Study Guide: Nervous System Structure and Function

Nervous System Overview

Divisions of the Nervous System

The nervous system is divided into three main parts, each responsible for specific functions and associated with distinct organs and tissues.

• Central Nervous System (CNS): Includes the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

• Enteric Nervous System (ENS): Governs the function of the gastrointestinal tract; sometimes considered part of the PNS.

Subdivisions of the Peripheral Nervous System

The PNS is further divided based on the types of neurons and their functions.

• Sensory Division: Transmits sensory information from receptors to the CNS.

• Motor Division: Transmits motor commands from the CNS to effectors (muscles and glands).

• Somatic vs. Visceral:

  • Somatic: Controls voluntary movements (skeletal muscles).

  • Visceral: Controls involuntary functions (smooth muscle, cardiac muscle, glands).

Sensory Receptors and Neuron Function

Types of Sensory Receptors

Sensory receptors detect changes in the environment and relay information to the nervous system.

• Interoceptors: Monitor internal body conditions (e.g., blood pressure, pH).

• Proprioceptors: Detect body position and movement (e.g., muscle stretch).

• Exteroceptors: Sense external stimuli (e.g., touch, temperature, pain).

Interneurons

Interneurons connect sensory and motor neurons within the CNS, facilitating communication and integration of information.

Types of Motor Effectors

• Somatic Effectors: Control voluntary muscles.

• Visceral Effectors: Control involuntary muscles and glands.

Neuron Structure and Classification

Regions of a Neuron

Neurons have specialized regions for receiving, processing, and transmitting signals.

• Dendrites: Receive incoming signals.

• Cell Body (Soma): Contains the nucleus and organelles; integrates signals.

• Axon: Transmits electrical impulses away from the cell body.

Synapse: The junction where a neuron communicates with another cell.

• Types of Synapses:

  • Electrical Synapse: Direct flow of ions between cells.

  • Chemical Synapse: Neurotransmitter-mediated communication.

Anatomical Classes of Neurons

Neurons are classified based on their structure and function.

• Anaxonic: No distinct axon; found in the brain.

• Bipolar: One axon and one dendrite; found in sensory organs.

• Unipolar: Single process; common in sensory neurons.

• Multipolar: One axon, multiple dendrites; most common type in CNS.

Neuroglia (Glial Cells)

Neuroglia support and protect neurons. Their functions vary between the CNS and PNS.

• CNS Neuroglia:

  1. Astrocytes: Maintain blood-brain barrier, support neurons.

  2. Oligodendrocytes: Form myelin sheaths in CNS.

  3. Microglia: Act as immune cells.

  4. Ependymal Cells: Produce cerebrospinal fluid.

• PNS Neuroglia:

  1. Schwann Cells: Form myelin sheaths in PNS.

  2. Satellite Cells: Support neuron cell bodies in ganglia.

Myelination and Nervous Tissue

Myelinated vs. Unmyelinated Neurons

Myelination affects the speed and efficiency of nerve impulse transmission.

• Myelinated Neurons: Conduct impulses rapidly via saltatory conduction.

• Unmyelinated Neurons: Conduct impulses slowly via continuous conduction.

White Matter vs. Grey Matter

• White Matter: Composed of myelinated axons; responsible for signal transmission.

• Grey Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; involved in processing and integration.

Membrane Potentials and Ion Channels

Membrane Potential

The membrane potential is the electrical difference across a neuron's plasma membrane.

• Resting Membrane Potential: The stable voltage in a resting neuron, typically around -70 mV.

• Maintenance: Maintained by ion gradients and selective permeability.

Gradients

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Electrochemical Gradient: Combined effect of chemical and electrical gradients.

Types of Ion Channels

Ion channels regulate the movement of ions across the membrane, affecting membrane potential.

• Leak Channels: Always open; maintain resting potential.

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically-Gated Channels: Open in response to binding of specific molecules.

Graded and Action Potentials

Graded Potentials

Graded potentials are small changes in membrane potential that occur in response to stimuli.

• Depolarization: Membrane potential becomes less negative.

• Local Current: Movement of ions that spreads the graded potential.

• Distance Effect: Graded potentials decrease with distance from the stimulus.

Action Potential

An action potential is a rapid, large change in membrane potential that propagates along the axon.

• Generation Steps:

  1. Resting state

  2. Depolarization

  3. Repolarization

  4. Hyperpolarization

  5. Return to resting state

• Refractory Periods:

  • Absolute Refractory Period: No new action potential can be generated.

  • Relative Refractory Period: A stronger stimulus can generate an action potential.

• Propagation:

  • Continuous Propagation: Occurs in unmyelinated axons.

  • Saltatory Propagation: Occurs in myelinated axons; faster due to jumping between nodes of Ranvier.

Synaptic Transmission

Synapse Types and Function

Synapses are specialized junctions for communication between neurons.

• Cholinergic Synapse: Uses acetylcholine as neurotransmitter.

• Synaptic Fatigue: Temporary inability to transmit signals due to neurotransmitter depletion.

• Synaptic Delay: Time required for neurotransmitter release and binding.

Information Processing in Neurons

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing likelihood of action potential.

Summation

• Temporal Summation: Multiple signals in quick succession add together.

• Spatial Summation: Signals from multiple locations add together.

Neurotransmitters and Effects

Direct and Indirect Effects

• Direct Effects: Neurotransmitter binds directly to ion channel, causing immediate change (e.g., acetylcholine).

• Indirect Effects: Neurotransmitter activates second messenger pathways, leading to changes in ion channel activity (e.g., epinephrine).

Key Equations

• Nernst Equation: Used to calculate equilibrium potential for an ion: $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Ohm's Law (for membrane current): $I = g(V_m - E_{ion})$

Comparison Table: Myelinated vs. Unmyelinated Neurons

Feature	Myelinated Neuron	Unmyelinated Neuron
Conduction Speed	Fast (saltatory)	Slow (continuous)
Energy Efficiency	High	Low
Location	White matter	Grey matter

Additional info: Some content and examples were inferred based on standard Anatomy & Physiology curriculum and the structure of the provided questions.