Nervous Tissue

Nervous Tissue

• Nervous system enables the body to respond to external changes and controls internal organ functions.
• Neurons: Specialized cells that conduct impulses, they are the functional unit of the nervous system.
• Supporting cells: Non-conducting cells that surround neurons and their processes.

Central Nervous System (CNS)

• Composed of the brain and spinal cord.
• Contains gray matter with neuron cell bodies and supporting cells.
• Contains white matter with myelinated neuron processes and a few supporting cells.

Peripheral Nervous System (PNS)

• Includes all nervous tissue outside the CNS.
• Contains cranial nerves, spinal nerves, and ganglia.

Neurons

• Consist of a cell body, axon, and dendrites.
• Axon: Transmits impulses away from the cell body.
• Dendrites: Shorter processes that transmit impulses toward the cell body.
• Communicate through junctions called synapses.

Neuron Types

• Motor neurons: Transmit impulses from the CNS or ganglia to effector cells.
• Sensory neurons: Transmit impulses from receptors to the CNS.
• Interneurons: Communicate between sensory and motor neurons.

Neuron Structures

• Multipolar neurons: Possess many dendrites and a single axon.
• Bipolar neurons: Have a single axon and a single dendrite extending from the cell body.
• Unipolar neurons: Sensory neurons with one short process that branches into two.

Supporting Cells: Neuroglia

• Called central neuroglia in the CNS.
• Astrocytes: Most common type, connect with capillaries for metabolic exchange between neurons and blood, have a star-shaped appearance.
• Oligodendrocytes: Generate the myelin sheath for multiple axons, increasing the speed of action potential conduction.
• Microglia: Play a role in immune defense and can become phagocytic.
• Ependymal cells: Line the ventricles of the brain and the central canal of the spinal cord, have microvilli and cilia.

Gray and White Matter

• Gray matter: Contains neuron cell bodies, unmyelinated dendrites, and supporting cells.
• White matter: Contains myelinated neuron processes and a small number of supporting cell bodies.

Supporting Cells in the Peripheral Nervous System (PNS)

• Schwann cells: Primary supporting cells in the PNS, form myelin sheaths around individual axons.
• Satellite cells: Maintain a controlled microenvironment for neurons in ganglia of the PNS.

Peripheral Nerve Structure

• Myelinated axons: One Schwann cell completely surrounds a single axon, forming segments of myelin sheath.
• Unmyelinated axons: Multiple axons are enveloped by a single Schwann cell, with no tightly wrapped layers, forming grooves on the cell surface.

Connective Tissue in Peripheral Nerves

• Endoneurium: Surrounds individual nerve fibers and Schwann cells.
• Perineurium: Surrounds a group of nerve fibers (nerve fascicles).
• Epineurium: Surrounds the entire peripheral nerve and fills spaces between fascicles.

Mechanoreceptors

• Lamellar corpuscles: Sensitive to vibration and pressure, oval-cylindrical in shape, wrapped by a layer of connective tissue with concentric layers.

Neuroregeneration

• Neurons do not divide, but can partially regenerate after injury.
• Regeneration process:
  • Macrophages phagocytize debris and stimulate Schwann cell growth.
  • Schwann cells form new myelin sheaths around axonal sprouts.
  • Regeneration can take up to a year.
  • Axonal growth rate is faster in larger nerves.

Nervous Tissue

• The nervous system allows the body to respond to changes in its environment and controls internal organ functions. It comprises neurons and supporting cells.
• Neurons are specialized cells that conduct impulses, while supporting cells provide support and do not conduct impulses.
• The neuron is the functional and structural unit of the nervous system.
• The central nervous system (CNS) consists of the brain and spinal cord, containing gray and white matter.
• The peripheral nervous system (PNS) includes all nervous tissue outside the CNS, like cranial nerves, spinal nerves, and ganglia.

Neuron Structure

• Each neuron has a cell body, an axon, and dendrites.
• The axon transmits impulses away from the cell body, while dendrites carry impulses towards it.
• Neurons communicate with each other through specialized junctions called synapses.

Neuron Types

• Motor neurons: Carry impulses from the CNS or ganglia to effector cells.
• Sensory neurons: Carry impulses from receptors to the CNS.
• Interneurons: Communicate between sensory and motor neurons.

Multipolar neurons

• They have multiple dendrites and a single axon, typical of motor neurons and interneurons.

Bipolar neurons

• They have a single axon and a single dendrite extending from the cell body, found in the retina and other sensory organs.

Unipolar neurons

• Also known as pseudounipolar neurons, are sensory neurons with one short process branching from the cell body.
• They are not easily seen in a single section due to their length.

Supporting Cells: Neuroglia (Central)

• Astrocytes: The most common supporting cell.
• They connect with capillaries through "end-feet," facilitating metabolic exchange between neurons and blood.
• They have a star-shaped appearance.

Supporting Cells: Neuroglia (Central)

• Oligodendrocytes: Generate the myelin sheath for up to 50 axons.
• The myelin sheath insulates axons, enhancing the speed of action potential conduction.
• Damage to the myelin sheath, as in multiple sclerosis, leads to demyelination and impaired axonal function.

Supporting Cells: Neuroglia (Central)

• Microglia: Less common type, they play a role in immune defense and can become phagocytic.

Supporting Cells: Neuroglia (Central)

• Ependymal cells: Form the ependyma, lining the ventricles of the brain and the central canal of the spinal cord.
• They have microvilli and cilia on their apical surface.

Gray and White Matter

• Gray matter contains neuron cell bodies, unmyelinated dendrites, and supporting cells.
• White matter contains myelinated axons and a small number of cell bodies from supporting cells.

Supporting Cells: Neuroglia (Peripheral)

• Schwann cells: The main supporting cell in the PNS.
• They wrap their plasma membrane around axons, forming segments of the myelin sheath.
• Each Schwann cell creates one myelin sheath segment between nodes of Ranvier.
• Gaps between segments allow saltatory conduction, enhancing nerve impulse propagation.

Supporting Cells: Neuroglia (Peripheral)

• Satellite cells: Maintain a controlled environment around nerve cell bodies in PNS ganglia.

Peripheral Nerve Structure

• In myelinated axons, one Schwann cell wraps around a single axon multiple times.
• In unmyelinated axons, a single Schwann cell surrounds several axons, but without tight wrapping.
• Both types of nerve fibers are found in nerves.

Peripheral Nerve Structure

• Individual nerve fibers are held together by connective tissue:
  • Endoneurium: Surrounds each nerve fiber and its associated Schwann cell.
  • Perineurium: Surrounds each nerve fascicle.
  • Epineurium: Surrounds a peripheral nerve and fills spaces between nerve fascicles.

Mechanoreceptor (Lamellar Corpuscles)

• These nerve endings are responsible for sensitivity to vibration and pressure.
• They are oval-cylindrical, about 1 mm long.
• The corpuscle is enclosed by a connective tissue capsule with 20-60 concentric lamellae.

Neuroregeneration

• Neurons do not divide, but they have a limited ability to regenerate after injury.
• Regeneration starts 20 days after injury.

Neuroregeneration

• After injury, macrophages phagocytize debris and stimulate Schwann cells to release growth factors.
• Schwann cells start forming myelin sheaths around axonal sprouts.
• Regeneration takes a year to complete.
• Axon growth rates can reach 2 mm/day in small nerves and 5 mm/day in large nerves.