Comprehensive Study Notes: Nervous System Structure and Function

Nervous System Overview

Central and Peripheral Nervous Systems

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS coordinates sensory data and motor commands, while the PNS delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• CNS: Brain and spinal cord

• PNS: All neural tissue outside the CNS

Somatic Nervous System

Function and Control

The somatic nervous system controls skeletal muscle contractions. These actions are typically under voluntary control, but some responses (e.g., reflexes, nociceptors for pain) can occur at the subconscious level.

• Reflexes: Automatic responses to stimuli

• Nociceptors: Pain receptors

Neurons: Structure and Classification

Specialized Cells

Neurons are specialized cells that do not divide and have a long life span. They are the functional units of the nervous system.

Structure of Neurons

• Perikaryon: Cytoplasm surrounding the nucleus

• Nissl bodies: Clusters of RER and free ribosomes (give a gray color)

• Dendrites: Slender processes that receive information

• Axon: Long cytoplasmic process that propagates electrical impulses (action potentials)

• Axoplasm: Cytoplasm of the axon

• Axolemma: Plasma membrane of the axon

• Axon hillock: Initial segment where electrical impulses begin

• Axon collaterals: Branches of an axon

• Telodendria: End branches of an axon

Axonal Transport

• Anterograde: Flow of materials from the cell body to the axon terminal

• Retrograde: Flow of materials from the axon terminal to the cell body

Structural Classification of Neurons

• Anaxonic: No obvious axons

• Bipolar: Two distinct processes

• Unipolar: Most sensory neurons

• Multipolar: Common in CNS; all motor neurons controlling skeletal muscle are multipolar

Neuroglia (Glial Cells)

Types and Functions

Neuroglia are supportive cells in the nervous system. They protect neurons and provide a supportive framework.

• Astrocytes: Largest and most numerous; star-shaped; maintain the blood-brain barrier; regulate the interstitial environment

• Ependymal cells: Assist in producing and monitoring CSF (cerebrospinal fluid)

• Oligodendrocytes: Provide myelin sheath to neurons in the CNS

• Microglia: Migrate through nervous tissue, acting as janitorial service and police force by engulfing cellular debris, wastes, and pathogens

• Schwann cells (PNS): Myelinate axons in the PNS

• Satellite cells (PNS): Surround neuron cell bodies in ganglia; regulate the interstitial fluid around neurons

Demyelination

• Multiple sclerosis (MS): Progressive destruction of myelin sheets, leading to loss of sensation and motor control (paralysis)

Membrane Potential and Ion Channels

Membrane Potential

The membrane potential is the difference in electric charges across the membrane, known as the electrochemical gradient.

• Inside and outside fluid differ in ionic composition

• Extracellular fluid: high Na+

• Cytosol: high K+

• Movement of charges to eliminate potential differences is called a current

• Cell membrane separates cytoplasm from the environment

• Inner membrane has a slightly negative charge

• Outer membrane has more Na+

Membrane Ion Channels

• Leak channels: Always open

• Regulated/gated ion channels: Open/close in response to specific stimuli

• Voltage-gated ion channels: Open/close in response to changes in membrane potential

• Chemically gated ion channels: Open/close in response to binding of specific chemicals

• Mechanically gated ion channels: Open/close in response to physical distortion

Sodium-Potassium Exchange Pump

• Makes the membrane more negative

$3 \text{Na}^+ \text{out of the cell}$ $2 \text{K}^+ \text{in}$

Graded (Local) Potential vs. Action Potential

Graded (Local) Potential	Action Potential (Nerve Impulses)
Small changes in the membrane potential; decrease with distance from stimulus	If the graded potential caused depolarization to threshold, an action potential is generated
Any stimulus that opens a gated channel produces a graded potential	All-or-none event; propagated along entire membrane surface without decrease in strength
Decreases with distance	Occurs in excitable membranes, such as neurons and muscle cells

Generation of an Action Potential

1. Resting membrane: -70 mV

2. Depolarization: Opening of Na+ channels; Na+ influx

3. Repolarization: Opening of K+ channels; K+ efflux

4. Hyperpolarization: K+ channels remain open longer, causing membrane potential to become more negative

The Refractory Period

• Prevents another action potential until the previous one finishes

• Ensures one-way propagation

Propagation Types

• Continuous propagation: Unmyelinated axon; relatively slow

• Saltatory propagation: Myelinated axon; action potential "jumps" from node to node; much faster

Synapses and Neurotransmitters

Synapse Structure and Function

A synapse is a specialized site where one neuron communicates with another cell. The synaptic cleft is the gap between pre- and post-synaptic membranes. Neurotransmitters are released into the synaptic cleft to transmit signals.

Electrical vs. Chemical Synapse

• Electrical: Action potential always reaches the next cell

• Chemical: Action potential may or may not reach the next cell

Cholinergic Synapses (ACh)

1. Arrival of action potential depolarizes the axon terminal membrane

2. Release of calcium ions causes exocytosis of ACh

3. ACh binds to postsynaptic membrane, depolarizing it

4. ACh is removed by acetylcholinesterase (AChE)

Synaptic Delay and Reflexes

• Synaptic delay: Time for a signal to cross a synapse

• Reflexes: Fewer synapses involved, faster response

Neurotransmitters

Types and Effects

Excitatory	Inhibitory
Depolarize and promote action potentials (e.g., Glutamate)	Hyperpolarize and suppress action potentials (e.g., GABA, Glycine)

Dissolved (Lipid Soluble) Gases

• Nitric oxide (NO): Generated by axon terminal; causes vasodilation

• Carbon monoxide: Exogenous; can affect neural function

Neuromodulators

• Substances that alter the rate of neurotransmitter release or change the postsynaptic cell's response

• Typically neuropeptides

• Slower, longer-term effects

Receptor Types

• Ionotropic: Direct effect through receptor

• Metabotropic: Indirect effect through GPCR

Sensory Receptors and Sensory Pathways

Neural Transduction and Transmission

• Neural transduction: Conversion of sensory stimulus into electrical signal

• Neural transmission: Process of signal traveling along the neuron and passing to another neuron at the synapse

Types of Sensory Receptors

• Tonic receptors: Always active; action potentials generated at a frequency reflecting background level of stimulation (e.g., pain receptors)

• Phasic receptors: Active only for a short time in response to change; adapt quickly (e.g., thermoreceptors)

Nociceptors (Pain Perception)

• Free nerve endings with large receptive fields

• Type A fibers: Fast pain (pricking pain)

• Type C fibers: Slow pain (burning, aching pain)

Thermoreceptors

• Detect changes in temperature

• Located in skin, skeletal muscle, liver, hypothalamus

Mechanoreceptors

• Detect mechanical pressure or distortion (e.g., touch, stretching, vibration)

Additional info:

• Some content inferred for completeness, such as definitions and examples of receptor types and propagation mechanisms.