Nervous System Pt 1 Lec Notes

Introduction to the Nervous System and Nervous Tissue Part 1

Overview of the Nervous System

  • Functions

    • Controls perception and experience of the world.

    • Directs voluntary movement.

    • Is the seat of consciousness, personality, learning, and memory.

    • Works with the Endocrine System to maintain homeostasis of various regulated variables:

    • Respiratory rate,

    • Blood pressure,

    • Body temperature,

    • Sleep/wake cycle,

    • Blood pH.

Major Divisions of the Nervous System

  • Central Nervous System (CNS)

    • Composed of:

    • Brain

      • Composed of approximately 100 billion cells called neurons or nerve cells.

      • Regulates all body functions.

    • Spinal Cord

      • Contains around 100 million neurons.

      • Merges with the brain at the foramen magnum and passes through the vertebral foramen of the first cervical vertebra, extending to the first or second lumbar vertebra.

      • Enables brain communication with the body below the head and neck; performs some functions autonomously.

  • Peripheral Nervous System (PNS)

    • Composed of:

    • Nerves

      • Carry signals to and from the CNS; consist of bundles of long neuron axons or "arms" packed with blood vessels and surrounded by connective tissue sheaths.

    • Cranial Nerves

      • Originate from or travel to the brain; a total of 12 pairs.

    • Spinal Nerves

      • Originate from or travel to the spinal cord; a total of 31 pairs.

Functions of the Nervous System

  • Sensory Functions

    • Involve gathering information about both internal and external environments of the body.

    • Carried out by the Sensory (Afferent) Division of the PNS.

  • Integrative Functions

    • Analyze and interpret detected sensory stimuli and determine an appropriate response.

    • Primarily executed by the CNS, particularly the brain.

  • Motor Functions

    • Actions performed in response to integrations.

    • Carried out by the Motor (Efferent) Division of the PNS.

Functional Divisions of the Nervous System

  • CNS and PNS

    • 99% of sensory input that the CNS receives is disregarded, leading to a response when necessary.

  • Sensory Receptors

    • Specialized structures that detect stimuli, varying from small neuron tips to complex receptors.

    • Somatic Sensory Division

    • Signals from skeletal muscles, bones, joints, skin, as well as special sensory receptors for vision, hearing, taste, smell, and balance.

    • Visceral Sensory Division

    • Signals from organs such as the heart, lungs, stomach, intestines, kidneys, and urinary bladder.

PNS Motor Division

  • Functions

    • Involve carrying out motor functions such as muscle contractions and gland secretion.

    • Effectors

    • Organs that execute the effects of the nervous system response.

  • Voluntary Motor Division

    • Somatic Motor Division

    • Neurons transmit signals to skeletal muscles.

  • Involuntary Motor Division

    • Visceral Motor Division (Autonomic Nervous System)

    • Neurons transmit signals to thoracic and abdominal viscera, regulate secretion from glands, and control smooth and cardiac muscles.

Summary of Nervous System Structure and Function

  • CNS

    • Brain and spinal cord integrate information.

  • PNS

    • Cranial and spinal nerves link CNS with the rest of the body; perform motor and sensory functions.

    • Motor (Efferent) Division

      • Distinct divisions:

      • Sensory (Afferent) Division

      • Somatic Sensory Division

      • Visceral Sensory Division

      • Somatic Motor Division

      • Autonomic Nervous System (ANS)

        • Divided into the Sympathetic Nervous System and Parasympathetic Nervous System.

Nervous Tissue

  • Composition

    • Nervous tissue consists of approximately 80% cells and about 20% extracellular matrix.

    • Neurons

    • Highly excitable cell type responsible for sending and receiving signals in the form of action potentials; essential for sensory, integrative, and motor functions of the nervous system.

    • Neuroglial Cells

    • Smaller, more numerous cells that do not transmit signals but perform various supportive functions.

Characteristics of Neurons

  • Lifespan

    • Long-lived but generally amitotic (do not divide after a certain development stage).

    • Vary in length from 1 mm in the CNS to over 1 meter in the PNS.

    • Composed of a central Cell Body (where organelles are housed), one or more Dendrites (receive signals), and one Axon (sends signals away from the cell body).

  • Cell Body (Soma)

    • Ranges from 5 to 100 µm in diameter; most metabolically active part of a neuron; key functions include maintaining a large cytoplasmic volume and manufacturing proteins.

    • Contains abundant Free Ribosomes and Rough Endoplasmic Reticulum (forming clusters known as Nissl Bodies), Golgi Apparatus, prominent Nucleoli, and numerous Mitochondria.

    • Neuronal Cytoskeleton

    • Composed of intermediate filaments (Neurofibrils) and microtubules that support the cell, extending into dendrites and axon.

Neuron Processes: Dendrites and Axons

  • Dendrites

    • Short, forked processes resembling tree branches that receive input from other neurons and transmit electrical impulses toward the cell body.

    • Have the same organelles as the cell body and undergo growth and pruning during maturation based on nervous system demands.

  • Axons (Nerve Fibers)

    • Structures capable of generating and conducting action potentials.

    • Key Components:

    • Axon Hillock: Region of the cell body where the axon arises.

    • Axon Collaterals: Branches of the axon.

    • Telodendria: Fine branches of the axon and its collaterals.

    • Axon Terminals (Synaptic Knobs): Ends of telodendria that communicate with target cells, which can number over 1,000 per neuron.

  • Axolemma and Axoplasm

    • Axolemma: Plasma membrane of the axon.

    • Axoplasm: Cytoplasm of the axon containing organelles except protein-making components.

Axonal Transport

  • Mechanism

    • Substances travel through the axoplasm toward or away from the cell body.

  • Types:

    • Slow Axonal Transport: Moves cytoskeletal and other proteins at 1-3 mm/day.

    • Fast Axonal Transport: Moves vesicles and organelles using motor proteins; ATP-dependent.

    • Retrograde Axonal Transport: Moves substances towards the cell body; can reach 200 mm/day.

    • Anterograde Axonal Transport: Moves substances away from the cell body; can reach 400 mm/day.

Classification of Neurons

  • Functional Regions

    • Receptive Region: Dendrites and cell body receive signals.

    • Conducting Region: Axon propagates the signal.

    • Secretory Region: Releases neurotransmitters to target cells.

  • Structural Classification of Neurons:

    • Multipolar Neurons: Over 99% of neurons; one axon with multiple highly branched dendrites.

    • Bipolar Neurons: One axon, one dendrite; typically sensory neurons found in eyes and olfactory epithelium.

    • Pseudounipolar Neurons: Single axon that bifurcates into two parts; involved in sensory signal transmission for touch, pressure, and pain.

  • Functional Classification of Neurons:

    • Sensory (Afferent) Neurons: Carry signals towards the CNS, either pseudounipolar or bipolar.

    • Motor (Efferent) Neurons: Carry signals away from CNS to muscles and glands; mostly multipolar.