CHAPTER 10 AND BIT OF 11

Overview of the Nervous System

  • The nervous system can be divided into two main parts:

    • Central Nervous System (CNS): Comprising the brain and spinal cord (indicated in gray).

    • Peripheral Nervous System (PNS): Comprised of nerves that branch out from the CNS (indicated in yellow).

Sensory and Motor Components of the Nervous System

  • The PNS is categorized based on the type of function it serves:

    • Sensory (Afferent): Refers to signals going to the CNS from sensory receptors in the body (e.g., external environment).

    • Definition: Afferent means 'going to' the CNS.

    • Motor (Efferent): Refers to signals traveling away from the CNS to effectors (muscles or glands).

    • Definition: Efferent means 'going away' from the CNS.

  • The motor division can further be divided into:

    • Somatic Nervous System (Voluntary control, e.g., movement of skeletal muscles).

    • Autonomic Nervous System (Involuntary control, e.g., heart rate, digestion, breathing).

    • Important Note: Cardiac signals and other automatic functions operate without conscious control.

Neuron Structure and Function

  • Basic Neuron Components:

    • Cell Body (Soma): Contains the nucleus and genetic material.

    • Dendrites: Branch-like structures that receive incoming signals.

    • Analogy: Think of dendrites as arms catching signals coming from all directions.

    • Axon: The long extension from the cell body, which transmits signals away from the neuron.

    • Ends in axon terminals, where neurotransmitters can be released to communicate with other cells.

    • Myelin Sheath: Fatty substance surrounding the axon, which increases the speed of signal transmission.

    • Function: Insulates axon to protect electrical signals.

    • Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission via saltatory conduction.

      • Clinical Relevance: Diseases such as Multiple Sclerosis (MS) involve degeneration of the myelin sheath affecting signal transmission speed.

Importance of Myelination

  • Essential for the development and speed of neural transmission:

    • Adequate nutritional intake of lipids during infancy is crucial for myelin formation.

    • Deficiencies can lead to slower signal transmission or neurological disorders.

Neuron Classification by Structure

  • Types of Neurons:

    • Multipolar Neurons: Most prevalent, characterized by multiple extensions.

    • Functionality: Serves as a connector, integrator, and transmitter of signals in the nervous system.

    • Bipolar Neurons: Involved in sensory systems (e.g., sight, smell).

    • Unipolar Neurons: Not emphasized in the discussion.

Types of Neurons by Functionality

  • Sensory Neurons (Afferent): Transmit information from sensory receptors to the CNS.

  • Motor Neurons (Efferent): Convey signals from the CNS to effectors, initiating action.

  • Interneurons: Function as connectors or operators within the CNS, directing the message to the appropriate destination.

Neuroglial Cells (Supporting Cells)

  • Neuroglia Overview: Support and nourish neurons; six major types exist:

    • Central Nervous System (CNS) Neuroglia:

    1. Microglial Cells: Phagocytic cells that clean up debris in the brain.

    2. Astrocytes: Form the blood-brain barrier (BBB) and regulate nutrient transport.

    3. Oligodendrocytes: Form myelin sheaths around multiple axons in the CNS.

    4. Ependymal Cells: Produce cerebrospinal fluid (CSF), which fills cavities in the brain.

    • Peripheral Nervous System (PNS) Neuroglia:

    1. Schwann Cells: Form myelin sheaths around single axons.

    2. Satellite Cells: Support and nourish neuron cell bodies in the PNS.

Action Potentials and Neuronal Communication

  • Action Potentials: Electrical impulses that allow neurons to communicate:

    • Resting potential is established at -70mV.

    • Threshold potential is at -55mV.

    • Signals must reach this threshold to propagate action potentials.

    • During depolarization, sodium ions ($Na^+$) flood into the neuron, making it positively charged (up to +30mV).

    • The process then includes repolarization where potassium ions ($K^+$) exit the neuron.

    • Hyperpolarization may occur, temporarily exceeding the resting state.

  • Sodium-Potassium Pump: Maintains concentration gradients; expels 3 sodium ions for every 2 potassium ions pumped in.

Summary of Key Terms and Concepts

  • Afferent: Sensory information TO the CNS.

  • Efferent: Motor output FROM the CNS.

  • Somatic: Controls voluntary actions.

  • Autonomic: Controls involuntary activities.

  • Multipolar Neurons: Predominant neuron type in the body.

  • Myelination: Insulation of axons which increases conduction speed.

  • Saltatory Conduction: Rapid transmission of action potentials jumping from node to node along myelinated axons.

  • Refractory Period: Time after an action potential during which a new action potential cannot be initiated.

Cranial Nerves Overview

  • 12 Cranial Nerves enumerated with their primary functions:

    1. Olfactory Nerve: Sense of smell (Sensory).

    2. Optic Nerve: Sense of vision (Sensory).

    3. Oculomotor Nerve: Eye movement (Motor).

    4. Trochlear Nerve: Eye movement (Motor).

    5. Trigeminal Nerve: Mastication and facial sensation (Both).

    6. Abducens Nerve: Lateral eye movement (Motor).

    7. Facial Nerve: Facial expressions and taste (Both).

    8. Vestibulocochlear Nerve: Hearing and balance (Sensory).

    9. Glossopharyngeal Nerve: Throat sensation and motor function (Both).

    10. Vagus Nerve: Parasympathetic control of heart, lungs, digestive tract (Both).

    11. Accessory Nerve: Controls sternocleidomastoid and trapezius muscles (Motor).

    12. Hypoglossal Nerve: Tongue movement (Motor).

Concluding Remarks

  • Understanding the components and functions of the nervous system is critical.

  • Each system, structure, and cell type plays a pivotal role in maintaining homeostasis and enabling complex bodily functions.