Nervous System: Structure and Function Study Notes

Nervous System Overview

Three Overlapping Functions of the Nervous System

The nervous system is responsible for sensory input, integration, and motor output. These functions work together to maintain homeostasis and coordinate body activities.

• Sensory Functions: Detect changes in the environment and send afferent signals to the brain or spinal cord.

• Integration Functions: Process and interpret sensory input, make decisions, and create motor instructions.

• Motor Functions: Send instructions from the integration center to muscles or glands to elicit a response.

Organization of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, which serve as the main control centers for processing information.

• Organs: Brain, spinal cord

• Nuclei: Clusters of neuron cell bodies within the CNS

Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body and is composed of cranial and spinal nerves, as well as ganglia.

• Organs: Cranial nerves, spinal nerves

• Ganglia: Clusters of neuron cell bodies in the PNS

Comparison of Nuclei and Ganglia

Structure	CNS	PNS
Nuclei	Clusters of somas	Not present
Ganglia	Not present	Clusters of somas

Types of Signals in the Nervous System

Visceral Sensory Signals

• Detect stretch, nociception (pain), hunger, and satiety

Somatic Motor Signals

• Control skeletal muscle (voluntary motor)

Visceral Motor Signals

• Control smooth and cardiac muscle (involuntary motor)

• Also known as the autonomic nervous system (ANS)

• Divided into:

  • Parasympathetic: Resting and digesting

  • Sympathetic: Fight or flight

Neurons and Neuroglia

Differences Between Neurons and Neuroglia

• Neurons: Can create and send electrical signals

• Neuroglia: Do not send electrical signals; support and protect neurons

Special Characteristics of Neurons

• Last entire life

• Do not divide by mitosis after birth

• High metabolism (require constant oxygen and glucose)

Parts of a Neuron and Their Functions

• Soma: Cell body; contains nucleus and most organelles; site of protein synthesis and repair

• Dendrites: Receive signals from other neurons

• Axon: Sends signals away from soma toward synapse

  • Axon hillock: Widest section, attaches to soma, trigger zone for signal initiation

  • Synaptic knob: End of axon, stores and releases neurotransmitters

  • Neurotransmitter: Chemical messenger secreted into synaptic cleft

• Myelin sheath: Insulates axon, increases speed of electrical signals

  • CNS: Oligodendrocytes

  • PNS: Schwann cells

  • Nodes of Ranvier: Gaps between myelin cells, exposed axon sections

Synapses and Neuronal Communication

Synapse

A synapse is the junction between a neuron and another cell (neuron, muscle, or gland) where communication occurs.

• Presynaptic neuron: Sends signal

• Postsynaptic neuron: Receives signal

• Synaptic cleft: Gap between presynaptic and postsynaptic cells

Types of Synapses

• Axoaxonic: Between axon of one neuron and axon of another

• Axosomatic: Between axon and soma

• Axodendritic: Between axon and dendrite

Structural and Functional Classification of Neurons

Structural Classification

Type	Structure	Location
Multipolar	Many dendrites, 1 axon	CNS and PNS (mainly CNS)
Bipolar	1 dendrite, 1 axon	Special sensory organs
Unipolar	One long process; one end is dendrite, other is axon	PNS
Pseudounipolar	Similar to unipolar	PNS

Functional Classification

• Sensory neurons: For afferent signals

• Motor neurons: For efferent signals

• Interneurons: Bridge between sensory and motor neurons; mostly multipolar

Neuroglia: Support Cells of the Nervous System

Neuroglia of the CNS

• Astrocytes: Secrete nutrients, surround blood capillaries, create blood-brain barrier, absorb excess neurotransmitters and potassium, create scar tissue

• Microglia: Form blood, digest pathogens, digest dead neurons

• Ependymal cells: Secrete cerebrospinal fluid (CSF)

• Oligodendrocytes: Create myelin sheath

Neuroglia of the PNS

• Schwann cells: Create myelin sheath

• Satellite cells: Support neuron cell bodies in ganglia

Gray Matter and White Matter

Gray Matter

• Composed of dendrites, somas, and small axons; not myelinated

• Location in brain: Inner nuclei and outer cortex

• Location in spinal cord: Inner core

White Matter

• Composed of myelinated axons; larger and longer axons

• Location in brain: Tracts and peduncles (middle and outer regions)

• Location in spinal cord: Outer layer, surrounds gray matter

Nerves and Connective Tissue Coverings

Nerve Structure

• Nerve: Bundle of fascicles

• Nerve fiber: Single axon

• Neurofibril: Bundle of fascicles

Connective Tissue Layers

• Endoneurium: Surrounds one nerve fiber (superficial to myelin sheath)

• Perineurium: Surrounds fascicle (group of axons)

• Epineurium: Surrounds entire nerve

Reflexes and Neural Circuits

Reflex

A reflex is an automatic response to a stimulus.

• Parts of the reflex arc:

  1. Receptor: Detects stimulus

  2. Sensory neuron: Afferent signals

  3. Integration center: Interneuron in spinal cord

  4. Motor neuron: Efferent signals

  5. Target/effector: Creates response

Types of Reflexes

• Monosynaptic reflex: 1 synapse, 2 neurons; simplest and fastest (e.g., knee-jerk reflex)

• Polysynaptic reflex: 2 synapses, 3 neurons; more complex (e.g., withdrawal reflex)

Neural Circuits

• Diverging: 1 neuron synapses with many

• Converging: Many neurons synapse with 1

• Reverberating: Repeating circuit (e.g., breathing)

Key Equations and Concepts

• Neuron Signal Transmission: $\text{Signal speed} \propto \text{Myelination}$ Myelinated fibers transmit impulses much faster than unmyelinated fibers.

• Reflex Arc: $\text{Reflex arc} = \text{Receptor} \rightarrow \text{Sensory neuron} \rightarrow \text{Integration center} \rightarrow \text{Motor neuron} \rightarrow \text{Effector}$

Additional info: Academic context and definitions have been expanded for clarity and completeness.