Nervous System Reading AS111

Introduction

  • An animal's body is complex, requiring coordination and communication among cells, tissues, organs, and systems to maintain homeostasis and health.

    • Two communication and control systems exist: the nervous system and the endocrine system.

  • Nervous System:

    • Uses neurotransmitters produced only by neurons (nerve cells).

    • Neurotransmitters travel short distances across synapses for quick responses, but cannot sustain activities for long.

  • Endocrine System:

    • Uses hormones secreted into the bloodstream.

    • Hormones travel long distances, result in slower responses, but can sustain activities for longer periods.

  • The nervous system is discussed in detail, while the endocrine system is covered in Chapter 11.

Structure of the Nervous System

  • The nervous system has two main divisions:

    1. Central Nervous System (CNS):

    • Composed of the brain and spinal cord.

    1. Peripheral Nervous System (PNS):

    • Composed of nerves that connect the CNS to the rest of the body.

  • Functional Categories of Nervous System Activities:

    1. Sensory Functions:

    • Sensing changes inside and outside the body.

    1. Integrating Functions:

    • Analyzing and integrating sensory information in the brain/spinal cord to produce a response.

    1. Motor Functions:

    • Instructing actions (e.g., muscle contraction or gland secretion).

  • Neurology - the study of the nervous system (neuro = nervous system; logos = study).

Neurons and Supporting Cells

  • Neurons:

    • Basic functional units of the nervous system, responsible for responding to stimuli and conducting impulses.

    • Have high oxygen requirements; can’t survive more than a few minutes without oxygen (important for CPR).

    • After birth, neurons lose ability to reproduce, but can regenerate cell processes if the cell body is intact.

    • Lack of reproductive ability explains the severity of nervous system injuries.

  • Neuroglia (Glial Cells):

    • Support, protect, and maintain neurons; outnumber neurons approximately 10:1.

    • Not directly involved in information transmission but are essential infrastructure for neuronal function.

Structure of a Neuron

  • Divisions of Neuron Structure:

    • Central cell body (soma/perikaryon).

    • Two types of processes:

    1. Dendrites:

      • Afferent processes that receive stimuli and conduct them toward the cell body.

      • May have modifications that allow them to act as sensory receptors.

      • Typically short, numerous, and branched (tree-like appearance).

    2. Axons:

      • Efferent processes that conduct impulses away from the cell body.

      • Can be extremely long; known as nerve fibers in bundled forms.

      • May be covered by myelin sheaths for faster conduction.

  • Myelin:

    • Fatty substance covering some axons, appearing white (white matter).

    • Enhances conduction speed, particularly through saltatory conduction at nodes of Ranvier.

Organization of the Nervous System

Anatomical Location: CNS vs. PNS

  • CNS:

    • Composed of brain and spinal cord.

  • PNS:

    • Comprised of nerves extending from the CNS to the periphery of the body.

  • Cranial nerves originate from the brain; spinal nerves emerge from the spinal cord.

Direction of Impulses: Afferent vs. Efferent

  • Afferent Nerve Fibers:

    • Conduct nerve impulses toward the CNS (sensory fibers).

  • Efferent Nerve Fibers:

    • Conduct impulses away from the CNS (motor fibers).

  • Mixed nerves contain both sensory and motor fibers; sensory nerves contain only sensory fibers; motor nerves contain only motor fibers.

Function: Autonomic vs. Somatic

  • Somatic Nervous System:

    • Involves voluntary control of skeletal muscles.

    • Afferent impulses from sensory receptors (skin, muscles) are consciously perceived.

  • Autonomic Nervous System:

    • Controls involuntary functions (e.g., digestion, heart rate).

    • Comprised of motor nerves to smooth muscle, cardiac muscle, and glands.

Neuron Function: Depolarization and Repolarization

Resting State and Membrane Potential

  • Resting Neuron:

    • It maintains polarization with a resting membrane potential (~ -70 mV).

    • Sodium-potassium pump: pumps sodium ions (Na⁺) out and potassium ions (K⁺) in, keeping the interior negatively charged.

Depolarization Process

  • Triggered by stimulation, causing sodium channels to open, allowing Na⁺ to flow into the neuron, thus reversing the charge.

  • Leads to an action potential which is the electrical signal transmitted down the axon.

Repolarization Process

  • After depolarization, sodium channels close and potassium channels open, allowing K⁺ to exit the cell, restoring negativity inside.

  • The sodium-potassium pump resets ion distributions after repolarization.

Depolarization Threshold and Action Potential

  • A stimulus must reach a threshold to provoke a nerve impulse.

  • All-or-Nothing Principle:

    • If a nerve impulse is generated, it is conducted along the entire neuron at the same strength.

Refractory Period

  • During and post-impulse, a neuron cannot fire again (absolute refractory), but can respond to a stronger stimulus (relative refractory).

Saltatory Conduction

  • In myelinated axons, action potentials jump between nodes of Ranvier, significantly speeding up conduction.

Synaptic Transmission

Structure of the Synapse

  • Synapse: Junction between two neurons or between a neuron and a target cell (presynaptic and postsynaptic neurons).

Neurotransmitter Release and Function

  • Neurotransmitters are released from synaptic knobs after depolarization, triggering changes in the postsynaptic cell via receptor binding.

Types of Neurotransmitters

  • Excitatory Neurotransmitters:

    • Cause depolarization in postsynaptic cells (e.g., acetylcholine).

  • Inhibitory Neurotransmitters:

    • Cause hyperpolarization in postsynaptic cells (e.g., GABA).

  • Effects depend on specific receptors present in the postsynaptic cell.