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, voluntary and involuntary movement, and maintaining homeostasis. It is essential for consciousness, personality, learning, and memory.

• Regulates homeostasis through control of body temperature, blood pressure, sleep/wake cycles, and blood pH.

• Seat of consciousness, personality, and experience of the world.

Master Controlling and Communicating System

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

• Rapid and specific communication via nerve impulses.

• Neurons are the main functional units.

• Neuroglia support and protect neurons.

Functions of the Nervous System

• Sensory Input: Receives information from internal and external environments via sensory receptors.

• Integration: Processes and interprets sensory input, deciding what should be done.

• Motor Output: Activates 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: Located in the cranial cavity.

• Spinal Cord: Located in the vertebral cavity.

Peripheral Nervous System (PNS)

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

• Spinal nerves: Carry impulses to and from the spinal cord.

• Cranial nerves: Carry impulses to and from the brain.

Functional Divisions of the Nervous System

Sensory (Afferent) Division

Transmits sensory information from receptors to the CNS.

• Somatic sensory fibers: Carry impulses from skin, skeletal muscles, and joints.

• Visceral sensory fibers: Carry impulses from visceral organs.

Motor (Efferent) Division

Transmits motor commands from the CNS to effector organs.

• Somatic Nervous System: Voluntary control of skeletal muscles.

• Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

Somatic and Autonomic Nervous Systems

Somatic Nervous System

• Conducts impulses from CNS to skeletal muscle.

• Voluntary nervous system.

Autonomic Nervous System

• Regulates smooth muscle, cardiac muscle, and glands.

• Involuntary nervous system.

• Two subdivisions:

  • Sympathetic: Mobilizes body systems during activity.

  • Parasympathetic: Conserves energy, promotes housekeeping functions.

Histology of Nervous Tissue

General Features

• Highly cellular, tightly packed tissue.

• Little extracellular space.

Principal Cell Types

• Neurons: Excitable cells that transmit electrical signals.

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

Neurons

Structure and Function

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

• Large, highly specialized cells.

• Conduct impulses (action potentials).

• Extreme longevity (over 100 years).

• Amitotic (do not divide), with few exceptions.

• High metabolic rate; require continuous supply of oxygen and glucose.

Neuron Anatomy

• Cell body (soma): Contains nucleus and organelles; biosynthetic center.

• Processes: Arm-like extensions from the cell body.

  • Dendrites: Short, branched processes; receptive region.

  • Axon: Long process; conducting region, transmits impulses away from cell body.

Axon Structure

• One axon per neuron, arising from axon hillock.

• May be short or long (up to 1 meter).

• Branches at end (axon terminals).

• Conducts impulses to other neurons or effector cells.

Axonal Transport

Substances travel through axoplasm by two types of axonal transport:

• Slow axonal transport: Moves substances slowly from cell body toward axon terminals.

• Fast axonal transport: Uses motor proteins and ATP to move organelles and vesicles rapidly.

Equation:

$\text{Rate of fast axonal transport} \approx 400 \text{ mm/day}$

$\text{Rate of slow axonal transport} \approx 0.5-2 \text{ mm/day}$

Classification of Neurons

Structural Classification

• Multipolar: Three or more processes (one axon, many dendrites); most common in CNS.

• Bipolar: Two processes (one axon, one dendrite); rare, found in special sensory organs.

• Unipolar: Single short process; mainly in PNS as sensory neurons.

Functional Classification

• Sensory (afferent) neurons: Transmit impulses from sensory receptors to CNS.

• Motor (efferent) neurons: Carry impulses from CNS to effectors.

• Interneurons (association neurons): Shuttle signals through CNS pathways; most abundant.

Neuroglia (Glial Cells)

Types of Neuroglia

• Astrocytes (CNS): Most abundant, versatile, and highly branched glial cells. Support neurons, regulate capillary exchange, and control chemical environment.

• Oligodendrocytes (CNS): Branched cells that form insulating myelin sheaths in CNS.

• Microglial cells (CNS): Defensive cells; phagocytize microorganisms and debris.

• Ependymal cells (CNS): Line cerebrospinal fluid-filled cavities; help circulate CSF.

• Satellite cells (PNS): Surround neuron cell bodies in PNS; similar to astrocytes.

• Schwann cells (PNS): Form myelin sheaths in PNS; vital to regeneration of damaged nerve fibers.

Astrocytes

• Support and brace neurons.

• Anchor neurons to capillaries.

• Control chemical environment.

• Respond to nerve impulses and neurotransmitters.

• Influence neuronal functioning.

Oligodendrocytes

• Form myelin sheaths around CNS nerve fibers.

• Insulate and increase speed of nerve impulse transmission.

Summary Table: Types of Neuroglia

Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate environment
Oligodendrocytes	CNS	Form myelin sheaths
Microglial cells	CNS	Phagocytosis, immune defense
Ependymal cells	CNS	Line CSF cavities, circulate CSF
Satellite cells	PNS	Support neuron cell bodies
Schwann cells	PNS	Form myelin sheaths, aid regeneration

Key Terms and Definitions

• Neuron: Excitable cell that transmits electrical signals.

• Neuroglia: Non-excitable cells that support, protect, and insulate neurons.

• Axon: Long process of neuron that conducts impulses away from cell body.

• Dendrite: Short, branched process that receives input.

• Myelin sheath: Insulating layer around axons, increases speed of impulse transmission.

• Synapse: Junction between two neurons or a neuron and an effector cell.

Example: Application of Nervous System Function

When you touch a hot surface, sensory receptors in your skin send impulses via sensory neurons to the CNS. The CNS processes the information and sends motor commands via motor neurons to your muscles, causing you to withdraw your hand.