Nervous Tissue Overview

Nervous Tissue Overview

• The nervous system helps maintain homeostasis by controlling internal conditions within healthy limits.
• The nervous system includes various branches and diverse cell types.

Introduction

• This chapter aims to explain how the nervous system helps maintain health.
• It will detail the different branches of the nervous system.
• It will categorize the various cells within nervous tissue.

Overview of Nervous System

• The nervous system is partitioned into the central nervous system (CNS) and peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS includes cranial nerves, spinal nerves, enteric plexuses, and sensory receptors in skin.

Nervous System Overview

• Sensory neurons transmit signals from sensory receptors to the CNS.
• Interneurons (in brain or spinal cord) connect sensory and motor neurons.
• Motor neurons transmit signals from the CNS to effectors (muscles or glands).
• Signals travel in a specific direction (receptor → sensory neurons → interneurons → motor neurons → effector).

Organization of the Nervous System

• The nervous system is organized into the CNS and PNS.
• The PNS comprises the sensory and motor divisions.
• The motor division branches into the somatic nervous system (skeletal muscles) and the autonomic nervous system (smooth, cardiac muscle, and glands).
• The autonomic nervous system further divides into sympathetic and parasympathetic divisions with differing effects on the target organs.
• Enteric plexuses (in the wall of the digestive tract) form a part of the nervous system.

Functions of the Nervous System

• Sensory function detects changes through sensory receptors.
• Integrative function analyzes sensory information, stores some aspects, and makes decisions to adjust behaviors.
• Motor function responds to stimuli via effectors.
• Neurons are electrically excitable cells.
• Neurons transmit nerve impulses.
• The impulse is known as an action potential.

Histology of Nervous Tissue

• Nervous tissue comprises neurons and neuroglia.
• Neurons are electrically excitable cells transmitting impulses.
• Neuroglia support and nourish the neurons, making up about half the volume of nervous tissue.
• Six types of neuroglia support nerve cells.

Neurons

• Neurons exhibit electrical excitability, meaning a stimulation can initiate a wave of depolarization.
• The nerve impulse (action potential) is a propagation of depolarization along the length of a neuron.

Structural Classification of Neurons

• A neuron's structure is classified according to the number of processes extending from the cell body.
• Multipolar neurons have many processes extending from their cell body.
• Bipolar neurons have two processes extending from the cell body.
• Pseudounipolar neurons have a single process extending from the cell body, which branches into two axon processes.

Examples of Dendritic Branching

• Neurons have diverse dendritic branching patterns.
• Purkinje cells and pyramidal cells are examples of neurons with distinctive dendritic branching.

Functional Classification of Neurons

• Neurons are classified based on the direction of nerve impulse propagation.
• Sensory (afferent) neurons transmit signals to the CNS.
• Motor (efferent) neurons transmit signals away from the CNS.
• Interneurons (association neurons) process information, connecting sensory and motor neurons.

Neuroglia

• Neuroglia are non-excitable cells.
• Neuroglia make up approximately half the volume of the nervous system.
• In the CNS, there are four types of neuroglia: astrocytes (support neurons in the CNS), oligodendrocytes (produce myelin in the CNS), microglia (participate in phagocytosis), and ependymal cells (form and circulate cerebrospinal fluid).
• In the PNS, there are two types of neuroglia: satellite cells (support neurons), and Schwann cells (produce myelin).

Myelination of Neurons

• Myelin sheaths are segmented fatty coatings around axons formed by Schwann cells (PNS) and oligodendrocytes (CNS).
• Myelin increases the speed of impulse propagation.

Gray Matter vs. White Matter

• CNS tissue is classified as gray or white matter.
• Gray matter is composed of cell bodies and unmyelinated axons.
• White matter is largely myelinated axons.

Electrical Signals in Neurons

• Excitation of neurons involves action potentials, graded potentials.
• Graded potentials allow communication over short distances; action potentials over longer distances.
• Action potentials (APs), occur when the membrane potential of a neuron reaches threshold.

Excitable Cells and the Resting Membrane Potential

• Resting membrane potential is the difference in electrical charge across a neuron's plasma membrane.

Graded Potentials & Action Potentials

• Graded potentials are small, local changes in membrane potential that vary in size depending upon the strength of the stimulus.

Ion Channels in Neurons

• There are three major categories of ion channels: leak channels, mechanically gated channels, and voltage-gated channels.

Resting Membrane Potential

• It's due to unequal ion distribution across the membrane.
• The membrane is selectively permeable to certain ions, primarily sodium (Na+) and potassium (K+).
• The Na+/K+ pump actively maintains this gradient.

Graded Potentials

• Small deviations in the resting membrane potential.
• Graded potentials arise from the activation of ligand-gated or mechanically gated ion channels.

Summation of Postsynaptic Potentials

• The sum of EPSPs and IPSPs determines whether the potential reaches threshold and initiates a nerve impulse at the trigger zone of the postsynaptic neuron.

Action Potentials

• Action potentials are rapid changes in membrane potential that propagate along the axon (all or none).

Propagation of Action Potentials

• Action potentials spread along the length of the neuron (do not "die out").

Continuous vs. Saltatory Conduction

• Continuous nerve impulse propagation involves continuous depolarization along the axon membrane whereas saltatory conduction involves impulse leaping from one Node of Ranvier to another.

Factors That Affect Propagation Speed

• Axon diameter
• Amount of myelination
• Temperature

Signal Transmission at Synapses

• Synapses are junctions where neurons connect with other neurons or effectors.
• Electrical synapses connect cells via gap junctions, enabling synchronized activity.
• Chemical synapses use neurotransmitters for one-way transfer of information from a presynaptic neuron to a postsynaptic neuron.

Synapses and Neurotransmitter Action

• Chemical synapses use neurotransmitters to transmit signals across the synaptic cleft.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs) are depolarizing.
• Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizing.

Structure of Neurotransmitter Receptors

• Receptors can be ionotropic (ligand-gated ion channels) or metabotropic (coupled to ion channels via G proteins).

Removal of Neurotransmitters

• Neurotransmitters are removed from the cleft by diffusion, enzymatic degradation, or re-uptake.

Summation

• Summation includes spatial summation (multiple inputs at once) and temporal summation (rapid, successive inputs).
• Combined inputs determine if threshold is reached and triggers a nerve impulse.

Neural Circuits

• Neural circuits are functional groups of neurons that process specific information.
• Types of neural circuits include simple series, diverging, converging, reverberating, and parallel after-discharge circuits.

Regeneration and Repair of Nervous Tissue

• The nervous system exhibits plasticity but regeneration varies between neurons of the CNS and PNS.

Neurogenesis in CNS

• The capacity of the CNS to regenerate is limited due to inhibitory influences from neuroglia and rapid scar tissue formation.

Damage and Repair in the CNS

• Damage to the PNS has greater potential for repair if the cell body remains intact.
• Repair involves chromatolysis, Wallerian degeneration, and formation of a regeneration tube.

Neural Disorders - Multiple Sclerosis

• Multiple sclerosis is an autoimmune disease characterized by progressive demyelination of axons.
• The cause is unknown but likely involves genetic and environmental factors.
• Symptoms typically include muscle weakness, abnormal sensations, and double vision.

Neural Disorders - Depression

• Depression encompasses various types, including major depression, dysthymia, bipolar disorder, and seasonal affective disorder.
• Common symptoms feature a lack of interest in activities, sadness, feelings of helplessness, and possible suicidal thoughts.
• Treatment frequently involves selective serotonin reuptake inhibitors (SSRIs).

Other Neural Disorders

• Epilepsy involves recurrent seizures, arising from abnormal electrical discharges in the brain.
• Excitotoxicity is neuronal damage from excessive stimulation, often caused by high levels of glutamate.

This quiz explores the essential functions and structure of the nervous system, explaining how it maintains homeostasis. It details the central and peripheral nervous systems and categorizes various types of neurons within nervous tissue.