Morphology of the Neuron. Types of Neurons.
Overview of Nerve Tissue and Neurons Nerve tissue is a highly cellular network specialised for communication throughout the body. It primarily transmits electrical signals from sensory receptors to effector organs like muscles and glands, controlling their activity. This tissue is crucial for internal controls (homeostasis), mental activity, movement, and general behavioural patterns. Nerve tissue consists mainly of two types of cells: nerve cells (neurons) and glial cells. Neurons are the functional units of the nervous system, typically featuring numerous long processes for signal transmission. Glial cells, or neuroglia, are more numerous than neurons but smaller, providing structural and mechanical support, maintaining homeostasis, immune responses, and forming myelination.
Morphology of the Neuron (Structural Features) Most neurons consist of three main parts: the cell body (perikaryon), dendrites, and the axon.
- 1. Cell Body (Perikaryon / Soma):
- Contains the nucleus and most of the cell's organelles.
- Acts as the synthetic or trophic centre for the entire neuron, producing cytoplasm for movement into its processes.
- Typically has a large, euchromatic (pale-staining) nucleus with a prominent nucleolus, reflecting intense synthetic activity.
- Contains a highly developed rough endoplasmic reticulum (RER) with many parallel cisternae and numerous polyribosomes, appearing as clumps of basophilic material called chromatophilic substance (Nissl substance or Nissl bodies). This is abundant in large nerve cells like motor neurons.
- The Golgi apparatus is located only in the cell body.
- Mitochondria are found throughout the cell but are abundant in axon terminals.
- Intermediate filaments (called neurofilaments) are abundant and can form neurofibrils visible with a light microscope after silver staining.
- May contain pigmented inclusions like lipofuscin.
- Responds to graded inputs.
- 2. Dendrites:
- Usually short and branched like tree branches (arborization), significantly increasing the receptive area of the cell.
- Are the principal signal reception and processing sites on neurons, covered with many synapses.
- Cytoplasm at the dendrite base is similar to the cell body, with cytoskeletal elements predominating.
- Become thinner as they subdivide.
- Many synapses occur on dendritic spines, which are short blunt structures projecting along dendrites. These spines are important for neural plasticity, adaptation, learning, and memory.
- Stimulated by inputs.
- 3. Axon (Efferent Projection):
- A single, long, cylindrical process specialised for generating and conducting nerve impulses (action potentials) away from the cell body to other cells (nerve, muscle, gland cells).