Introduction to the Nervous System and Nervous Tissue: Study Guide

Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals between different parts. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct structures and functions.

• Major Functions: Sensory input, integration of information, motor output, and regulation of homeostasis.

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

• Peripheral Nervous System (PNS): Consists of nerves and ganglia outside the CNS; transmits sensory and motor signals.

• Functional Divisions of PNS: Somatic (controls voluntary movements) and Autonomic (regulates involuntary functions).

Example: The CNS interprets sensory information from the PNS and sends motor commands back to muscles.

Nervous System Structure and Function

Neurons and neuroglial cells are the primary cellular components of nervous tissue. Their structure is closely related to their function in signal transmission and support.

• Neuron Components: Dendrites (receive signals), cell body (integrates signals), axon (transmits signals).

• Types of Neurons: Sensory (afferent), motor (efferent), and interneurons (integration).

• Neuroglial Cells: Support and protect neurons. Four types in CNS (astrocytes, oligodendrocytes, microglia, ependymal cells) and two types in PNS (Schwann cells, satellite cells).

• Structure-Function Relationship: Myelinated axons conduct signals faster; dendritic branching increases input capacity.

Example: Schwann cells in the PNS form myelin sheaths, increasing conduction velocity.

Electrophysiology of Neurons

Neurons communicate via electrical signals generated by ion movement across membranes. Action potentials are the primary means of long-distance communication.

• Voltage-Gated Ion Channels: Essential for action potential generation; allow selective ion flow.

• Refractory Periods: Absolute (no new action potential possible), Relative (stronger stimulus required).

• Conduction Types: Continuous (unmyelinated axons), Saltatory (myelinated axons, faster).

• Factors Affecting Velocity: Axon diameter (larger = faster), myelination (increases speed).

Example: Saltatory conduction allows rapid signal transmission in myelinated nerves.

Equation:

$\text{Conduction Velocity} \propto \text{Axon Diameter} \times \text{Degree of Myelination}$

Neuronal Synapses

Synapses are specialized junctions where neurons communicate. They can be electrical or chemical, each with unique properties and mechanisms.

• Electrical Synapses: Direct ion flow via gap junctions; fast, bidirectional.

• Chemical Synapses: Use neurotransmitters; slower, unidirectional.

• Synaptic Structure: Presynaptic terminal, synaptic cleft, postsynaptic membrane.

• Neurotransmitter-Receptor Relationship: Specific binding triggers postsynaptic effects.

• Synaptic Transmission Steps: Neurotransmitter release, receptor binding, postsynaptic response.

• EPSP: Excitatory postsynaptic potential; depolarizes membrane.

• IPSP: Inhibitory postsynaptic potential; hyperpolarizes membrane.

Example: Glutamate produces EPSPs, while GABA produces IPSPs in the CNS.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses. Their effects depend on the receptor type and location.

• Excitatory vs. Inhibitory: The same neurotransmitter can have different effects depending on the receptor.

• Major Classes: Amino acids, monoamines, peptides, and others.

• Common CNS Neurotransmitters: Excitatory: Glutamate; Inhibitory: GABA.

• Structural Properties: Small molecules (fast action), peptides (modulatory).

Example: Dopamine can be excitatory or inhibitory depending on the synapse.

Table: Types of Neuroglial Cells

The following table summarizes the main types of neuroglial cells in the CNS and PNS, their structure, and function.

Cell Type	Location	Function
Astrocyte	CNS	Support, blood-brain barrier, nutrient regulation
Oligodendrocyte	CNS	Myelination of CNS axons
Microglia	CNS	Immune defense, phagocytosis
Ependymal Cell	CNS	Produce and circulate cerebrospinal fluid
Schwann Cell	PNS	Myelination of PNS axons
Satellite Cell	PNS	Support and regulate environment of PNS neurons

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