Introduction to the Nervous System and Nervous Tissue: Study Notes

Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is responsible for controlling perception, experience, and voluntary movement. It is essential for maintaining homeostasis and regulating many bodily functions.

• Functions: Controls voluntary movement, consciousness, personality, learning, and memory.

• Homeostasis: Regulates body temperature, blood pressure, sleep/wake cycle, and blood pH.

Master Controlling and Communicating System

The nervous system is the primary system for communication and control in the body, using electrical and chemical signals.

• Functions: Sensory input, integration, and motor output.

• Development: Nervous tissue forms from the embryonic ectoderm; the neural tube becomes the CNS.

Functions of the Nervous System

Sensory Input

Sensory receptors detect changes in the internal and external environment and send signals to the nervous system.

• Integration: Processing and interpretation of sensory input.

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Structural Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord, serving as the integration and command center.

• Brain: Controls higher functions and processes sensory information.

• Spinal Cord: Conducts signals to and from the brain; controls reflexes.

Peripheral Nervous System (PNS)

The PNS consists of nerves and ganglia outside the CNS, connecting the CNS to limbs and organs.

• Nerves: Bundles of axons that transmit signals.

• Ganglia: Clusters of neuron cell bodies in the PNS.

Functional Divisions of the Nervous System

Sensory (Afferent) Division

Transmits sensory information from receptors to the CNS.

• Somatic Sensory Fibers: Carry information from skin, skeletal muscles, and joints.

• Visceral Sensory Fibers: Carry information from organs.

Motor (Efferent) Division

Transmits motor commands from the CNS to effector organs.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

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

Somatic and Autonomic Nervous Systems

Somatic Nervous System

Conducts impulses from the CNS to skeletal muscles, allowing voluntary control.

• Voluntary Nervous System: Conscious control of skeletal muscles.

Autonomic Nervous System

Regulates involuntary functions and consists of two subdivisions:

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

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

Histology of Nervous Tissue

General Features

Nervous tissue is highly cellular with little extracellular space. It contains two principal cell types:

• Neuroglia (glial cells): Support and protect neurons.

• Neurons: Excitable cells that transmit electrical signals.

Neurons

Structure and Function

Neurons are the structural units of the nervous system, specialized for rapid communication.

• Cell Body (Soma): Contains the nucleus and organelles; biosynthetic center.

• Processes: Arm-like extensions (dendrites and axons) for receiving and transmitting signals.

• Longevity: Neurons can live over 100 years.

• Amitotic: Most neurons do not divide after development.

Neuron Structure

• Dendrites: Short, branched processes that receive signals and convey them to the cell body.

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

• Axon Hillock: Cone-shaped area where the axon originates from the cell body.

• Axon Terminals: Distal endings that form synapses with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse transmission.

Axonal Transport

Substances travel through the axoplasm by two types of axonal transport:

• Slow Axonal Transport: Moves substances slowly from the cell body toward the axon terminal.

• Fast Axonal Transport: Uses motor proteins and ATP to move organelles and vesicles rapidly in both directions.

Equation:

$\text{Rate of fast axonal transport} \approx 400 \text{ mm/day}$ $\text{Rate of slow axonal transport} \approx 0.5-10 \text{ mm/day}$

Classification of Neurons

Structural Classification

Neurons are classified by the number of processes extending from the cell body:

• Multipolar: Three or more processes (most common type in CNS).

• Bipolar: Two processes (rare, found in special sensory organs).

• Unipolar: One process (mainly in PNS, sensory neurons).

Functional Classification

Neurons are grouped by the direction in which nerve impulses travel relative to the CNS:

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors to the CNS.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors (muscles/glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neuroglia (Glial Cells)

Types of Neuroglia

Neuroglia are supportive cells in the nervous system, with several types found in the CNS and PNS.

• Astrocytes (CNS): Most abundant; support neurons, regulate the blood-brain barrier, and maintain the chemical environment.

• Oligodendrocytes (CNS): Form myelin sheaths around CNS nerve fibers.

• Microglial Cells (CNS): Act as phagocytes, removing debris and pathogens.

• Ependymal Cells (CNS): Line brain ventricles and spinal cord central canal; produce cerebrospinal fluid.

• Schwann Cells (PNS): Form myelin sheaths around PNS nerve fibers.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia; regulate the environment.

Astrocytes

• Support and brace neurons.

• Anchor neurons to capillaries.

• Control the chemical environment.

• Respond to nerve impulses and neurotransmitters.

• Influence neuronal functioning and information processing.

Oligodendrocytes

• Branched cells that form insulating myelin sheaths in the CNS.

• Increase the speed of electrical signal transmission.

Table: Comparison of Neuroglia Types

Cell Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate blood-brain barrier, maintain environment
Oligodendrocytes	CNS	Form myelin sheaths
Microglial Cells	CNS	Phagocytosis of debris and pathogens
Ependymal Cells	CNS	Produce cerebrospinal fluid
Schwann Cells	PNS	Form myelin sheaths
Satellite Cells	PNS	Regulate environment around neuron cell bodies

Summary

• The nervous system is divided into the CNS and PNS, each with structural and functional subdivisions.

• Neurons are specialized for rapid communication and are supported by various types of neuroglia.

• Understanding the structure and function of neurons and neuroglia is essential for studying the nervous system.