Chapter 12

Introduction to Nervous Tissue

  • Chapter 12 focuses on the nervous system as an introductory chapter.
  • Function of the nervous system in the human body:
    • Allows communication and control of other systems.
    • Collects information through receptors that detect stimuli.
    • Processes, integrates, and evaluates information to determine a response.
    • Initiates responses via effectors (muscles or glands).
  • Key components:
    • Sensory neurons: bring information into the CNS.
    • Central Nervous System (CNS): consists of the brain and spinal cord.
    • Peripheral Nervous System (PNS): consists of nerves and ganglia.

Nervous System Organization

  • Structurally organized into:
    • CNS: brain and spinal cord.
    • PNS: nerves and ganglia.
  • Defined cell types in nervous tissue:
    • Neurons: functional cells of nervous tissue.
    • Glial cells: supportive cells.
  • Parts of a neuron:
    • Dendrites: receiving segment.
    • Cell body: contains nucleus and organelles.
    • Axon: relays signals away from the cell body.
  • Understanding the terminology:
    • Nerves: bundles of axons.
    • Ganglia: bundles of cell bodies.

Functional Organization of the Nervous System

  • Divided into sensory (afferent) and motor (efferent) divisions:
    • Sensory nervous system:
    • Receives sensory information from receptors.
    • Divided into:
      • Somatic sensory: consciously perceived stimuli (sight, hearing, etc.).
      • Visceral sensory: internal stimuli (not consciously perceived).
    • Motor nervous system:
    • Commands leaving the CNS.
    • Divided into:
      • Somatic motor: voluntary signals to skeletal muscles.
      • Autonomic motor (visceral motor): signals to cardiac muscle, smooth muscle, and glands.
  • Autonomic nervous system has two branches:
    • Sympathetic: fight or flight response.
    • Parasympathetic: rest and digest, balances homeostasis.

Peripheral Nervous System Structure

  • Nerves in PNS: bundles of parallel axons wrapped in connective tissue.
    • Epineurium: outer covering (dense irregular connective tissue).
    • Fascicles: bundles of axons wrapped in perineurium.
    • Endoneurium: wraps each individual axon (areolar connective tissue).
  • Nerves are well vascularized, requiring significant ATP for operation.

Synapses

  • Definition: connection point between neurons or between a neuron and an effector.
  • Two types of synapses:
    • Chemical synapse: most common.
    • Utilizes neurotransmitters.
    • Involves pre-synaptic neuron, synaptic cleft, and post-synaptic neuron with receptors.
    • Electrical synapse: less common, faster neural communication via gap junctions.

Neurotransmitter Communication

  • Process at chemical synapse involves:
    • Release of neurotransmitters into synaptic cleft.
    • Binding to post-synaptic receptors and initiating graded potentials.
  • Graded potentials result from neurotransmitter influence:
    • Depolarization: makes the inside of the neuron more positive.
    • Hyperpolarization: makes the inside of the neuron more negative.

Ion Channels and Membrane Potentials

  • Pumps and channels play vital roles in maintaining resting membrane potential:
    • Sodium-potassium pump: pumps 3 sodium out and 2 potassium in, maintaining concentration gradients.
    • Leak channels allow ions to move down their concentration gradient.
    • Types of channels:
    • Leak channels: continuously open, allowing diffusion.
    • Chemically gated channels: open upon neurotransmitter binding.
    • Voltage-gated channels: open in response to changes in electrical charge.
    • Modality-gated channels: respond to sensory stimuli (e.g., light, pressure).
  • Resting membrane potential: typically around -70 mV.
    • Threshold potential: around -55 mV where an action potential can be generated.

Action Potentials

  • Occur when graded potentials sum to reach threshold:
    • All-or-none principle: if threshold is reached, an action potential is fired.
  • Phases of action potential:
    • Depolarization: influx of sodium ions due to opening of voltage-gated sodium channels.
    • Repolarization: efflux of potassium ions as voltage-gated potassium channels open.
    • Hyperpolarization: sometimes occurs as potassium channels remain open longer than necessary.

Propagation of Action Potentials

  • Sequential change occurs along the axon in two main conduction types:
    • Continuous conduction: occurs in unmyelinated axons.
    • Saltatory conduction: occurs in myelinated axons with nodes of Ranvier; faster than continuous conduction.
  • Important for understanding conduction speed:
    • Larger diameter axons conduct faster.

Synaptic Transmission

  • At the transmissive segment:
    • Voltage-gated calcium channels open, allowing calcium influx.
    • Triggers pre-synaptic vesicles to perform exocytosis, releasing neurotransmitters into the synaptic cleft.
  • Importance of neurotransmitters:
    • Bind to target cell receptors, causing a physiological response.
    • Examples include acetylcholine, biogenic amines, amino acids, and neuropeptides.
  • Modulation of neurotransmitter effects:
    • Facilitation: increases response of postsynaptic neuron.
    • Inhibition: decreases response of postsynaptic neuron.

Summary and Study Tips

  • Review the physiological processes of neurons and create diagrams or concept maps for visual learning.
  • Understand the roles of different ion channels and neurotransmitters in neuronal action.
  • Revisit the importance of action potentials, graded potentials, threshold, and the all-or-none response in neural communication.
  • Pay attention to the distinction between graded and action potentials and their roles in neural activity.