Fundamentals of the Nervous System and Nervous Tissue: Structured Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master control and communication system of the body, responsible for monitoring changes, processing information, and dictating responses. It is essential for maintaining homeostasis and coordinating bodily functions.

• Sensory receptors monitor changes inside and outside the body.

• Stimulus: Any change detected by receptors.

• Sensory input: Information gathered by receptors.

• Integration: Processing and interpretation of sensory input.

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

Divisions of the Nervous System

The nervous system is divided into two main anatomical and functional regions:

• Central Nervous System (CNS): Composed of the brain and spinal cord. It serves as the integrating and command center.

• Peripheral Nervous System (PNS): Consists of nerves (cranial and spinal) extending from the CNS, linking all regions of the body to the CNS.

Subdivisions of the PNS

• Sensory (afferent) division: Carries signals from sensory receptors to the CNS.

  • Somatic sensory: Skin, muscles, bones.

  • Visceral sensory: Visceral organs.

• Motor (efferent) division: Carries signals away from the CNS to muscles and glands.

  • Somatic motor: Voluntary control of skeletal muscles.

  • Visceral motor: Involuntary control of smooth and cardiac muscle; forms the autonomic nervous system.

Organization of the Nervous System

The nervous system is organized hierarchically, with the CNS at the center and the PNS connecting the rest of the body. Sensory information flows into the CNS, and motor commands flow out.

Sensory Subdivision of the PNS

Somatic Sensory

• General somatic senses: Touch, pain, vibration, pressure, temperature.

• Proprioceptive senses: Detect stretch in tendons and muscles; provide information about body position and movement.

• Special somatic senses (covered in Ch. 16): Hearing, balance/equilibrium, vision, taste, smell.

Visceral Sensory

• General visceral senses: Stretch, internal pain, nausea, hunger.

• Widely felt in digestive, urinary, and reproductive organs.

Motor Subdivision of the PNS

Somatic Motor

• Signals contraction of skeletal muscles.

• Under voluntary control.

• Also called the voluntary or somatic nervous system.

Visceral Motor

• Regulates contraction of smooth and cardiac muscle.

• Makes up the autonomic nervous system (involuntary).

• Controls function of visceral organs.

Nervous Tissue

Nervous tissue is composed of densely packed and intertwined cells, primarily of two types:

• Neurons: Transmit electrical signals and communicate all information in the nervous system.

• Neuroglia: Non-excitable support cells that surround and wrap neurons.

Neurons: Structure and Function

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

• Conduct electrical signals along the plasma membrane (neurilemma) as action potentials.

• Do not divide (except neural stem cells).

• Longevity: Can live and function for a lifetime.

• High metabolic rate: Require abundant oxygen and glucose; die quickly without oxygen.

Neuron Structure: Cell Body (Soma)

• Contains usual organelles plus:

  • Chromatophilic bodies (Nissl bodies): Clusters of rough ER and free ribosomes; renew membranes and proteins.

  • Neurofibrils/neurofilaments: Bundles of intermediate filaments; help hold the cell together.

• Most cell bodies are in the CNS (nuclei); in the PNS, they are in ganglia.

Neuron Processes

• Dendrites: Extensively branching; function as receptive sites for signals from other neurons; transmit signals toward the cell body.

• Axons: Each neuron has one; generates and conducts impulses away from the cell body.

  • Branches infrequently; if present, called axon collaterals.

  • Multiple branches at the end (terminal arborization), ending in terminal boutons (axon terminals).

  • Contains neurofilaments, actin microfilaments, and microtubules for strength and transport (axonal transport).

  • Action potentials are generated at the axon hillock and conducted to the terminal boutons, where neurotransmitters are released.

Synapses

Synapses are specialized sites where neurons communicate, primarily via chemical messengers called neurotransmitters.

• Presynaptic neuron: Conducts signal toward the synapse via axon; releases neurotransmitters.

• Postsynaptic neuron: Receives neurotransmitters via dendrites; transmits electrical activity away from the synapse.

• Synaptic vesicles: Membrane-bound sacs in axon terminals containing neurotransmitters; mitochondria are abundant.

• Synaptic cleft: Space between the plasma membranes of the two neurons.

Classification of Neurons

Structural Classification

• Multipolar: More than two processes (numerous dendrites, one axon); most common, e.g., motor neurons.

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

• Unipolar (pseudounipolar): One short, single process; start as bipolar during development.

Functional Classification

• Sensory (afferent) neurons: Transmit impulses toward the CNS; virtually all are unipolar; cell bodies in ganglia in the PNS.

• Motor (efferent) neurons: Carry impulses away from the CNS to effector organs; most are multipolar; cell bodies within the CNS.

• Interneurons: Association neurons; most are multipolar; lie between motor and sensory neurons; typically confined to the CNS.

Table: Comparison of Structural Classes of Neurons

Neuron Type	Processes	Location/Function
Multipolar	Many dendrites, one axon	Most common; motor neurons in CNS
Bipolar	One dendrite, one axon	Rare; special sensory organs (retina, olfactory epithelium)
Unipolar (Pseudounipolar)	Single process splits into peripheral and central branches	Sensory neurons in PNS; cell bodies in ganglia

Neuroglia (Glial Cells)

Neuroglia are supportive cells in nervous tissue, outnumbering neurons and providing essential functions for neuron health and activity.

• Six types:

  • Four in CNS: Astrocytes, Microglia, Ependymal cells, Oligodendrocytes

  • Two in PNS: Satellite cells, Schwann cells

• Functions: Support neurons, cover nonsynaptic regions, some surround synapses.

Neuroglia in the CNS

• Branching processes and central cell body; can divide throughout life (can cause gliomas).

• Astrocytes: Most abundant; regulate neurotransmitter levels, increase blood flow, maintain blood-brain barrier, control ionic environment, produce growth factors, propagate calcium signals.

• Microglia: Smallest and least abundant; phagocytes (macrophages of CNS); engulf microorganisms and dead neurons; derived from monocytes.

• Ependymal cells: Line central cavities of brain and spinal cord; help circulate cerebrospinal fluid.

• Oligodendrocytes: Wrap axons in CNS with myelin sheaths; create white matter.

Neuroglia in the PNS

• Satellite cells: Surround neuron cell bodies in ganglia.

• Schwann cells: Surround axons; form myelin sheath in PNS.

Key Terms and Concepts

• Action potential: Large nerve impulse conducted along axons.

• Myelin sheath: Insulating layer around axons, formed by oligodendrocytes (CNS) and Schwann cells (PNS); increases speed of impulse transmission.

• Gray matter: Contains neuronal cell bodies, dendrites, and unmyelinated axons; found in CNS.

• White matter: Contains myelinated axons; forms tracts in CNS.

• Reflex arc: Simple chain of neurons responsible for rapid, automatic responses to stimuli; can be monosynaptic (one synapse) or polysynaptic (multiple synapses).

Example: Reflex Arc Components

1. Receptor: Site where stimulus is detected.

2. Sensory neuron: Transmits afferent impulse to CNS.

3. Integration center: One or more synapses in CNS.

4. Motor neuron: Conducts efferent impulse to effector.

5. Effector: Muscle or gland responding to stimulus.

Additional info:

• Neuronal regeneration is limited in the CNS due to inhibitory factors and lack of supportive glial bands, but some regeneration is possible in the PNS.

• Diseases such as multiple sclerosis involve immune attacks on myelin sheaths, leading to neurological dysfunction.