Fundamentals of the Nervous System and Nervous Tissue

Fundamentals of the Nervous System and Nervous Tissue Chapter 11

Introduction n n 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 n The nervous system has three overlapping functions l l l Gathering of sensory input Integration or interpretation of sensory input Causation of a response or motor output

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

Organization of the Nervous System

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

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

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

Innervation of Visceral Organs

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

Peripheral Nervous System n n n 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 n Nervous tissue is highly cellular l l n 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 l Neurons • Excitable cells that transmit electrical signals l Support cells • Smaller cells that surround and wrap the delicate neurons l These same cells are found within CNS and PNS

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

Supporting Cells in the CNS n n n 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 n n 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 n n n Cells function as antigen presenting cells of the immune response Control chemical environment around neurons, recapturing potassium ions and released neurotransmitters Astrocytes signal each other via intracellular calcium pulses

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

Ependymal n n 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

Oligodendrocytes n n n 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 n There are two supporting cells in the PNS l l n Satellite cells Schwann cells These cells are similar in type and differ mainly in location

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

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

Neuron Cell Body

Neuron Cell Body n n n 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 n n n 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 n n 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 n n n 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 Motor neuron

Neuron Processes n n n 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 l l Dendrites Axons Note: Convention of “typical” neuron

Dendrites n n 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 n n n 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 n n Motor neuron Each neuron has a single axon The axon arises Axon from the cone hillock 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

Axons n n n 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 Motor neuron Axon hillock

Myelinated Axon n n 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 n 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 n n 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 n 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 n n 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 n n n 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 n n 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 n n n 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 n n 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 n n 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 n n n 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

Structural Classification n Multipolar - many processes extend from cell body, all dendrites except one axon Bipolar - Two processes extend from cell, one a fused dendrite, the other an axon Unipolar - One process extends from the cell body and forms the peripheral and central process of the axon

Multipolar Neurons n n Multipolar neurons have three or more processes Most common type in humans Major neuron of the CNS Most have many dendrites and one axon, some neurons lack an axon

Bipolar Neurons n n n Bipolar neurons are rare in the human body Found only in special sense organs where they function as receptor cells Examples include those found in the retina of the eye and in the olfactory mucosa

Unipolar Neuron n n Unipolar neurons have a single process that emerges from the cell body The central process is more proximal to the CNS and the peripheral is closer to the PNS Unipolar neurons are chiefly found in the ganglia of the peripheral nervous system Function as sensory neurons

Functional Classification n n The functional classification scheme groups neurons according to the direction in which the nerve impulse travels relative to the CNS Based on this criterion there are three neurons l l l Sensory neurons Motor neurons Association neurons

Sensory Neurons n n Neurons that transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS are called sensory or affective neurons Virtually all primary sensory neurons of the body are unipolar

Sensory Neurons: Bipolar n n Bipolar sensory neurons are only found in the special sensory organs of the eye or olfactory mucosa Nuerons convey sensory input to higher CNS levels (eye to occipital lobe)

Sensory Neurons: Bipolar

Motor Neurons n n n Neurons that carry impulses away from the CNS to effector organs (muscles and glands) is called a motor or efferent neuron Upper motor neurons are in the brain Lower motor neurons are in PNS

Association Neurons or Interneurons n n These neurons lie between the motor and sensory neurons These neurons are found in pathways where integration occurs Confined to CNS Make up 99% of the neurons of the body and are the principle neuron of the CNS

Turn to Basic Concepts of Neural Integration Page 419

Neural Integration n The organization of the nervous system is hierarchical The parts of the system must be integrated into a smoothly functioning whole Neuronal pools represent some of the basic patterns of communication with other parts of the nervous system

Neuronal Pools n Neuronal pools are functional groups of neurons that process and integrate incoming information from other sources and transmit it forward One incoming presynaptic fiber synapses with Several different neurons in the pool. When Incoming fiber is excited it will excite some Postsynaptic neurons and facilitate others.

Neuronal Pools n n Neurons most likely to generate impulses are those most closely associated with the incoming fiber because they receive the bulk of the synaptic contacts These neurons are in the discharge zone Discharge Zone

Neuronal Pools n n Neurons farther away from the center are not excited to threshold by the incoming fiber, but are facilitated and can easily brought to threshold by stimuli from another source The periphery of the pool is the facilitated zone Facilitated zone

Neuronal Pools n n Note: The illustrations presented are a gross oversimplification of an actual neuron pool Most neuron pools consist of thousands of neurons and include inhibitory as well as excitatory neurons

Types of Circuits n n n Individual neurons in a neuron pool send and receive information and synaptic contacts may cause either excitation or inhibition The patterns of synaptic connections in neuronal pools are called circuits and they determine the functional capabilities of each type of circuit There are four basic types of circuits l Diverging, converging, reverberating, and parallel discharge circuits

Diverging Circuits n n In diverging circuits one incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit Diverging circuits are often called amplifying circuits because they amplify the response

Diverging Circuits n n n These circuits are common in both sensory and motor systems Input from a single receptor may be relayed up the spinal cord to several different brain regions Impulses from the brain can activate a hundred neurons and thousands of muscle fibers

Converging Circuits n n n The pattern of converging circuits is opposite to that of diverging circuits Common in both motor and sensory pathways In these circuits, the pool receives inputs from several presynaptic neurons, and the circuit as a whole has a funneling or concentrating effect

Converging Circuits n Incoming stimuli may converge from many different areas or from the same source, which results in strong stimulation or inhibition

Reverberating (oscillating) Circuits n n In reverberating circuits the incoming signal travels through a chain of neurons, each Reverberating of which makes circuit collateral synapses with neurons in the previous part of the pathway As a result of this positive feedback, the impulses reverberate through the circuit again and again

Reverberating (oscillating) Circuits n n n Reverberating circuits give a continuous signal until one neuron in the circuit is inhibited and fails to fire These circuits are involved in the control of rhythmic activities such as the sleep-wake cycle and breathing The circuits may oscillate for seconds, hours, or years

Parallel After-Discharge Circuits n n The incoming fiber stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell Impulses reach the output cell at different times, creating a burst of impulses called an after discharge that may last 15 ms after initial input ends

Parallel After-Discharge Circuits n n This circuit has no positive feedback and once all the neurons have fired, circuit activity ends These circuit may be involved with complex problem solving activities

Patterns of Neural Processing n n n Processing of inputs in the various circuits is both serial and parallel In serial processing, the input travels along a single pathway to a specific destination In parallel processing, the input travels along several different pathways to be integrated in different CNS regions Each pattern has its advantages The brain derives its power from its ability to process in parallel

Serial Processing n n n In serial processing the whole system works in a predictable all-or-nothing manner One neurons stimulates the next in sequence, producing a specific, anticipated response Reflexes are examples of serial processing but there are others

Reflexes n n n Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same motor response Reflex activity is stereotyped and dependable Some your are born with and some you acquire as a consequence of interacting with your environment

Serial Processing: A Reflex Arc n Reflexes occurs over neural pathways called reflex arcs that contain five essential components l l l Receptor Sensory neuron CNS integration center Motor neuron Effector

Parallel Processing n n n In parallel processing inputs are segregated into many different pathways Information delivered by each pathway is dealt with simultaneously by different parts of neural circuitry During parallel processing several aspects of the stimulus are processed l n Barking dog The same stimulus can hold common or unique meaning to different individuals

Parallel Processing n n Parallel processing is not repetitious because the circuits do different things with more information Each parallel path is decoded in relation to all the others to produce a total picture of the stimulus

Parallel Processing n n n Even simple reflex arcs do not operate in complete isolation As an arc moves through an association neuron this activates parallel processing of the same input at higher brain levels The reflex arc may cause you to pull away from a negative stimulus while parallel processing of the stimulus initiates problem solving about what need to be done

Parallel Processing n n Parallel processing is extremely important for higher level mental functioning An integrated look at the whole problem allows for faster processing Parallel processing allows you to store a large amount of information in a small volume This allows logic systems to work much faster

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue End of Chapter