Fundamentals of the Nervous System and Nervous Tissue

Fundamentals of the Nervous System and Nervous Tissue Chapter 11

Introduction • The nervous system is the master controlling and communicating system of the body • It is responsible for all behavior • Along with the endocrine system it is responsible for regulating and maintaining body homeostasis • Cells of the nervous system communicate by means of electrical signals

Nervous System Functions • The nervous system has three overlapping functions • Gathering of sensory input • Integration or interpretation of sensory input • Causation of a response or motor output

Introduction • Sensory input • The nervous system has millions of sensory receptors to monitor both internal and external change • Integration • It processes and interprets the sensory input and makes decisions about what should be done at each moment • Motor output • Causes a response by activating effector organs (muscles and glands)

Organization of the Nervous System

Organization • There is only one nervous system; however, for convenience the nervous system is divided into two parts • The central nervous system • Brain and spinal cord • Integrative and control centers • The peripheral nervous system • Spinal and cranial nerves • Communication lines between the CNS and the rest of the body

Organization • The peripheral nervous system has two fundamental subdivisions • Sensory (afferent) division • Somatic and visceral sensory nerve fibers • Consists of nerve fibers carrying impulses to the central nervous system • Motor (efferent) division • Motor nerve fibers • Conducts impulses from the CNS to effectors • (glands and muscles)

Organization • The motor division of the peripheral nervous system has two main subdivisions • The somatic nervous system • Voluntary (somatic motor) • Conducts impulses from the CNS to skeletal muscle • The autonomic nervous system (ANS) • Involuntary • Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

Innervation of Visceral Organs

Organization • The autonomic nervous system has two principle subdivisions • Sympathetic division • Mobilizes body systems during emergency situations • Parasympathetic division • Conserves energy • Promotes non-emergency functions • The two subdivisions bring about opposite effects on the same visceral organs • What one subdivision stimulates, the other inhibits

Peripheral Nervous System • Visceral organs are served by motor fibers of the autonomic nervous system and by visceral sensory fibers • The somata (limbs and body wall) are served by motor fibers of the somatic nervous system and by sensory somatic sensory fibers • Arrows indicate the direction of impulses

Histology of the Nervous Tissue • Nervous tissue is highly cellular • Less that 20% of the CNS is extracellular space • Cells are densely packed and tightly intertwined • Nervous tissue is made up of two cell types • Neurons • Excitable cells that transmit electrical signals • Support cells • Smaller cells that surround and wrap the delicate neurons • These same cells are found within CNS and PNS

Supporting Cells • All neurons associate closely with nonnervous support cells of which there are 6 types • Support cells of the CNS • Astrocytes • Microglial • Ependymal • Oligodendrocyte • Support cells of the PNS • Schwann cells • Satellite cells

Supporting Cells in the CNS • The supporting cells of the CNS are collectively called neuroglia or simply, glial cells • Like neurons, glial cells have branching processes and a central cell body • Neuroglia can be distinguished by their much smaller size and by their darker staining nuclei • They outnumber neurons in the CNS by a ration of 10 to 1 • Make up half of the mass of the brain

Astrocytes • Star shaped • Most abundant type of glial cell • Radiating projections cling to neurons and capillaries, bracing the neurons to their blood supply • Astrocytes play a role in exchanges between capillaries and neurons

Astrocytes • Cells function as antigen presenting cells of the immune response • Control chemical environment around neurons, recapturing potassium ions and released neuro- transmitters • Astrocytes signal each other via intracellular calcium pulses

Microglial • Small ovid cells with relatively long “thorny” processes • Their branches touch nearby neurons to monitor health of the neuron • Microglial migrate toward injured neurons

Microglial • When invading microorganisms are present or damaged neurons have died, the microglial transforms into a special type of macro- phage that protects the CNS by phagocytizing the microorganisms or neuronal debris • Important because cells of the immune system can enter CNS

Ependymal • Range in shape from squamous to columnar and many are cilated • Line the central cavities of the brain and spinal cord • Form a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS • Beating cilia circulates cerebrospinal fluid

Oligodendro- cytes • Fewer branches than astrocytes • Cells wrap their cytoplasmic extensions tightly around the thicker neurons in the CNS • Produce insulating coverings called myelin sheaths

Supporting Cells of the PNS • There are two supporting cells in the PNS • Satellite cells • Schwann cells • These cells are similar in type and differ mainly in location

Satellite Cells • Somewhat flattened satellite cells surround cell bodies within ganglia • Thought to play some role in controlling the chemical environment of neurons with which they are associated, but function is largely unknown

Schwann Cells • Surround and form myelin sheaths around the larger nerve fibers in PNS • Similar to the oligodendrocytes of CNS • Schwann cells are vital to peripheral nerve fiber regeneration

Neurons • Neurons are the structural units of the nervous system • Neurons are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another

Neuron Characteristics • Extreme longevity • Live and function optimally for a lifetime • Amitotic • As neurons assume their role in the nervous system they lose their ability to divide • Neurons cannot be replaced if destroyed • High metabolic rate • Require continuous and abundant supplies of oxygen and glucose • Homeostatic deviations often first appear in nervous tissue which has specific needs

Neurons • The plasma membrane of neurons is the site of electrical signaling, and it plays a crucial role in most cell to cell interaction • Most neurons have three functional components in common • A receptive component • A conducting component • A secretory or output component • Each component is associated with a particular region of a neuron’s anatomy

Neuron structure • Typically large, complex cells, they all have the following structures • Cell body • Nuclei • Nissl bodies • Axon hillock • Cell processes • Dendrites • Axon • Myelin sheath or neurilemma

Neuron Cell Body

Neuron Cell Body • The cell body consists of a large, spherical nucleus with a prominent nucleolus surrounded by cytoplasm • The cell ranges from 5 to 140m in diameter • The cell body is the biosynthetic center of the neuron

Neuron Cell Body • The cell body contains the usual organelles with the exception of centrioles (not needed in amitotic cells) • The rough endoplasmic reticulum or Nissl bodies is the protein and membrane making machinery of the cell • The cell body is the focal point for neuron growth in development

Neuron Cell Bodies • Clusters of cell bodies in the CNS are called nuclei • The relatively rare collection of cell bodies in the PNS are called ganglia

Neuron Processes Motor neuron • Cytoplasmic extension called processes extend from the cell body of all neurons • The CNS contain both neuron cell bodies and their processes • The PNS consists chiefly of processes

Neuron Processes Motor neuron • Bundles of neuron processes are called tracts in the CNS • Bundles of neuron processes in the PNS are called nerves • Two types of neuron processes • Dendrites • Axons Note: Convention of “typical” neuron

Dendrites • Dendrites are short, tapering diffusely branching extensions • Motor neurons have hundreds of dendrites clustering close to the cell body • Dendrites are receptive to input and provide an enormous surface area for the reception of signals • In many areas of the brain the finer dendrites are highly specialized for information collection

Dendrites • Dendritic spines represent areas of close contact with other neurons • Dendrites convey information toward the cell body • These electrical signals are not nerve impulses but are short distance signals call graded potentials

Axons Motor neuron • Each neuron has a single axon • The axon arises from the cone shaped axon hillock • It narrows to form a slender process that stays uniform in diameter the rest of its length • Length varies; short or absent to 3 feet in length Axon hillock

Axons Motor neuron • Each axon is called a nerve fiber • Each neuron has only one axon but may possess a collateral branch • It branches profusely at its end to form more than 10,000 telodendria Axon hillock

Myelinated Axon • Many nerve fibers, particularly those that are long or large in diameter, are covered with a whitish, fatty segmented myelin sheath • Myelin protects and electrically insulates fibers from one another

Myelinated Axon • Myelin increase the speed of transmission of nerve impulses • Myelinated axons transmit nerve impulses rapidly; 150 meters/second • Unmyelinated axons transmit quite slowly; 1 meter/second

Myelinated Processes • Myelin sheaths are associated only with axons and their collaterals as these are impulse conducting fibers and need insulation • Dendrites which carry only graded potentials are always unmyelinated

Myelination of an Axon • Myelin sheaths in the PNS are formed by Schwann cells • The cells first become indented to receive the axon and then wrap themselves around it in a jelly roll fashion • Initially the wrappings are loose, but the cell cytoplasm is squeezed out between layers

Myelination of an Axon • When the wrapping process is complete many concentric layers wrap the axon • Plasma membranes of myelinating cells have less protein which makes them good electrical insulators

Myelinated Axons • The nucleus and most of the cytoplasm of the Schwann cell is located just beneath the outer layer of the plasma membrane • The outer layer is called the sheath of Schwann • Gaps, called Nodes of Ranvier, occur between Schwann cell

Myelinated Axons • Nodes of Ranvier occur at regular intervals along the axon • Since the axon is only exposed at these nodes nerve impulses are forced to jump from one node to the next which greatly increases the rate of conduction

Myelinated Axons • Schwann cells that surround but do not coil around peripheral fibers are considered unmyelinated • Each axon occupies a separate tubular recess • Fibers are typically thin

CNS Axons • Oligodendrocytes form the CNS myelin sheaths • In contast to Schwann cells, oligodendrocytes can form the sheaths of as many as 60 processes at one time • Nodes are spaced more widely than in PNS • Axons can be myelinated or unmyelinated

CNS Axons • Regions of the brain containing dense collections of myelinated fibers are referred to as white matter and are primarily fiber tracts • Gray matter contains mostly nerve cell bodies and unmyelinated fibers

Classification of Neurons • Neurons can be classified structurally or functionally • Both classifications are described in the text • Functional classification is usually used to describe how the neurons work within us

Fundamentals of the Nervous System and Nervous Tissue