Unit 4: Nervous Tissues
1. Neurons: Structure, Types, and Neuron Doctrine
Neurons are the basic structural and functional units of the nervous system. They are specialized for the reception of stimuli and the transmission of electrical impulses.
Structure of a Neuron
- Cell Body (Soma/Perikaryon): Contains the nucleus and metabolic machinery.
- Dendrites: Short, branched processes that receive incoming signals.
- Axon: A single long process that carries impulses away from the cell body towards other neurons or muscles.
Types of Neurons
The Neuron Doctrine
The Neuron Doctrine states that the nervous system is composed of discrete individual cells (neurons) rather than a continuous network or syncytium.
2. Classification and Properties of Nerve Fibres
Nerve fibres are classified based on their diameter and degree of myelination.
Classification (Erlanger-Gasser)
- Type A: Thick, myelinated, and have the fastest conduction speed (e.g., motor fibres).
- Type B: Medium diameter, myelinated (e.g., preganglionic autonomic fibres).
- Type C: Thin, unmyelinated, and the slowest (e.g., pain sensations).
Properties of Nerve Fibres
- Excitability: Ability to respond to a stimulus and generate an action potential.
- Conductivity: Ability to transmit an impulse along its length.
- All-or-None Law: If a stimulus is strong enough to reach threshold, a complete response is generated; if not, no response occurs.
3. Glia Cells: Structure and Functions
Glia cells (or neuroglia) are non-neuronal support cells that maintain homeostasis and provide protection for neurons.
Types of Glia Cells
- Astrocytes: Star-shaped cells that form the blood-brain barrier.
- Oligodendrocytes: Form myelin sheaths in the Central Nervous System (CNS).
- Schwann Cells: Form myelin sheaths in the Peripheral Nervous System (PNS).
- Microglia: Small phagocytic cells that act as the immune defense of the brain.
- Ependymal Cells: Line the ventricles and help produce cerebrospinal fluid (CSF).
4. Synapse and Neuromuscular Junction
Synapse: Structure and Types
A synapse is the site of communication between two neurons or a neuron and an effector.
- Chemical Synapse: Uses neurotransmitters (e.g., Acetylcholine) to bridge the synaptic cleft.
- Electrical Synapse: Uses gap junctions for direct ion flow between cells.
Neuromuscular Junction (NMJ)
A specialized synapse between a motor neuron and a skeletal muscle fibre.
- Motor End Plate: The specialized region of the sarcolemma containing receptors.
- Synaptic Vesicles: Store neurotransmitters in the axon terminal.
5. Exam Focus Enhancements
Exam Tips
- Differentiation: Be prepared to differentiate between Schwann cells (PNS) and Oligodendrocytes (CNS).
- The Gap: The small gap between myelin segments is called the Node of Ranvier; saltatory conduction happens here.
- Direction: Remember that dendrites carry impulses towards the soma, and axons carry them away.
Common Mistakes
- Confusing Unipolar with Bipolar neurons. Bipolar has two distinct processes; Unipolar (pseudounipolar) has one process that splits.
- Assuming glia cells conduct electricity. Only neurons are primary conductors of action potentials.
Frequently Asked Questions
- What is the difference between electrical and chemical synapses?
- Explain the significance of the All-or-None law in nerve conduction.
- Discuss the role of Astrocytes in the Central Nervous System.