Nervous System Structure and Function: Study Notes for Anatomy & Physiology

Chapter 11: Nervous System

Overview and Function of the Nervous System

The nervous system is responsible for receiving sensory input, integrating information, and producing motor output. It allows the body to respond to changes in both the external and internal environments.

• Sensory Input: Detection of changes in the environment by sensory receptors.

• Integration: Processing and interpretation of sensory input in the central nervous system (CNS).

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

• Example: Touching a hot object triggers sensory input, CNS integration, and motor output to withdraw the hand.

Organization of the Nervous System

The nervous system is organized into several major divisions, each with specific functions and components.

• Afferent Peripheral System: Carries sensory information from receptors to the CNS.

• Somatic Nervous System: Controls voluntary movements via motor neurons.

• Autonomic Nervous System (ANS): Regulates involuntary functions; includes sympathetic (fight or flight) and parasympathetic (rest and digest) divisions.

• Efferent Peripheral System: Transmits motor commands from the CNS to effectors.

Division	Function	Example
Afferent Peripheral	Sensory input to CNS	Touch, pain, temperature
Somatic Nervous	Voluntary motor control	Moving limbs
Autonomic Nervous	Involuntary control	Heart rate, digestion
Efferent Peripheral	Motor output from CNS	Muscle contraction

Neuron Structure

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

• Cell Body: Main portion containing the nucleus and organelles.

• Dendrites: Branch-like extensions that receive signals.

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

• Neurotransmitter: Chemical messenger released at synapses.

• Schwann Cells: Neuroglial cells in the PNS that produce myelin.

• Myelin Sheath: Insulating layer of proteins and lipids around axons.

• Nodes of Ranvier: Gaps in myelin sheath facilitating rapid signal transmission.

Classification of Neurons

Neurons are classified by structure and function.

• Multipolar: Many processes; one axon, multiple dendrites (most common in CNS).

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

• Unipolar: Single process that divides into two branches (sensory neurons in ganglia).

Type	Structure	Location
Multipolar	Many dendrites, one axon	Brain, spinal cord
Bipolar	One dendrite, one axon	Sensory organs
Unipolar	Single process splits	Sensory ganglia

Classification by Function

• Sensory Neurons: Detect changes and send signals to CNS; mostly unipolar.

• Interneurons: Connect neurons within CNS; multipolar.

• Motor Neurons: Transmit impulses from CNS to effectors; multipolar.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, essential for neuron function.

• Astrocytes: Regulate movement of substances in the blood-brain barrier (CNS).

• Oligodendrocytes: Produce myelin in CNS, wrap around multiple axons.

• Microglia: Phagocytic cells that remove debris and pathogens.

• Ependymal Cells: Line brain ventricles and spinal canal, regulate cerebrospinal fluid.

• Satellite Cells: Surround neuron cell bodies in PNS, similar function to astrocytes.

• Schwann Cells: Myelinate axons in PNS.

Cell Type	Location	Function
Astrocytes	CNS	Blood-brain barrier, substance regulation
Oligodendrocytes	CNS	Myelination of axons
Microglia	CNS	Phagocytosis
Ependymal Cells	CNS	CSF regulation
Satellite Cells	PNS	Support neuron cell bodies
Schwann Cells	PNS	Myelination of axons

White Matter vs. Gray Matter

White and gray matter are distinct regions in the CNS, each with specific components and functions.

• White Matter: Contains myelinated axons; found in brain and spinal cord.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; found in cortex and nuclei.

Type	Components	Location
White Matter	Myelinated axons	Brain, spinal cord
Gray Matter	Cell bodies, dendrites, unmyelinated axons	Cortex, nuclei

Synapses and Neurotransmission

Synapses are specialized junctions for one-way transmission of signals between neurons or between neurons and muscles.

• Action Potential: Electrical impulse reaches axon terminal.

• Calcium Influx: Ca2+ enters axon terminal.

• Neurotransmitter Release: Vesicles fuse with membrane, releasing neurotransmitter.

• Receptor Binding: Neurotransmitter binds to postsynaptic receptors, opening ion channels.

Resting Membrane Potential

The resting potential is the electrical charge difference across the neuron membrane at rest.

• Typical Value: -70 mV

• Na+ ions: Higher concentration outside the cell

• Inside Cell: Negatively charged

• Maintained by: Voltage-gated Na+/K+ pumps and leak channels

Equation:

$V_{rest} \approx -70\ \text{mV}$

Myelination and Nerve Impulse Transmission

Myelination increases the speed of nerve impulse transmission by allowing action potentials to jump between nodes of Ranvier (saltatory conduction).

• Myelinated Axons: Action potential only at nodes of Ranvier; faster conduction (120 m/s).

• Unmyelinated Axons: Action potential along entire axon; slower conduction (0.5 m/s).

• Clinical Note: Multiple sclerosis is caused by loss of myelination.

• Axon Diameter: Larger diameter increases impulse speed.

Propagation of Action Potential

Action potentials propagate along axons by sequential depolarization of membrane segments.

• Na+ influx opens voltage-gated channels, causing local currents and depolarization.

• Depolarization spreads, triggering action potential in adjacent areas.

• Myelination restricts action potential to nodes, increasing speed and directionality.

Neurotransmitters

Neurotransmitters are chemical signals released by neurons to communicate across synapses.

• Acetylcholine: Excitatory/inhibitory; involved in muscle activation.

• Norepinephrine/Epinephrine: Released during stress; regulates fight-or-flight response.

• Serotonin: Regulates mood, anxiety, happiness.

• Histamine: Increases capillary permeability; involved in immune response.

• GABA (Gamma Amino Butyric Acid): Major inhibitory neurotransmitter; regulates behavior and cognition.

• Glutamate (MSG): Major excitatory neurotransmitter; involved in cognition, memory, learning.

• Enkephalins, Endorphins: Natural painkillers.

Neurotransmitter	Main Function
Acetylcholine	Muscle activation, autonomic functions
Norepinephrine/Epinephrine	Fight-or-flight response
Serotonin	Mood regulation
Histamine	Immune response
GABA	Inhibition, behavior regulation
Glutamate	Excitation, learning
Enkephalins/Endorphins	Pain relief

Additional info: These notes expand on brief points from the slides, providing definitions, examples, and context for each major concept in nervous system anatomy and physiology.