Lecture #12

Nerve Cells and Neurotransmitters

Introduction

This study guide covers the fundamental aspects of nerve cells (neurons), supporting glial cells, and the mechanisms of neuronal signaling, with a focus on long-distance communication and neurotransmitter function in the nervous system.

Long Distance Neuronal Signaling

Overview of Long Distance Signaling

• Long distance signaling in the body is achieved through the use of extracellular chemical messages that act on target cells.

• Two major systems are involved: the Nervous System and the Endocrine System.

Nervous System

• Signaling is rapid (milliseconds to seconds).

• Controls both voluntary and involuntary actions.

• Uses neurotransmitters for communication (cell-to-cell, close proximity).

• All neurotransmitters have extracellular receptors.

• Neurotransmitters can be excitatory or inhibitory.

Endocrine System

• Signaling is slow and long-lasting (minutes to hours).

• Controls involuntary processes.

• Uses hormones that travel long distances via blood vessels or lymph vessels.

Comparison Table: Nervous vs. Endocrine Signaling

Feature	Nervous System	Endocrine System
Speed	Rapid (ms–s)	Slow (min–hours)
Control	Voluntary & Involuntary	Involuntary
Signal Molecule	Neurotransmitters	Hormones
Distance	Short (cell-to-cell)	Long (via blood/lymph)

Cell to Cell Chemical Signaling

Types of Chemical Signaling

• A chemical signal is released outside the cell and binds to a receptor on the target cell.

• Receptors can be located on the plasma membrane (extracellular) or within the cytoplasm (intracellular).

Modes of Chemical Signaling

• Autocrine: Chemicals act on the same cells that secrete them.

• Paracrine: Chemicals affect nearby cells other than those that secrete them.

• Endocrine: Chemicals (hormones) are secreted into the blood to signal distant cells.

• Exocrine: Chemicals are secreted into the external environment, usually through ducts in epithelial tissue.

Example

• Neurotransmitters are released from the axon terminal of a neuron and bind to receptors on the postsynaptic cell, mediating rapid communication.

Cells of the Nervous System

Main Cell Types

• Neurons: The primary signaling cells responsible for transmitting electrical and chemical signals.

• Glia (Glial Cells): Supporting cells that outnumber neurons (approximate ratio 15:1) and provide structural, metabolic, and functional support.

Neurons

• Exhibit extreme longevity (can live over 100 years).

• Are amitotic (most do not divide or regenerate if destroyed).

• Have a high metabolic rate (require continuous oxygen and glucose supply).

• Comprise only 2% of body mass but use 20% of the body's energy.

Glial Cells (Supporting Cells)

• Most glia retain the ability to divide (mitotic ability).

• Some glia possess electrical properties similar to neurons.

Types of Glial Cells in the CNS

• Astrocytes: Most abundant; regulate extracellular fluid composition, remove excess K+ and neurotransmitters, provide nutrients, and help form the blood-brain barrier.

• Microglia: Act as macrophages; phagocytize debris and remodel synapses.

• Oligodendrocytes: Provide myelin sheaths for axons, enabling electrical insulation.

• Ependymal cells: Line cavities of the CNS; cilia circulate cerebrospinal fluid (CSF) to cushion and nourish the CNS.

Types of Glial Cells in the PNS

• Schwann cells: Surround and form myelin sheaths around axons (analogous to oligodendrocytes in the CNS).

• Satellite cells: Surround neuron cell bodies and regulate the external chemical environment.

Myelination

Structure and Function

• Myelin sheath: A multilayered lipid covering that insulates axons, increasing the speed of action potential conduction.

• Formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

• Myelinated fibers conduct impulses faster than unmyelinated fibers.

• Nodes of Ranvier: Gaps between myelin sheaths along the axon, crucial for saltatory conduction.

Neuronal Structure and Function

Neuronal Cell Body (Soma)

• The cell body (soma) is the biosynthetic center of the neuron, containing the nucleus and most organelles (mitochondria, Golgi apparatus, endoplasmic reticulum).

• Nissl bodies (rough ER) are highly active in protein synthesis.

• Neurofilaments (intermediate filaments) maintain cell shape.

• Multipolar neurons are the most common type in the nervous system.

Neuronal Processes

• Dendrites: Receive input from other neurons; highly branched to increase surface area for synaptic contacts. They transmit graded potentials toward the cell body.

• Axon: Conducts output signals away from the cell body. Each neuron has a single axon, which may be very short or up to a meter long. Axons may be myelinated or unmyelinated.

• The axon hillock is the cone-shaped region where the axon originates from the cell body.

• The axon terminal (synaptic terminal) is the endpoint where neurotransmitters are released.

Summary Table: Neuron vs. Glial Cell

Feature	Neuron	Glial Cell
Function	Signal transmission	Support, protection, insulation
Mitotic Ability	Generally amitotic	Most are mitotic
Longevity	Very long-lived	Variable
Electrical Properties	Yes	Some types

Key Terms and Definitions

• Neurotransmitter: A chemical messenger released by neurons to transmit signals across a synapse to another cell.

• Myelin: A fatty substance that insulates axons and increases the speed of nerve impulse conduction.

• Node of Ranvier: A gap in the myelin sheath of a nerve, between adjacent Schwann cells or oligodendrocytes.

• Astrocyte: A star-shaped glial cell of the central nervous system.

• Microglia: Glial cells that act as the main form of active immune defense in the CNS.

• Oligodendrocyte: A type of glial cell in the CNS that produces myelin.

• Schwann cell: A glial cell in the PNS that forms myelin sheaths.

• Satellite cell: A glial cell that surrounds neuron cell bodies in the PNS.