Nervous Tissue and Divisions

Overview of the Nervous System

The nervous system is the primary communication and control system of the body, responsible for detecting changes in the environment, processing information, and initiating responses. It is composed of specialized nervous tissue, including neurons and neuroglial (glial) cells.

• Neurons: Electrically excitable cells that transmit nerve impulses.

• Neuroglial (Glial) Cells: Support, protect, and nourish neurons; not excitable.

Anatomical Divisions of the Nervous System

The nervous system is divided into two main anatomical regions:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main processing center for the entire nervous system.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS, such as cranial nerves and spinal nerves.

PNS Terminology

• Nucleus: Group of neuron cell bodies in the CNS.

• Tract: Group of axons in the CNS.

• Ganglion: Group of neuron cell bodies in the PNS.

• Nerve: Group of axons in the PNS.

Functional Divisions of the Nervous System

The nervous system is also divided functionally based on the direction and type of information flow:

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Integrative Function: Processes and interprets sensory input, making decisions for appropriate responses.

• Motor (Efferent) Division: Transmits commands from the CNS to effectors (muscles or glands).

Divisions of the PNS

• Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands.

• Enteric Nervous System (ENS): Controls the gastrointestinal tract independently but can communicate with the CNS.

Autonomic Nervous System Subdivisions

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

Neurons: Structure and Function

Neurons are the structural and functional units of the nervous system, specialized for the conduction of nerve impulses.

• Dendrites: Highly branched processes that receive information from other neurons or sensory receptors.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Axon: Long process that transmits impulses away from the cell body to other neurons or effectors.

Axon-Related Structures

• Axon Hillock: Junction between the cell body and axon; site where action potentials are initiated.

• Axoplasm: Cytoplasm of the axon.

• Axolemma: Plasma membrane of the axon.

• Telodendria: Fine terminal branches of the axon.

• Synaptic Knobs (Axon Terminals): Expanded tips of telodendria that form synapses with other cells.

Structural Classification of Neurons

• Unipolar Neurons: Single short process that branches like a T; primarily sensory neurons.

• Bipolar Neurons: One dendrite and one axon; found in special sensory organs (e.g., retina).

• Multipolar Neurons: Many dendrites and one axon; most common type, includes motor neurons and interneurons.

Synapses: Communication Between Neurons

A synapse is the junction between a neuron and another cell, allowing for the exchange of information.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Cell: Receives the signal (can be another neuron, muscle cell, or gland cell).

Types of Synapses

• Electrical Synapses: Cells are physically connected by gap junctions, allowing direct passage of electrical signals.

• Chemical Synapses: Cells are not physically connected; neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell.

Transmission Across a Chemical Synapse

1. Nerve impulse arrives at the synaptic knob of the presynaptic neuron.

2. Voltage-gated Ca2+ channels open, allowing Ca2+ to enter the synaptic knob.

3. Ca2+ triggers the release of neurotransmitters into the synaptic cleft.

4. Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic cell.

5. This binding initiates a response in the postsynaptic cell (excitation or inhibition).

Types of Neurotransmitters

• Excitatory Neurotransmitters: Trigger nerve impulses in the receiving cell.

• Inhibitory Neurotransmitters: Decrease the likelihood of nerve impulses in the receiving cell.

Neuroglia (Glial Cells): Types and Functions

Neuroglia are non-excitable cells that support, protect, and nourish neurons. They are more numerous than neurons.

• Ependymal Cells (CNS): Cuboidal/columnar cells that line ventricles and central canal; produce cerebrospinal fluid and form barriers.

• Astrocytes (CNS): Star-shaped cells; maintain the blood-brain barrier, support neurons, guide neuronal development, and repair damaged tissue.

• Microglia (CNS): Small cells with slender processes; act as phagocytes to remove debris and pathogens.

• Oligodendrocytes (CNS): Few processes; form myelin sheaths around CNS axons.

• Satellite Cells (PNS): Flat cells that support neuron cell bodies in ganglia.

• Schwann Cells (PNS): Flat cells that surround axons and form myelin sheaths in the PNS.

Myelin Sheath and Nervous Tissue Organization

• Myelin Sheath: Lipid-rich covering that insulates axons and increases the speed of nerve impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission.

• White Matter: Composed of myelinated axons.

• Gray Matter: Contains unmyelinated axons, neuron cell bodies, and dendrites.

Example: The rapid reflex response to touching a hot object involves sensory neurons (afferent), interneurons in the CNS, and motor neurons (efferent), all working together through the structures and divisions described above.