres
• Regulate voluntary motor control, procedural learning, and emotional responses

Diencephalon

• Contains the thalamus and hypothalamus
• Thalamus: Relay station for sensory information
• Hypothalamus: Regulates homeostasis, hunger, temperature, and circadian rhythms
• Pineal gland: Produces melatonin, regulating sleep-wake cycles

Brainstem

• Controls reflexes and pathways for visual and auditory information
• Connects different parts of the brain
• Regulates sleep and autonomic functions (e.g., breathing, heart rate)

Cerebellum

• Coordinates voluntary movements, balance, and posture

Spinal Cord Dimensions

• Approximate length: 42-45 cm in adults
• Starts at the base of the skull (foramen magnum)
• Ends around L1-L2 vertebrae
• Regions: Cervical, thoracic, lumbar, sacral, and coccygeal
• Cauda equina: Bundle of spinal nerves below the end of the spinal cord

Spinal Cord Parts (cont.)

• Dorsal Horn: Contains sensory neurons
• Ventral Horn: Contains motor neurons
• Lateral Horn: Found in thoracic and upper lumbar regions, contains autonomic neurons
• Central Canal: Contains cerebrospinal fluid (CSF)

Meninges

• Protective membranes surrounding the brain and spinal cord
• Dura mater: Outermost layer, tough
• Arachnoid mater: Middle layer, web-like
• Pia mater: Innermost layer, adheres to brain/spinal cord
• Subdural space: Between dura and arachnoid mater
• Subarachnoid space: Contains cerebrospinal fluid (CSF)
• Cerebrospinal fluid (CSF): Cushions the brain and spinal cord, provides buoyancy, and transports nutrients to the nervous tissues
• Dural sinuses: Venous channels that drain blood from the brain

Ventricles

• Cavities within the brain filled with cerebrospinal fluid
• Lateral ventricles: Two large cavities in each hemisphere
• Septum pellucidum: Thin membrane separating lateral ventricles
• Interventricular foramen: Connects lateral ventricles to the third ventricle
• Third ventricle: Connects to the fourth ventricle via the cerebral aqueduct
• Fourth ventricle: Located between the brainstem and cerebellum
• Medial/Lateral Apertures: Openings allowing CSF to flow into the subarachnoid space

Cerebrospinal Fluid (CSF)

• Produced by choroid plexuses in the ventricles
• Clear fluid containing glucose, electrolytes, and proteins
• Circulates through ventricles and subarachnoid space
• Absorbed into venous system via arachnoid villi
• Provides cushioning, buoyancy, and nutrient transport for the brain and spinal cord

Blood- Brain Barrier

• A selective permeability barrier that protects the brain from harmful substances.
• Composed of tightly packed endothelial cells and astrocytic end-feet.