Bio250-13 Nervous System Organization

Nervous System Organization

Central Nervous System (CNS) Components:

  • Meninges: Protects the brain and spinal cord; consists of three layers:

    • Dura Mater: The outermost layer, composed of dense, fibrous connective tissue that provides strength and stability; it also contains blood vessels and forms dural sinuses which return venous blood from the brain to the heart.

    • Arachnoid Mater: The middle layer featuring a web-like structure; it has tight junctions that create a barrier to protect the CNS from pathogens and substances in the blood.

    • Pia Mater: The innermost layer composed of areolar connective tissue, intimately adheres to the surface of the brain and spinal cord, accommodating blood vessels that supply the CNS.

  • Cerebrospinal Fluid (CSF): A clear fluid that surrounds the CNS and acts as a cushion, effectively protecting the brain and spinal cord from trauma.

  • White Matter: Composed primarily of myelinated axons, it is organized into tracts (bundles of axons) that facilitate communication between different areas of the CNS. White matter regions are involved in relaying signals and coordinating various functions.

  • Grey Matter: Comprised of neuronal cell bodies, dendrites, and unmyelinated axons; it is organized into the cortex (outer layer) of the brain and deeper nuclei within the CNS, where processing and synaptic connections occur.

Cerebrospinal Fluid Functions

  • Primary Functions:

    • Cushions and supports the brain, reducing the potential for injury during movement.

    • Transports essential nutrients and waste products, maintaining homeostasis in the CNS.

  • Formation:

    • Produced by the Choroid Plexus, a network of blood vessels in the ventricles of the brain, aided by Ependymal Cells which line the ventricle walls and facilitate the production and movement of CSF.

    • Normal volume: Approximately 150 mL in the adult brain, with about 500 mL produced daily, constantly circulating to ensure optimal conditions.

  • Circulation Pathways:

    • Flows through the central canal, ventricles, and subarachnoid space, returning to the bloodstream through the arachnoid granulations.

  • Constituents of CSF:

    • Contains glucose, proteins, lactic acid, ions (such as sodium, potassium, and chloride), which are crucial for neuronal function and health.

Ventricles & Choroid Plexus

  • Components:

    • Includes lateral ventricles, 3rd & 4th ventricles, and Cerebral Aqueduct, which are interconnected cavities that house CSF and facilitate its movement throughout the CNS.

Spinal Cord Anatomy

  • Structure:

    • Divided into segments corresponding to head, neck, shoulder, arms, trunk, abdominal organs, and legs, allowing regional control of motor and sensory functions.

  • Conus Medullaris & Cauda Equina:

    • The spinal cord terminates at around L1 vertebra; below this, the nerve roots collectively form the Cauda Equina, which innervates the lower body.

    • Filum Terminale: A fibrous extension of the pia mater anchoring the spinal cord to the coccyx, providing stability.

Spinal Cord Cross-Sectional Anatomy

  • Key Features:

    • Dorsal Horn: Contains sensory neuron cell bodies that receive incoming sensory information.

    • Ventral Horn: Houses motor neuron cell bodies that send impulses to stimulate skeletal muscle contraction.

  • Root Organization:

    • Dorsal Root: Comprised of sensory (afferent) axons that transmit information to the CNS.

    • Ventral Root: Contains motor (efferent) axons that convey commands from the CNS to the muscles.

  • Functionality:

    • Various ascending (sensory) and descending (motor) tracts are mapped out based on their tracts, designating their specific roles in reflexes and voluntary movements.

Spinal Cord Functions

  • Descending Tracts (Motor):

    • Lateral Corticospinal Tract: The main pathway for voluntary motor control, particularly for upper extremities; mediates fine motor skills.

    • More central (medial) pathways are involved in gross motor control and balance.

  • Ascending Tracts (Sensory):

    • Dorsal Columns (posterior funiculi): Responsible for processing deep touch, proprioception (awareness of body position), and vibratory sensations.

    • Lateral Spinothalamic Tract: Transmits information regarding pain and temperature sensations.

    • Ventral Corticospinal Tract: Facilitates voluntary motor control of the body's trunk and proximal muscles.

    • Ventral Spinothalamic Tract: Sensory pathway responsible for light touch sensations.

Spinal Reflexes

  • Types of Reflexes:

    • Autonomic Reflexes: Involved in regulating the activity of smooth muscle, heart rate, blood pressure, and digestive processes.

    • Somatic Reflexes: Activate skeletal muscles for voluntary and reflexive movements.

  • Reflex Positions:

    • Found in various brain regions including the hindbrain, midbrain, and forebrain, demonstrating the integration of complex reflex actions.

Brain Regions

  • Cerebral Cortex:

    • Comprised of two hemispheres, it is responsible for conscious thought, perception, and voluntary movement; it is further divided into lobes (frontal, parietal, temporal, occipital).

    • Contains also the basal nuclei and limbic system, which play roles in motor control and emotional processing, respectively.

  • Limbic System:

    • Integrates emotional responses with memory formation, incorporating structures such as the hippocampus and amygdala among others from various brain regions.

The Brain Lobes

  • Structures include gyri (ridges), sulci (grooves), and fissures (deep grooves) that increase surface area for neuronal connections and processing efficiency.

Basal Ganglia/Nuclei

  • Functions:

    • Consist of grey matter masses within the cerebrum; crucial for the regulation of voluntary motor control, procedural learning, and the direction of attention towards stimuli.

  • Components:

    • Include the caudate nucleus, putamen, and globus pallidus.

The Diencephalon

  • Thalamus:

    • Acts as the major relay center for all motor and sensory pathways, processing sensory information (except for olfactory signals) before transmitting it to the cerebral cortex.

  • Hypothalamus:

    • Maintains homeostasis by regulating bodily functions such as temperature, hunger, thirst, and emotional responses; interacts with the pituitary gland to control hormonal balance and endocrine functions.

The Midbrain

  • Key Structures:

    • Reticular Formation: Responsible for alertness and consciousness; regulates wakefulness and sleep-wake cycles.

    • Substantia Nigra: Critical for smooth body movements through the regulation of motor function; its degeneration is associated with Parkinson's disease.

    • Ventral Tegmental Area: Central to the brain's reward circuitry, playing vital roles in mood, motivation, and addiction pathways by releasing dopamine.

The Hindbrain

  • Components:

    • Pons: Serves as a communication pathway between the cerebrum and cerebellum; involved in modulating breath control and autonomic functions.

    • Medulla Oblongata: Regulates vital autonomic functions such as heartbeat, blood pressure, and breathing rhythm, critical for survival.

    • Cerebellum: Coordinates voluntary movements, balance, and posture; its internal structure known as arbor vitae enhances processing and relay for motor functions.

Peripheral Nervous System

  • Nerves:

    • Bundled structures of neurons composed of axons and dendrites of varying diameters (1-20 mm) and lengths (micrometers to meters), responsible for peripheral sensory and motor functions.

  • Organization:

    • Fascicle: A bundle of neurons wrapped together.

    • Epineurium: The tough fibrous connective tissue that encases the entire nerve, providing structural integrity.

    • Perineurium: A protective layer that surrounds individual fascicles and helps maintain the appropriate environment for neural conduction.

    • Endoneurium: A delicate layer surrounding each nerve fiber (axon), facilitating the nourishment and protection of the individual neuronal axons.

Autonomic Nervous System Functions

  • Homeostasis Functions:

    • Activated during rest and digest activities; controls physiological processes such as digestion, heart rate, glucocorticoid metabolism, and respiratory rate while managing urine production and blood flow.

  • Fight or Flight Response:

    • Prepares the body for urgent physical activities by increasing heart rate, enhancing respiration, releasing adrenaline, mobilizing glucose stores for energy, and inhibiting non-essential functions such as digestion and urinary processes to optimize body performance in emergencies.

Sympathetic Trunk Functions

  • Regulates various bodily responses including:

    • Adjustments of heart rate and blood pressure to cope with stress.

    • Triggers sweating in response to thermal stimuli or emotional responses.

    • Controls pupil dilation to enhance vision under low-light conditions.

    • Receives signals from the spinal cord to activate autonomic/sympathetic responses essential for survival.