The Nervous System

7.1 Neurons and Supporting Cells

A. Introduction

1. The nervous system is divided into two main parts:

   • Central Nervous System (CNS): Includes the brain and spinal cord.

   • Peripheral Nervous System (PNS): Includes all the nerves outside the brain and spinal cord (cranial and spinal nerves).

2. Nervous system tissue has two types of cells:

   • Neurons: These cells transmit electrical signals (impulses). Most cannot divide but can repair themselves or grow new branches.

   • Supporting cells (neuroglia): These cells help neurons and can divide, but they don't transmit impulses.

B. Neurons

1. Neurons are the basic building blocks and working units of the nervous system.

2. Key functions of neurons:

   • Respond to different kinds of signals (chemical and physical).

   • Send electrochemical signals.

   • Release chemical messengers.

   • Help us sense things, learn, remember, and control our muscles and glands.

3. While most neurons can't divide, they are good at repairing themselves and growing new connections.

C. General Structure of Neurons

1. Neurons come in various sizes and shapes, but they all generally have:

   • Cell Body: Contains the nucleus and other cell parts (like Nissl bodies).

   • Dendrites: These tree-like extensions receive incoming signals and carry them towards the cell body.

   • Axon: This long extension carries signals (action potentials) away from the cell body.

D. Components of a Neuron

• Dendrites

• Axon hillock: The starting point for an action potential.

• Axon: Carries the action potential.

• Node of Ranvier: Gaps in the myelin sheath that help signals travel faster.

E. Axons

1. Axons can be very short (a millimeter) or very long (over a meter).

2. They connect to the cell body at a special region called the axon hillock. This is where the action potential usually begins.

3. Axons can have branches called axon collaterals.

4. Many axons are covered by myelin, a fatty layer, with gaps along it called nodes of Ranvier.

F. Axonal Transport

1. Axonal transport is an active process that moves cell parts and proteins from the cell body down to the axon ends.

   • Fast Component: Moves membrane-bound sacs (vesicles).

   • Slow Component: Moves internal structures like microfilaments, microtubules, and various proteins.

2. Direction of Transport:

   • Anterograde Transport: Moves materials from the cell body towards the axon terminals, using kinesin motor proteins. This carries things like synaptic vesicles.

   • Retrograde Transport: Moves materials from the axon terminals back to the cell body, using dynein motor proteins. This carries waste products (lysosomes) and can also be used by some viruses to infect the nervous system.

G. Classification of Neurons and Nerves

1. Functional Classification of Neurons: Based on the direction of their signals:

   • Sensory (Afferent) Neurons: Carry signals from sensory organs (like skin or eyes) to the CNS.

   • Motor (Efferent) Neurons: Carry signals from the CNS to muscles or glands (effector organs) to cause a response.

   • Interneurons: Found entirely within the CNS, they connect other neurons and help process information.

2. Motor Neurons:

   • Somatic Motor Neurons: Control voluntary movements and reflexes of skeletal muscles.

   • Autonomic Motor Neurons: Control involuntary functions of smooth muscle, heart muscle, and glands. They are further divided into:

     • Sympathetic: Activated during stress or emergencies (“fight or flight”).

     • Parasympathetic: Active during rest and digestion (“rest and digest”).

3. Structural Classification of Neurons: Based on how many extensions (processes) come off the cell body:

   • Pseudounipolar: Have one short process that immediately splits into two long ones. Typically sensory neurons.

   • Bipolar Neurons: Have two processes, one at each end. Found in places like the retina of the eye.

   • Multipolar Neurons: Most common type, with many dendrites and one axon. Typically motor neurons.

H. Identification of Nerves

1. Nerves are bundles of axons located outside the CNS. Most nerves contain both sensory and motor neurons.

2. Tract: A bundle of axons located inside the CNS.

3. Ganglion: A group of neuron cell bodies outside the CNS. When a similar group is inside the CNS, it's called a nucleus.

I. Neuroglia

1. Neuroglial cells don't conduct signals but provide support to neurons. They originate from the ectoderm during development.

2. Types of neuroglia in the PNS:

   • Schwann Cells: Form myelin sheaths around axons in the PNS.

   • Satellite Cells: Support the cell bodies of neurons in ganglia.

3. Neuroglia of the CNS:

   • Oligodendrocytes: Create myelin sheaths around axons in the CNS, forming the “white matter” of the brain and spinal cord.

   • Microglia: Function as immune cells (phagocytes), clearing away debris and foreign invaders in the CNS.

   • Astrocytes: Help regulate the environment around neurons and contribute to the blood-brain barrier.

   • Ependymal Cells: Line the brain's fluid-filled cavities (ventricles) and produce cerebrospinal fluid (CSF).

J. Functions of Microglia

1. Microglia develop from cells in the embryonic yolk sac and are similar to macrophages (a type of immune cell).

2. Characteristics:

   • Small cell body with thin branches.

   • Engulf and remove pathogens and debris (phagocytosis).

   • Can move around and become active when there's an infection or injury.

3. Activated microglia: Detect signals (like ATP from damaged cells), then move to the site, multiply, and clean up the damaged tissue.

K. Neuroglial Cells and Their Functions

L. Myelin Sheath and Neurilemma

1. In the PNS, every axon has a neurilemma, an outer protective layer formed by Schwann cells. This layer is crucial for nerve regeneration.

2. Myelinated Axons vs. Unmyelinated Axons:

   • Myelin makes nerve signals travel much faster.

   • The gaps in myelin (Nodes of Ranvier) allow signals to jump, speeding up transmission.

   • Unmyelinated axons are usually slower