Neuroscience: Neuron Types and Characteristics

Nervous Tissue (Part 1)

• Nervous tissue is fundamental in the human body for communication.
• It includes neurons and neuroglial cells.
• Neurons are classified according to the number of processes and function.
• Neurons have a cell body (perikaryon), dendrites (multiple processes), and an axon (single process).
• The cell body is rounded in pseudo-unipolar neurons, fusiform in bipolar neurons, and stellate, pyramidal, or pyriform in multipolar neurons.
• Neuron size varies depending on location, ranging from 4 μm to 100 μm. For instance, granular cells in the cerebellar cortex are smaller than motor neurons.
• The cell body contains a large, euchromatic nucleus with a prominent nucleolus, characteristic of an active cell.
• The cytoplasm includes a well-developed rough endoplasmic reticulum (rER) and numerous polyribosomes. These structures, appearing as basophilic granular areas called Nissl bodies, are variable according to the neuronal function.
• Golgi complexes are centrally located around the nucleus.
• Mitochondria are dispersed throughout the cytoplasm.
• Neurofilaments (intermediate filaments) are prevalent in the perikaryon (cell bodies) and processes, visible through light microscopy (LM) stained with silver. These filaments provide structural support.
• Microtubules are arranged in parallel bundles within the perikaryon and processes, and are critical for axonal transport of neurotransmitter substances and enzymes.
• Centrioles are absent in neurons since neurons do not exhibit cell division.
• Inclusions present in neurons include lipofuscin pigment. It is a residue of undigested material from lysosomes, increasing with aging. Melanin, a dark brown or black pigment, is found in substantia nigra neurons and may be a byproduct of abnormal metabolism.
• Lipid droplets are found in the cytoplasm of neurons and represent stored energy.

Classification of Neurons

• Pseudo-unipolar: The cell body is rounded, and the single process splits into peripheral and central branches shortly after leaving the perikaryon (cell body). The peripheral branch functions as a dendrite, and the central branch acts as an axon. Found in dorsal root ganglia & mesencephalic nucleus of the trigeminal nerve.

• Bipolar: possess a single dendrite and single axon. Present in the cochlea and vestibular ganglia of the inner ear, and in the retina and olfactory mucosa.

• Multipolar: have one axon and many dendrites. They appear as stellate (star-shaped) cells in the anterior horn of the spinal cord; pyramidal in the cerebral cortex; and pyriform in the cerebellar cortex (Purkinje cells).

Classifications of Neurons based on Function

• Sensory (Afferent): Receive sensory stimuli and transmit them to the spinal cord & brain via the dorsal root ganglion.
• Motor (Efferent): Transmit impulses from the central nervous system to muscles and glands, such as those in the anterior horn of the spinal cord.
• Interneurons: Connect neurons within the central nervous system. They are abundant in the brain and spinal cord and also in the retina.

Classification Based on Axon Length

• Golgi type I: neurons possess long axons extending beyond the grey matter into white matter. Examples include motor neurons in the spinal cord, pyramidal cells in the cerebral cortex, and Purkinje cells in the cerebellar cortex.
• Golgi type II: neurons possess short axons remaining confined within the grey matter. Examples include the vast majority of interneurons in the brain and cerebellum.

Nerve Fiber

• A nerve fiber is an axon (the projection of the neuron) surrounded by myelin and Schwann cells.
• The axon is encased by an axolemma, containing axoplasm, which is the cytoplasm within the axon.
• The axon emerges from the axon hillock, which is a conical extension from the cell body.

Types of Nerve Fibers

• Unmyelinated: Lack a myelin sheath and are subdivided into (a) naked fibers, present in gray matter; and (b) fibers with a Schwann cell sheath, prevalent in sympathetic postganglionic fibers.
• Myelinated: Possess a myelin sheath and are subdivided into (a) fibers without a Schwann cell sheath, found in white matter; and (b) fibers with a Schwann cell sheath, found in peripheral nerves.

The Schwann Cell Sheath

• The Schwann cell sheath consists of flattened cells with flattened nuclei that enwrap the myelin of the nerve fiber.
• Functions: Formation of the myelin sheath in the peripheral nervous system; insulation to speed up the nerve impulse; allowing for regeneration.

Myelin Sheath

• The myelin sheath is constructed from many modified cell membranes of Schwann cells. 
• It possesses higher lipid content compared to cell membranes.
• Under the light microscope (LM), it appears as a dark structure stained by osmic acid.
• Under the electron microscope (EM), it's evident as fused spiral laminae of the plasmalemma.
• The myelin sheath is interrupted by Nodes of Ranvier, gaps between Schwann cells.
• The space between nodes are called inter-nodal segments.

Peripheral Nervous System

• Consists of nerves, ganglia (clusters of neuron cell bodies), and nerve endings.
• The components involved in the nervous system are also well organized into bundles that are structurally supported. 
• Components of the bundles include, but are not limited to: epineurium, perineurium, and endoneurium.

Peripheral Nerve Structure

• The whole nerve is covered by a dense connective tissue sheath called the epineurium.
• Nerve fibers are organized into bundles, each surrounded by perineurium.
• The endoneurium, a connective tissue layer, surrounds individual nerve fibers.

Test Yourself Questions

• Question 1: Nerve fibers are the axon covered by the myelin and Schwann cells sheath.
• Question 2: Pseudo-unipolar neurons have rounded cell bodies and a single divided process.

This quiz focuses on distinguishing between various types of neurons, their structures, and functions. Test your knowledge on characteristics of pseudo-unipolar, bipolar, and other neuron types as well as their roles in the nervous system. Ideal for students studying neuroscience or related fields.