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Chapter 12: Nervous Tissue and the Nervous System – Mini-Study Guide

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Nervous System Overview

Structures of the Nervous System

The nervous system is composed of specialized organs and tissues responsible for communication and control throughout the body.

  • Organs: Brain and spinal cord

  • Other Structures:

    • Nerves: Connect the nervous system with other systems

    • Sensory receptors: Detect sensations (touch, temperature, vision, taste, etc.)

  • Neural tissue: Contains two main cell types:

    • Neurons: Cells that send and receive signals for intercellular communication

    • Neuroglia (glial cells): Support and protect neurons; essential for neuron survival and function

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, containing nervous tissue, connective tissue, and blood vessels.

  • Functions:

    • Receives, processes, and coordinates sensory information

    • Issues motor commands to control peripheral organs

    • Higher functions: intelligence, memory, learning, emotion

Peripheral Nervous System (PNS)

The PNS includes all neural tissue outside the CNS, such as cranial nerves, spinal nerves, and sensory receptors.

  • Functions:

    • Delivers sensory information to the CNS

    • Carries motor commands from CNS to peripheral tissues and systems

  • Divisions:

    • Sensory (Afferent) Division: Carries sensory information from receptors to CNS

    • Motor (Efferent) Division: Carries motor commands from CNS to effectors (muscles, glands, adipose tissue)

  • Motor Division Subdivisions:

    • Somatic Nervous System (SNS): Controls skeletal muscle contractions (voluntary and involuntary)

    • Autonomic Nervous System (ANS): Controls subconscious actions (smooth/cardiac muscle, glands)

      • Sympathetic Division: "Fight or flight"

      • Parasympathetic Division: "Resting & digesting"

      • Enteric Nervous System: "Brain of the gut"; controls digestive functions independently of CNS

Neurons

Structure and Function

Neurons are the basic functional units of the nervous system, responsible for communication and information processing.

  • Cell Body (Soma):

    • Perikaryon: Cytoplasm

    • Contains nucleus, nucleolus, mitochondria

    • Nissl bodies: Dense areas of RER and ribosomes; produce proteins and neurotransmitters; give "gray matter" its color

    • Cytoskeleton: Neurofilaments and neurotubules; support dendrites and axon; involved in axoplasmic transport

    • Axon hillock: Connects axon to cell body; action potential generator

  • Cell Processes:

    • Dendrites: Short, branched; receive information from other neurons

    • Axon: Single, long process; carries action potentials to targets

      • Axoplasm: Cytoplasm of axon

      • Axolemma: Specialized plasma membrane

      • Initial segment: Attaches axon to axon hillock

      • Collaterals: Branches of axon

      • Telodendria: Fine extensions at distal axon

      • Axon terminals: Tips of telodendria; synaptic terminals

Structural Classification of Neurons

  • Anaxonic neurons: Small, processes look similar; found in brain and sense organs

  • Unipolar neurons: Axon and dendrite fused; cell body off to side; most sensory neurons in PNS

  • Bipolar neurons: One dendrite, one axon; rare; found in special sensory organs

  • Multipolar neurons: One long axon, two or more dendrites; most common in CNS; all motor neurons controlling skeletal muscle

Functional Classification of Neurons

  • Sensory (afferent) neurons: Deliver information from sensory receptors to CNS; typically unipolar or bipolar; cell bodies in sensory ganglia

    • Somatic sensory neurons: Monitor external environment

    • Visceral sensory neurons: Monitor internal environment

  • Interneurons: Located in brain and spinal cord; multipolar; connect sensory and motor neurons; responsible for distribution of sensory info, coordination of motor activity, and higher functions (memory, planning, learning)

  • Motor (efferent) neurons: Deliver instructions from CNS to effectors

    • Somatic motor neurons: Innervate skeletal muscles; multipolar

    • Visceral motor neurons: Innervate smooth/cardiac muscle, glands, adipose tissue

Neuroglia (Glial Cells)

Types and Functions

Neuroglia support and protect neurons, preserving the structure and function of neural tissue. They make up half the volume of the nervous system.

Neuroglia in the CNS

  • Astrocytes: Maintain blood–brain barrier, create framework, repair tissue, guide development, control environment

  • Oligodendrocytes: Myelinate CNS axons; form myelin sheath; increase speed of action potentials; create white matter

  • Ependymal cells: Line central canal and ventricles; produce and monitor cerebrospinal fluid (CSF); cilia/microvilli circulate CSF

  • Microglia: Smallest, least numerous; migrate and clean up debris, waste, pathogens

Neuroglia in the PNS

  • Satellite cells: Surround and protect neuron cell bodies in ganglia; regulate interstitial fluid

  • Schwann cells: Myelinate peripheral axons; one cell sheaths one segment; outer surface is neurilemma

White Matter vs. Gray Matter

  • White matter: Regions with many myelinated axons; white color due to myelin

  • Gray matter: Regions with cell bodies, unmyelinated axons, dendrites; gray color due to Nissl bodies

Neural Responses to Injury

  • PNS: Wallerian degeneration (distal axon degenerates), Schwann cells form path, axon regrows, Schwann cells remyelinate

  • CNS: Limited regeneration due to astrocyte-released chemicals and scar tissue

Membrane Potential

Concepts and Mechanisms

Membrane potential is the difference in electrical charge across the cell membrane, crucial for neuron signaling.

  • Measured in millivolts (mV)

  • Key ions: Sodium (Na+), Potassium (K+)

  • Current: Movement of ions

  • Potential difference: Separation of charges by membrane barrier

  • Factors establishing membrane potential:

    • ECF and cytosol differ in ionic composition (Na+ high outside, K+ high inside)

    • Selective permeability; leak channels (always open), gated channels (open in response to stimuli)

    • Membrane more permeable to K+ than Na+

    • Passive forces (chemical/electrical gradients), active forces (sodium-potassium pump)

Resting Membrane Potential

  • Typical value: -70 mV

  • More permeable to K+ (more K+ leak channels)

  • Passive and active forces balance at rest

  • Sodium-potassium pump: Ejects 3 Na+ for every 2 K+ brought in

  • Equilibrium potentials:

    • If freely permeable to Na+: +66 mV

    • If freely permeable to K+: -90 mV

Graded Potentials

  • Localized changes in membrane potential

  • Caused by opening of gated channels (chemical, mechanical, voltage-gated)

  • Depolarization: Opening Na+ channels; membrane potential becomes more positive

  • Repolarization: Return to resting potential after stimulus removed

  • Hyperpolarization: Opening K+ channels; membrane potential becomes more negative

  • Change proportional to stimulus size; effects diminish with distance

Action Potential

Generation and Propagation

Action potentials are nerve impulses propagated along axons, linking graded potentials at the cell body with actions at the axon terminal.

  • Threshold: -60 to -55 mV; must be reached to trigger action potential

  • All-or-none principle: Action potential is always the same magnitude if threshold is exceeded

  • Steps:

    1. Voltage-gated Na+ channels open; Na+ enters, depolarizing membrane

    2. At +30 mV, Na+ channels inactivate; K+ channels open, K+ exits (repolarization)

    3. Brief hyperpolarization as K+ channels close with lag

  • Refractory Period:

    • Absolute: No new action potential possible

    • Relative: Only strong stimulus can trigger action potential

  • Sodium-potassium pump: Restores ion gradients after action potential

Propagation Methods

  • Continuous propagation: Unmyelinated axons; slow (1 m/sec); sequential segments

  • Saltatory propagation: Myelinated axons; fast; depolarization only at nodes (Nodes of Ranvier); local current "jumps" between nodes

Axon Fiber Types

Type

Diameter

Myelination

Speed

Function

Type A

Large

Myelinated

120-140 m/sec

Critical, time-dependent info (sensory, motor)

Type B

Medium

Myelinated

18 m/sec

Non-urgent sensory, peripheral effectors

Type C

Small

Unmyelinated

1 m/sec

Non-urgent sensory, peripheral effectors

Synapses

Types and Mechanisms

A synapse is a specialized site where a neuron communicates with another cell.

  • Presynaptic neuron: Sends the message

  • Postsynaptic cell: Receives the message; may be neuron, muscle (neuromuscular junction), or gland (neuroglandular junction)

  • Types:

    • Electrical synapses: Direct contact via gap junctions; ions pass directly; found in brain, eye, ciliary ganglia

    • Chemical synapses: Most abundant; separated by synaptic cleft; neurotransmitters transmit signal

  • Steps at chemical synapse:

    1. Action potential arrives at axon terminal

    2. Voltage-gated Ca2+ channels open; Ca2+ enters

    3. Ca2+ triggers exocytosis of synaptic vesicles

    4. Neurotransmitter diffuses across cleft; binds to postsynaptic receptors

    5. Neurotransmitter broken down by enzyme; recycled

  • Synaptic delay: 0.2–0.5 msec; due to Ca2+ influx and neurotransmitter release

  • Synaptic fatigue: Neurotransmitter cannot recycle fast enough during intense stimuli

Neurotransmitters

Classes and Functions

Neurotransmitters are chemical messengers released by neurons to transmit signals across synapses.

  • Functional classes:

    • Excitatory: Cause depolarization; promote action potentials

    • Inhibitory: Cause hyperpolarization; suppress action potentials

  • Effect depends on receptor type, not just neurotransmitter

  • Structural classes:

    • Biogenic amines: Norepinephrine (NE), dopamine, serotonin

    • Amino acids: Gamma aminobutyric acid (GABA)

    • Neuropeptides: Neuromodulators (e.g., opioids like endorphins)

    • Dissolved gases: Nitric oxide (NO), carbon monoxide (CO)

    • Acetylcholine (ACh): Common in CNS and PNS; all neuromuscular junctions use ACh

Information Processing

Postsynaptic Potentials and Summation

Neurons integrate information from thousands of synapses, determining whether to generate an action potential.

  • Postsynaptic potentials:

    • EPSP (excitatory postsynaptic potential): Graded depolarization

    • IPSP (inhibitory postsynaptic potential): Graded hyperpolarization

  • Summation: EPSPs and IPSPs combine to produce net effect

    • Temporal summation: Rapid, repeated stimuli at same synapse

    • Spatial summation: Multiple stimuli at different synapses

  • Facilitation: Net effect brings cell closer to threshold

  • Inhibition: Net effect brings cell farther from threshold

  • Neuromodulators and hormones: Can alter membrane sensitivity, shifting balance between EPSPs and IPSPs

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