Ch 4.5 - Nervous Tissue Mediates Perception & Response

What is the Structure of Nervous Tissue?

• Nervous tissue is the main component of the nervous system, which includes the brain, spinal cord, and nerves. It is specialized to detect stimuli, process information, initiate responses, and control bodily functions. It is composed of two main classes of cells:

  • Neurons & Neuroglia

What are Neurons (Nerve Cells)?

• These are the fundamental functional units of the nervous system.

• They are the conducting cells that are highly specialized to generate, transmit, and propagate electrochemical signals in the form of nerve impulses or action potentials.

• Neurons are typically long-lived and amitotic (do not divide).

What is Neuroglia (Glia Cells)

• These are non-conducting, supportive cells that outnumber neurons and play crucial roles in assisting neuron function and maintaining the neural environment.

  • They provide physical support, insulation, metabolic support, and protection for neurons, as well as influencing synaptic transmission.

What is the Function of Neurons?

• Propagate Information: Neurons receive, integrate, and transmit information throughout the body.

  • They communicate via rapid electrical signals (action potentials) along their axons and by releasing chemical messengers (neurotransmitters) at specialized junctions called synapses.

What is the Structure of a Neuron?

• A neuron consists of three main parts: the cell body (soma), dendrites, and an axon. The cell body contains the nucleus and cellular organelles, dendrites receive signals from other neurons, and the axon transmits impulses away from the cell body.

What is the Cell body of a Neuron? (Soma or Perikaryon)

• The main biosynthetic and metabolic center of the neuron, containing the nucleus and most organelles (e.g., Nissl bodies, a specialized rough ER for protein synthesis).

What is the Dendrite of a Neuron?

• Short, highly branched extensions that typically cover a large surface area emanating from the cell body.

  • They are the primary receptive regions of the neuron, specialized to receive incoming signals (neurotransmitters) from other neurons and convey them towards the cell body.

What is the axon of a neuron?

• A single, long, slender projection that extends from the cell body at a specialized region called the axon hillock.

• The axon is specialized to generate and transmit action potentials away from the cell body towards other neurons or effector cells (muscles, glands).

• Axons can range from micrometers to over a meter in length.

What is Myelin Insulation?

• Many axons are covered by a fatty insulating layer called the myelin sheath, which is formed by glial cells.

• The myelin sheath increases the speed of action potential conduction (saltatory conduction) and conserves energy.

  • Gaps in the myelin sheath are called Nodes of Ranvier.

  • The distal end of the axon branches into axon terminals that form synapses

What is a Synapse

• The specialized junction between a neuron and another cell (another neuron, muscle cell, or gland cell) where neurotransmitters are released to transmit a signal. This can be electrical or chemical.

What are the Types of Neurons?

Neurons are classified structurally based on the number of processes extending from the cell body: Multipolar, Bipolar, Unipolar

What are Multipolar Neurons?

• The most common type of neuron, possessing multiple dendrites and a single axon.

  • Found extensively in the CNS (e.g., motor neurons, interneurons) and involved in complex integration and motor output.

What are Bipolar Neurons?

• Possess one main dendrite and one axon extending from opposite ends of the cell body.

  • Relatively rare, found in specialized sensory pathways such as the retina of the eye and the olfactory epithelium.

What are Unipolar Neurons (Pseudounipolar)

• Possess a single, short process that emerges from the cell body and then splits into a peripheral process (functioning as a dendrite) and a central process (functioning as an axon).

  • Primarily found in sensory nerves (afferent neurons) of the PNS, transmitting sensory information from the body to the CNS.

What are the functions of neuroglia?

Neuroglia (glial cells) provide essential support and protection for neurons and maintain the neural environment. They do not generate or transmit action potentials.

What are astrocytes?

• The most abundant and versatile glia in the CNS.

  • They regulate the chemical environment around neurons (e.g., regulating ion concentrations, taking up excess neurotransmitters),

  • participate in forming the blood-brain barrier (by wrapping around capillaries),

  • provide structural support,

  • and guide neuron migration during development.

What are Oligodendrocytes?

• Found in the CNS, these cells produce the myelin sheath that insulates multiple axons within the brain and spinal cord, increasing the speed of nerve impulse transmission.

What are Schwann Cells?

• Found in the PNS, these cells form the myelin sheath around a single axon of a peripheral neuron.

  • They also play a role in regeneration of damaged peripheral nerve fibers.

What are Microglia?

• Small, thorny-looking cells that function as the resident immune defense cells of the CNS.

  • They monitor neuron health, sense damage, migrate to injured areas, and phagocytize invading microorganisms and cellular debris.

What are Ependymal Cells?

• Line the central cavities of the brain and spinal cord (ventricles and central canal), forming a permeable barrier between cerebrospinal fluid (CSF) and the brain tissue. Their cilia help circulate CSF.

What are Satellite Cells?

• Surround neuron cell bodies in the PNS, providing support and regulating the chemical environment around neurons, similar to astrocytes in the CNS.