Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Nervous System Overview

Introduction to the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for monitoring internal and external environments, integrating sensory information, and coordinating voluntary and involuntary responses.

• Sensory input: Monitoring stimuli inside and outside the body.

• Integration: Interpretation of sensory input and decision-making.

• Motor output: Activation of effector organs (muscles and glands) in response to integration.

Cells in the nervous system communicate via rapid and specific electrical and chemical signals.

Organization of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integration and command center.

• Peripheral Nervous System (PNS): Composed of paired cranial and spinal nerves; carries messages to and from the CNS and links all parts of the body to the CNS.

Functional Divisions of the PNS

• Sensory (afferent) division: Transmits impulses from sensory receptors to the CNS. Includes:

  • Somatic afferent fibers: From skin, skeletal muscles, and joints.

  • Visceral afferent fibers: From visceral organs.

• Motor (efferent) division: Transmits impulses from the CNS to effector organs (muscles and glands).

Motor Division Subdivisions

• Somatic Nervous System: Voluntary control of skeletal muscles.

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

  • Sympathetic division

  • Parasympathetic division

Histology of Nerve Tissue

Principal Cell Types

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (Glial cells): Supporting cells that surround and wrap neurons, providing support, insulation, and nourishment.

Types of Neuroglia

• Astrocytes (CNS): Most abundant glial cells; support and brace neurons, anchor them to capillaries, and regulate the chemical environment.

• Microglia (CNS): Small, ovoid cells with spiny processes; act as macrophages to monitor neuron health and phagocytize debris.

• Ependymal Cells (CNS): Line the central cavities of the brain and spinal cord; cilia help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes (CNS): Form myelin sheaths around CNS nerve fibers, increasing conduction speed.

• Schwann Cells (PNS): Form myelin sheaths around PNS nerve fibers; vital for regeneration of damaged fibers.

• Satellite Cells (PNS): Surround neuron cell bodies in the PNS; function is not fully understood.

Neurons (Nerve Cells)

Structure and Function

Neurons are the structural and functional units of the nervous system. They are long-lived, amitotic, and have a high metabolic rate. Each neuron consists of a cell body (soma), dendrites, and an axon.

• Cell body (Soma): Major biosynthetic center; contains the nucleus and organelles.

• Dendrites: Receptive regions that carry impulses toward the soma.

• Axon: Conducts impulses away from the soma to target cells; may branch extensively.

Myelin Sheath

The myelin sheath is a whitish, protein-lipid covering formed by Schwann cells in the PNS and oligodendrocytes in the CNS. It protects and electrically insulates axons, increasing the speed of action potential conduction.

• Node of Ranvier: Gaps between myelinating cells where action potentials are regenerated.

Neurophysiology

Action Potentials

Neurons are highly excitable and communicate via action potentials (APs), which are brief reversals of membrane potential that travel along axons without decreasing in strength.

• Resting state: All Na+ and K+ channels are closed.

• Depolarization: Na+ channels open, causing influx of Na+.

• Repolarization: K+ channels open, causing efflux of K+.

• Hyperpolarization: Some K+ channels remain open, Na+ channels reset.

Saltatory Conduction

In myelinated axons, action potentials jump from node to node (Nodes of Ranvier), greatly increasing conduction speed. This is known as saltatory conduction.

Synaptic Transmission

Chemical Synapses

At chemical synapses, the axon terminal of the presynaptic neuron releases neurotransmitters into the synaptic cleft, which bind to receptors on the postsynaptic cell, causing excitatory or inhibitory effects.

Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the likelihood of an action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the membrane, decreasing the likelihood of an action potential.

Summation

• Temporal summation: Rapid, repeated stimulation by one presynaptic neuron.

• Spatial summation: Simultaneous stimulation by multiple presynaptic neurons.

Neuronal Pools and Circuits

Neuronal Pools

Neuronal pools are groups of neurons that integrate incoming information and forward processed information to other destinations. They have a discharge zone (most likely to fire) and a facilitated zone (less likely to fire).

Types of Circuits in Neuronal Pools

• Diverging circuit: One input, many outputs; amplifies signals.

• Converging circuit: Many inputs, one output; concentrates signals.

• Reverberating circuit: Signal travels through a chain of neurons, each feeding back to previous neurons; controls rhythmic activity.

• Parallel after-discharge circuit: Signal stimulates neurons in parallel arrays that converge on a single output cell; involved in complex processes.

Development of Neurons

The nervous system originates from the neural tube and neural crest. The neural tube becomes the CNS. Neuron development involves proliferation, migration, and differentiation into neuroblasts.