Nervous Tissue: Structure, Function, and Physiology (BIO 2111 – A&P I, Chapter 11 Study Guide)

Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts. It is divided into central and peripheral components, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and coordinates activity throughout the body.

• Peripheral Nervous System (PNS): Includes all nervous tissue outside the CNS, except for the enteric nervous system. It connects the CNS to limbs and organs.

Functional Divisions of the Nervous System

• Sensory (Afferent) Division: Brings information to the CNS from receptors in peripheral tissues and organs. Detects stimuli such as touch, pain, temperature, and internal conditions.

• Motor (Efferent) Division: Carries motor commands from the CNS to peripheral tissues and systems, enabling responses.

Somatic and Autonomic Nervous Systems

• Somatic Nervous System (SNS): Also called the voluntary nervous system. Allows conscious control of movement and carries sensory information to the CNS.

• Autonomic Nervous System (ANS): Regulates involuntary activities such as smooth muscle, cardiac muscle, and glands. Includes the sympathetic and parasympathetic divisions.

Sympathetic vs. Parasympathetic Divisions

• Sympathetic Division: Prepares the body for stress ("fight or flight"). Increases heart rate, breathing rate, and blood flow; decreases digestion and energy storage.

• Parasympathetic Division: Promotes "rest and digest" activities. Calms the body, conserves energy, slows heart rate and breathing, and stimulates digestion.

Structure of a Neuron

Neurons are the functional units of the nervous system, specialized for communication.

• Cell Body (Soma): Contains the nucleus and organelles. The perikaryon is the cytoplasm around the nucleus, containing organelles for energy and neurotransmitter synthesis.

• Neurofilaments and Neurofibrils: Provide structural support within the neuron.

• Dendrites: Highly branched processes that receive information, primarily at dendritic spines.

• Axon: Long process that transmits signals away from the cell body. Includes:

  • Axon Hillock: Origin of the axon from the cell body.

  • Initial Segment: Site where action potentials are initiated.

  • Axolemma: Specialized plasma membrane surrounding the axon.

  • Axoplasm: Cytoplasm of the axon, containing organelles and enzymes.

• Telodendria: Fine extensions at the end of the axon, terminating at axon terminals (synaptic terminals) for communication with other cells.

Myelin and Nodes of Ranvier

• Myelin: Fatty insulating layer around axons, produced by Schwann cells (PNS) and oligodendrocytes (CNS). Increases the speed of nerve impulse conduction.

• Nodes of Ranvier: Gaps between myelin segments along the axon. Enable saltatory conduction, allowing action potentials to jump from node to node, greatly increasing transmission speed.

Nervous System Terminology

Term	Location	Description
Nuclei	CNS	Clusters of neuron cell bodies
Ganglia	PNS	Clusters of neuron cell bodies
Tracts	CNS	Bundles of axons; carry signals up/down brain and spinal cord
Nerves	PNS	Bundles of axons; carry sensory/motor signals between body and CNS
White Matter	CNS/PNS	Mostly myelinated axons; fast signal conduction
Gray Matter	CNS/PNS	Cell bodies, dendrites, unmyelinated axons; processing/integration

Functional Classes of Neurons

• Sensory (Afferent) Neurons: Carry information from receptors to CNS; detect internal and external stimuli.

• Interneurons (Association Neurons): Located within CNS; process, integrate, and relay information.

• Motor (Efferent) Neurons: Carry commands from CNS to effectors (muscles/glands).

Receptors and Afferent Neurons

• Exteroceptors: Respond to external stimuli (e.g., touch, temperature).

• Interoceptors: Respond to internal conditions (e.g., pH, blood pressure).

• Proprioceptors: Detect body position and movement.

• Somatic Sensory Neurons: Carry signals from skin, muscles, joints.

• Visceral Sensory Neurons: Carry signals from internal organs.

Efferent Neurons and Effectors

• Somatic Motor Neurons: Control skeletal muscle (voluntary movement).

• Autonomic Motor Neurons: Control smooth muscle, cardiac muscle, glands (involuntary responses).

Neuroglial (Glial) Cells

Neuroglia support and protect neurons, making up about half the volume of the nervous system.

Cell Type	Location	Function
Astrocytes	CNS	Support neurons, form blood-brain barrier, regulate nutrients/ions
Oligodendrocytes	CNS	Form myelin sheath, increase impulse speed
Microglia	CNS	Immune defense, remove debris/pathogens
Ependymal Cells	CNS (ventricles, central canal)	Produce/circulate cerebrospinal fluid
Schwann Cells	PNS	Form myelin sheath, aid axon regeneration
Satellite Cells	PNS (ganglia)	Support/regulate environment around neurons

Membrane Potentials: Resting, Graded, and Action Potentials

• Resting Membrane Potential: The electrical potential across the membrane of an undisturbed cell (typically -70 mV in neurons).

• Graded Potential: Temporary, localized change in membrane potential. Decreases with distance from the stimulus. Can be depolarizing or hyperpolarizing.

• Action Potential: Large, rapid change in membrane potential that propagates along the axon. Triggered if graded potential reaches threshold (about -55 mV).

Ion Gradients and Equilibrium Potentials

• Chemical Gradient: Difference in concentration of a substance across a membrane (e.g., more Na+ outside than inside).

• Electrical Gradient: Difference in charge across a membrane (e.g., inside of neuron is more negative).

• Electrochemical Gradient: Combined effect of chemical and electrical gradients; determines ion movement direction.

Equilibrium Potential: The membrane potential at which there is no net movement of a particular ion across the membrane.

• Sodium (Na+): Equilibrium potential ≈ +60 mV. Both gradients drive Na+ into the cell.

• Potassium (K+): Equilibrium potential ≈ -90 mV. Chemical gradient pushes K+ out; electrical gradient pulls it in.

Gated Ion Channels

• Chemically Gated Channels: Open when specific chemicals (e.g., neurotransmitters) bind. Abundant on dendrites and cell body.

• Voltage-Gated Channels: Open/close in response to changes in membrane potential. Found in excitable membranes (axon, sarcolemma).

• Mechanically Gated Channels: Open in response to physical distortion (stretch, pressure, vibration). Important in sensory receptors.

Phases of Membrane Potential Changes

• Depolarization: Membrane potential becomes less negative (more positive), usually due to Na+ influx.

• Repolarization: Return to resting negative value, typically due to K+ efflux.

• Hyperpolarization: Membrane potential becomes more negative than resting, often due to excess K+ leaving the cell.

• Refractory Period: Time during which a neuron cannot (absolute) or is less likely (relative) to fire another action potential.

Graded Potentials and Action Potentials

• Graded Potentials: Local changes in membrane potential; can be depolarizing or hyperpolarizing. Occur in dendrites and cell body. If strong enough to reach threshold at the axon hillock, they trigger an action potential.

• Action Potentials: All-or-none electrical events that propagate along the axon. Initiated if threshold is reached, opening voltage-gated Na+ channels.

Summation: Temporal and Spatial

• Temporal Summation: Multiple graded potentials from the same neuron in rapid succession add together over time.

• Spatial Summation: Graded potentials from multiple neurons at different locations combine at the same time.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons. Action potential moves in small steps along the axolemma. Each segment depolarizes and repolarizes sequentially.

• Saltatory Propagation: Occurs in myelinated axons. Action potential "jumps" from node to node (nodes of Ranvier), greatly increasing conduction speed.

Comparison of Propagation Types

Type	Location	Speed	Mechanism
Continuous	Unmyelinated axons	Slower	Stepwise depolarization along entire axon
Saltatory	Myelinated axons	Faster	Action potential jumps between nodes of Ranvier

Key Equations

• Nernst Equation (for equilibrium potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Resting Membrane Potential (Goldman-Hodgkin-Katz equation):

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Example

• Example of Saltatory Conduction: In a myelinated axon, an action potential generated at the initial segment causes local currents that depolarize the next node of Ranvier, skipping the myelinated internodes and resulting in rapid signal transmission.

Additional info: The above notes expand on the study guide by providing definitions, context, and key equations relevant to nervous tissue physiology, suitable for exam preparation in an introductory Anatomy & Physiology course.