Anatomy & Physiology Study Guide: Nervous System Structure and Function

Nervous System Overview

Main Components of the Nervous System

The nervous system is a complex network responsible for communication and control throughout the body. It is divided into central and peripheral components, each with specialized functions.

• Brain and Spinal Cord: Central processing units of the nervous system.

• Receptors of Sense Organs: Specialized cells that detect stimuli.

• Nerves: Connect the central nervous system (CNS) to other body systems.

Types of Nervous Cells

• Neurons: Functional units of the nervous system, actively sending signals.

• Neuroglia (Glial Cells): Supporting cells, packing material to hold neurons in place, recycling neurotransmitters, and maintaining the environment.

Organization of the Nervous System

Categories Based on Function or Structure

• Central Nervous System (CNS): Brain and spinal cord; responsible for information processing and integration.

• Peripheral Nervous System (PNS): Nerves outside the CNS, including sensory input and motor commands.

• Enteric Nervous System: Network in the gut, directly connected to the CNS.

Divisions of the Peripheral Nervous System

• Afferent Division: Sends sensory information from receptors to the CNS.

  • Special Sensory Receptors: Senses (vision, hearing, taste, smell).

  • Visceral Sensory Receptors: Internal organs.

  • Somatic Sensory Receptors: Skeletal muscles, joints, skin.

• Efferent Division: Sends motor commands from CNS to effectors.

  • Somatic Nervous System: Skeletal muscle control.

  • Autonomic Nervous System: Involuntary and subconscious control.

    • Parasympathetic Division: Rest and digest.

    • Sympathetic Division: Fight or flight.

Neuron Structure and Function

Basic Anatomy of a Neuron

• Soma: Cell body containing the nucleus.

• Nissl Bodies: Protein synthesis of neurotransmitters.

• Dendrite: Receives signals from other neurons and transmits them to the cell body.

• Axon Hillock: Connection point between soma and axon; integrates signals.

• Axon: Conducts action potentials; cytoskeleton runs the length of the axon.

• Axolemma: Excitable membrane of the axon; regulates flow of ions.

• Axon Terminals: Transmit signals to postsynaptic cells by releasing neurotransmitters.

Types of Neurons (Structural Classification)

• Anaxonic Neuron: No axon, just cell body and dendrites; found in special sensory organs.

• Bipolar Neuron: Two processes outside cell body; one axon and one dendrite; found in special sensory organs.

• Unipolar Neuron: Single process; cell body off to the side; sensory neurons of the PNS.

• Multipolar Neuron: Cell body, lots of dendrites, one axon; most common type, especially motor neurons.

Types of Neurons (Functional Classification)

• Interneuron (Multipolar): Processes information from sensory neurons and coordinates motor output.

• Sensory Neuron (Unipolar): Picks up information from receptors and sends it to the CNS.

• Motor Neuron (Multipolar): Sends commands from CNS to effectors (muscles, glands).

Neuroglia (Glial Cells)

Central Nervous System Neuroglia

• Astrocytes: Abundant neuroglia; hold neurons in place, recycle neurotransmitters, maintain blood-brain barrier.

• Ependymal Cells: Line ventricles of the brain and central canal in spinal cord; create and regulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Create myelin, myelinate CNS axons, provide structural framework.

• Microglia: Clean up debris, migrate through neural tissue, remove cell debris and pathogens by phagocytosis.

Peripheral Nervous System Neuroglia

• Satellite Cells: Surround cell bodies of neurons in ganglia; regulate oxygen, nutrients, and neurotransmitter levels.

• Schwann Cells: Surround all axons in PNS; myelinate peripheral axons; assist in repair after injury.

Gray Matter vs. White Matter

• Gray Matter: Regions where nuclei of neurons are congregating.

• White Matter: Insulates axons, making them appear white.

Myelinated vs. Unmyelinated Neurons

• Myelinated Neurons: Axons are surrounded by myelin, called nodes; allows for faster signal transmission.

• Unmyelinated Neurons: Axons lack myelin, slower signal transmission.

Neuron Injury and Repair

• Fragmentation: Axon and myelin break up in the distal stump.

• Schwann Cells: Divide and form cord, guiding axon regrowth.

• Macrophages: Remove debris from injury site.

• Axon Regrowth: Schwann cells guide axon to original location.

Membrane Transport and Resting Membrane Potential

Types of Solute Movement

• Passive Transporters (with open channels): Driven by concentration gradient; includes simple diffusion and facilitated diffusion.

• Passive Transporters (with voltage-sensitive gated channels): Requires a stimulus to open channels.

• Active Transporters: Move against gradient, require energy (ATP).

Key Example: Sodium-Potassium Pump

• Actively transports 3 sodium ions out and 2 potassium ions in.

• Maintains concentration gradients of sodium and potassium across the plasma membrane.

$\text{Na}^+/\text{K}^+ \text{ Exchange Pump:}~3~\text{Na}^+~\text{out},~2~\text{K}^+~\text{in}$

Membrane Permeability

• More permeable to potassium than sodium.

• Negative charge inside the cell due to protein molecules and potassium efflux.

Electrochemical Gradient

• Combined effect of ion concentration (chemical gradient) and electrical difference (electrical gradient) across the cell membrane.

• Resting membrane potential: inside of neuron is negatively charged compared to outside; typical value is -70 mV.

Graded Potentials and Action Potentials

Graded Potentials

• Small, localized changes in membrane potential.

• Caused by opening of ligand- or mechanically-gated ion channels in response to a stimulus.

• Can be depolarizing or hyperpolarizing.

• Do not fade or travel long distances; action potentials are always the same size and travel long distances.

Action Potentials

• Generated at the axon hillock when threshold is reached.

• Voltage-gated sodium channels open, sodium rushes into the cell, depolarizing the membrane.

• Potassium channels open, potassium leaves the cell, repolarizing the membrane.

• Action potential propagates along the axon like a wave.

$\text{Resting membrane potential} \approx -70~\text{mV}$

Synaptic Transmission

Synapse Structure and Function

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives neurotransmitters, which bind to receptors and initiate a response.

• Neurotransmitters: Chemical messengers (e.g., acetylcholine, dopamine) that transmit signals across the synapse.

Summary Table: Types of Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier, recycle neurotransmitters
Ependymal Cells	CNS	Produce and regulate CSF
Oligodendrocytes	CNS	Myelinate CNS axons
Microglia	CNS	Phagocytosis of debris and pathogens
Satellite Cells	PNS	Regulate environment around neuron cell bodies
Schwann Cells	PNS	Myelinate PNS axons, assist in repair

Key Terms and Definitions

• Neuron: Cell specialized for communication via electrical impulses.

• Neuroglia: Supporting cells in nervous tissue.

• Action Potential: Rapid change in membrane potential that travels along the axon.

• Graded Potential: Localized change in membrane potential.

• Synapse: Junction between two neurons.

• Myelin: Insulating layer around axons, increases speed of signal transmission.

• Resting Membrane Potential: The electrical potential difference across the membrane of a resting neuron.

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.