Fundamentals of the Nervous System and Nervous Tissue Chapter 12

Fundamentals of the Nervous System and

Nervous Tissue

Chapter 12

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 There is only one, highly integrated nervous system

Basic divisions of the nervous system

Central Nervous Systems

Peripheral Nervous System

Organization In order to discuss the nervous in smaller portions, 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 of the Nervous System

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 of the Nervous System

Organization The motor division of the peripheral nervous system has two main subdivisions The Somatic motor

•Voluntary motor•Conducts impulses from the CNS to skeletal muscle

The Visceral motor•Involuntary motor•Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

•Equivalent to the autonomic nervous system (ANS)

Organization of the Nervous System

Peripheral Nervous System

Organization of the Nervous System

Somatic sensory The sensory receptors that are spread widely throughout the outer tube of the body

These include the many senses experienced on the skin and in the body wall, such as touch, pain, pressure, vibration and temperature

Proprioception provides feedback from the stretch of the muscles, tendons and joint capsules - your “body sense”

Organization of the Nervous System

Somatic sensory The special somatic senses are receptors are more localized and specialized

The special senses include; sight, hearing, balance, smell and taste.

Organization of the Nervous System

Visceral sensory The general visceral senses include stretch, pain, and temperature which can be felt widely in the digestive and urinary tracts, reproductive organs, and other viscera

Sensations such as hunger and nausea are also general visceral sensations

The chemical senses such as taste and smell are considered by some as special visceral senses

Organization of the Nervous System

Somatic motor The general somatic motor is part of the PNS that stimulates contraction of the skeletal muscle in the body

Also referred to as voluntary nervous system

Skeletal muscles are widely distributed throughout the body, and therefore there is no special somatic motor category

Organization of the Nervous System

Visceral motor The general visceral motor part of the PNS regulates the contraction of smooth and cardiac muscle and secretion by the body’s many glands

General visceral motor neurons make up the autonomic nervous (ANS) which controls the function of the visceral organs

Organization of the Nervous System

Visceral motor Because we generally have no voluntary control over such activities as the pumping of the heart and movement of food through the digestive tract

The ANS is also called the involuntary nervous system

Organization of ANS 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

Nervous Tissue The nervous system consists mostly of nervous tissue whose cells are densely packed and tightly intertwined

Nervous tissue is made up just two main types of cells Neurons - the excitable cells that transmit electrical signals

Neuroglia - nonexcitable supporting cells that surround and wrap the neurons

Both cell types develop from the same embryonic tissues: neural tube and crest

The Neuron The human body contains many billions of neurons which are the basic structural units of the nervous system

Neurons are highly specialized cells that conduct electrical signals from one part of the body to another

These signals are transmitted along the plasma membrane in the form of nerve impulses or action potentials

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

The Neuron Special characteristics of neurons They have extreme longevity. Neurons can and must function over a lifetime

They do not divide•As fetal neurons assume their roles as communication links in the nervous system, they lose their ability to undergo mitosis

•Cells cannot be replaced if destroyed - Some limited exceptions do exist in the CNS as neural stem cells have been identified

The Neuron Special characteristics of neurons They have an exceptionally high metabolic rate requiring continuous and abundant supplies of oxygen and glucose

Neurons cannot survive for more than a few minutes without oxygen

The Neuron Neurons are typically large, complex cells

Neurons vary in their structure but they all have two fundamental components Neuron cell body One or more processes

The Cell Body The cell body of the neuron is also called a soma

The cell bodies of different neurons vary widely in size (from 5 to 140 m in diameter)

However, all consist of a single nucleus surrounded by cytoplasm

The Cell Body Typically large, complex cells, neurons have the following structures Cell body

•Nuclei•Chromatophilic (Nissl) bodies•Neurofibrils•Axon hillock

Cell processes•Dendrites•Axon•Myelin sheath or neurilemma

The Cell Body Cell Body

Nuclei Chromatophilic (Nissl) bodies

Neurofibrils Axon hillock

Neuron Processes

Dendrites Axons Myelin sheaths Axon terminals

The Cell Body In all but the smallest neurons, the nucleus is spherical and clear and contains a nucleolus near its center

The Cell Body The cytoplasm contains all the usual cellular organelles with the exception of centrioles (not needed in amitotic cells) as well as Nissl bodies

These cellular organelles continually renew the membranes of the cell

The Cell Body Neurofibrils are bundles of intermediate filaments that run in a network between chromatophilic bodies

These filaments keep the cell from being pulled apart when it is subjected to tensile forces

The Cell Body The cell body is the focal point for the outgrowth of the neuron processes during embryonic development

The Cell Body In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons

The Cell Body Most neuron cell bodies are located within the CNS

However, clusters of cell bodies called ganglia (singular ganglion) lie along the nerves in the PNS

Neuron Processes Bundles of neuron processes in the CNS are called tracts

Bundles of neuron processes in the PNS are called nerves

Neuron Processes Armlike processes extend from the cell bodies of all neurons

There are two types of processes

Dendrites Axons

Motor neuron

Neuron Processes

The cell processes of neurons are described here using a motor neuron

Motor neurons represent a typical neuron, but sensory neurons differ from the typical pattern

Motor neuron

Dendrites Dendrites are short, tapering, diffusely branching extensions from the cell body

Motor neurons have hundreds of dendrites clustering close to the cell body

Dendrites function as receptive cites providing an enlarged area for the reception of signals from other neurons

By definition, dendrites conduct electrical signals toward the cell body

Dendrites Dendritic spines represent areas of close contact with other neurons

These electrical signals are not nerve impulses but are short distance signals call graded potentials

Axons Each neuron has only one 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

Axons Each axon is called a nerve fiber

Axons are impulse generators and conductors that transmit nerve impulses away from the cell body

Axons Chromatophilic bodies (Nissl) and the Golgi apparatus are absent from the axon and the axon hillock

Axons lack ribosomes and all organelles involved in protein synthesis, so they must receive their proteins from the cell body

Axons Neurofilaments, actin microfilaments, and microtubules are especially evident in axons, where they provide structural strength

These cytoskeletal elements also aid in the transport of substances to and from the cell body as the axonal cytoplasm is recycled and renewed

The movement of substances along axons is called axonal transport

Axons The axon of some neurons are short, but in others it can be extremely long

Motor neurons in the CNS have axons that must reach to the musculature that they control that might be 3-4 feet away

Any long axon is called a nerve fiber and travels in a group of fibers composing a nerve

Axons Although axons branch far less frequently than dendrites, occasional branches do occur along their length

These branches, called axon collaterals, extend from the axon at almost right angles

Axons Axons branch profusely at its terminus

Ten thousand of these terminal branches per neuron is not unusual

These branches end in knobs called axon terminals or boutons

Axon A nerve impulse is typically generated at the axon’s initial segment and is conducted along the axon to the axon terminals, where it causes the release of chemicals called neurotransmitters into the extracellular space

The neurotransmitters excite or inhibit the neurons or target organs with which the axon is in close contact

Axon Axon diameter varies considerably among the different neurons of the body

Axons with larger diameters conduct impulses faster than those with smaller diameters

Neurons follow the law of physics: The resistance to the passage of an electrical current decreases as the diameter of any “cable” increases

Synapses The site at which neurons communicate is called a synapse

Most synapses in the nervous system transmit information through chemical messengers

Synapses Some neurons in certain areas of the CNS transmit signals electrically through gap junctions

Synapses Because signals pass across most synapses in one direction only, synapses determine the direction of information flow through the nervous system

Synapses The neuron that conducts signals toward a synapse is called the presynaptic neuron; the neuron that transmits signals away from the synapse is called the postsynaptic neuron

Synapses Most neurons in the CNS function as both presynaptic (information sending) and postsynaptic (information receiving) neurons, getting information from some neurons and dispatching it to others

Synapses There are two main types of synapses

Most synapses occur between the axon terminals of one neuron and the dendrites of another neuron

These are called axondendritic synapses

Synapses Many synapses also occur between axons and neuron cell bodies

These synapses are called axosomatic synapses

Synapses Synapses are elaborate cell junctions

This section shows axodendritic synapses because its structure is representative of both types of synapses

Synapses Structurally synapses are elaborate cell junctions

At the typical axodendritic synapse the presynaptic axon terminal contain synaptic vesicles

Synapses Synaptic vesicles are membrane bound sacs filled with molecular neurotransmitters

These molecules transmit signals across the synapse

Synapses Mitochondria are abundant in the axon terminal as the secretion of neurotransmitters requires a great deal of energy

Synapses At the synapse, the plasma membranes of the two neurons are separated by a synaptic cleft

On the under surfaces of the opposing cell membranes are dense materials; the pre- and post- synaptic densities

Synapses When an impulse travels along the axon of the presynaptic neuron, it signals the synaptic vesicles to fuse with the presynaptic membrane at the presynaptic density

The fused area then ruptures releasing neurotransmitter molecules to diffuse across the synaptic cleft and bind to the postsynaptic membrane at the post synaptic density

Synapse The binding of the two membranes changes the membrane charge on the postsynaptic neuron, influencing the membrane’s ability to generate a nerve impulse

Signals Carried by Neurons

Neurons carry information via electrical signals called nerve impulses, or action potentials

Signals are relayed from neuron to neuron via chemical neurotransmitters

In essence an impulse is a reversal of electrical charge that travels rapidly along the neuronal membrane

Signals Carried by Neurons

In a resting (un-stimulated) neuron, the membrane is polarized which means that the inner cytoplasmic side is negatively charged with respect to its outer, extracellular side

Signals Carried by Neurons

In addition, the concentration of potassium ions (K+) is higher inside the neuron and the concentration of sodium ions (Na+) is higher outside the neuron

Signals Carried by Neurons

When a neuron is stimulated the permeability of the plasma membrane changes at the site of the stimulus, allowing Na+ ions to rush in.

As a result, the inner face of the membrane becomes less negative or depolarized

Signals Carried by Neurons

If the stimulus initiating the depolarization is strong enough, the membrane at the axon’s initial segment is depolarized, so that it is positively charged inside the axon and negatively charged outside

Signals Carried by Neurons

Once begun, this depolarization occurs all along the axon length

It is this wave of charge reversal that constitutes the nerve impulse

Signals Carried by Neurons

The impulse travels rapidly down the entire length of the axon without decreasing in strength

Signals Carried by Neurons

After the impulse has passed the membrane repolarizes itself

Signals Carried by Neurons

Neurons in the body receive stimuli either directly from the environment or from signals received at synapses

In signals received at synapses neurotransmitters released by presynaptic neurons alter the permeability of the postsynaptic membrane to certain ions

Signals Carried by Neurons

Synapses that result in an influx of positive ions into the postsynaptic neuron depolarize the neuron’s membrane and bring the neuron closer to impulse generation

These synapses are called excitatory synapses because they stimulate the postsynaptic neuron

Signals Carried by Neurons

Other synapses increase membrane polarization, making the external surface of the postsynaptic cell even more positive than it was

This makes the postsynaptic cell less likely to generate a nerve impulse

These types of synapses are called inhibitory synapses because they reduce the ability of the postsynaptic neuron to generate a nerve impulse

Signals Carried by Neurons

Thousands of excitatory and inhibitory synapses act on every neuron, competing to determine whether or not that neuron will generate an impulse

Classification of Neurons

Neurons can be classified structurally or functionally

Neurons are grouped structurally according to the number of processes that extend from the cell body

By this classification there are three types of neurons; Multipolar Bipolar Unipolar

Classification of Neurons 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

Classification of Neurons

Multipolar neurons usually have a single axon and many dendrites

This type of neuron constitutes 99% of the neurons in the body

Classification of Neurons

Multipolar neurons have more than two processes

Most common type in humans

Major neuron of the CNS

Some neurons lack an axon

Classification of Neurons

Bipolar neurons have two processes that extend from opposite sides of the cell body

This rare type of neuron occurs in the special sensory organs

Classification of Neurons

Bipolar neurons are found only in special sense organs where they function as receptor cells

Examples include those found in the retina of the eye, inner ear, and epithelium of the olfactory mucosa

They are primarily sensory neurons

Classification of Neurons

Unipolar neurons have a short, single process that emerges from the cell body and divides like a “T” into two long branches

Classification of Neurons

Unipolar neurons have a single process that emerges from the cell body

The central process (axon) 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 primarily as sensory neurons

Functional Classification

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 types of neurons Sensory neurons Motor neurons Interneurons

Functional Classification

Sensory Neurons

These afferent neurons make up the sensory division of the PNS

They transmit impulses toward the CNS from sensory receptors in the PNS

Sensory Neurons

Sensory neurons have their cell bodies in ganglia outside of the CNS

The single (unipolar) process is divided into the central process and the peripherial process

Sensory Neuron The central process is clearly an axon because it carries a nerve impulse and carries that impulse away from the cell body which meet the criteria which define an axon

The peripheral by contrast carries nerve impulses toward the cell body which suggests that it is a dendrite

However, the basic convention is that the central process and the peripheral process are parts of a unipolar neuron

Motor Neurons

Neurons that carry impulses away from the CNS to effector organs (muscles and glands) are called motor or efferent neurons

Upper motor neurons are in the brain

Lower motor neurons are in PNS

Motor Neurons

Motor neurons are multipolar and their cell bodies are located in the CNS (except autonomic)

Motor neurons form junctions with effector cells, signaling muscle to contract or glands to secrete

Interneuron or Association Neurons These neurons

lie between the motor and sensory neurons

Form complex neural pathways

Confined to CNS Make up 99.98% of the neurons of the body and are the principle neuron of the CNS

Interneuron Neurons

Almost all interneurons are multipolar Interneurons show great diversity in the size and branching patterns of their processes

Interneurons The Pyramidal cell is the large neuron found in the primary motor cortex of the cerebrum

The Purkinje cell is from the cerebellum

Supporting Cells All neurons associate closely with non-nervous supporting cells called neuroglia Support cells of the CNS

•Astrocytes•Microglial•Ependymal•Oligodendrocyte

Support cells of the PNS•Schwann cells•Satellite cells

Supporting Cells While each support cell has a unique specific function, in general these cells provide a supportive scaffolding for neurons

In addition, they all cover nonsynaptic parts of the neurons thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other

Supporting Cells in the CNS

Like neurons, glial cells have branching processes and a central cell body

Neuroglia can be distinguished from neurons by their much smaller size and darker staining nuclei

They outnumber neurons in the CNS by a ratio of 10 to 1

Make up half of the mass of the brain

Unlike neurons, glial cells divide throughout one’s lifetime

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 of ions between capillaries and neurons

Astrocytes Astrocytes take up and release ions to control the environment around neurons

Concentrations of ions must be kept within narrow limits for nerve impulses to be generated & conducted

Astrocytes recapture and recycle potassium ions and released neuro- transmitters

Astrocytes Astrocytes contact both the neuron and the capillary in order to sense when the neuron are highly active and releasing large amounts of neurotransmitters (glutamate)

Astrocytes then extract blood sugar from the capillaries they contact to obtain the energy they need to fuel the process of glutamate uptake

Astrocytes Astrocytes also are involved with synapse formation in developing neural tissue, produce molecules necessary for neural growth (brain-derived trophic factor BDTF) and propagate calcium signals that may be involved in memory

Understanding the role of these abundant glial cells in neural functioning is an area of ongoing research

Microglial Smallest and least abundant type of neuroglial cell

The elongated cells have relatively long “thorny” processes

They are phagocytes, the macrophages of the CNS

Microglial Microglial derive from blood cells and migrate to the CNS during embryonic and fetal development

Microglial engulf invading microogranisms and injured or dead neurons

Microglial When invading micro- organisms are present or damaged neurons have died, the micro- glial 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 CNS tissue originates in the embryo as a hollow neural tube and retains a central cavity throughout life

Form a simple epithelium that lines the central cavity of the spinal cord and brain

Ependymal Forms a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS

Ependymal cells bear cilia that helps circulate the 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

Neuroglia in 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

Myelin Sheaths Myelin sheaths are produced by oligo dendrocytes in the CNS and Schwann cells in the PNS

Myelin sheaths are segmented structures, each composed of the lipoprotein myelin and surround the thicker axons of the body

Myelin Sheaths

Each segment of myelin consists of a plasma membrane of a supporting cell rolled in concentric layers around the axon

Myelin Sheaths

Myelin sheaths form an insulating layer that… Prevents the leakage of electrical current from the axon

Increases the speed of impulse conduction

Makes impulse propagation more energy efficient

Myelin Sheaths in PNS Myelin sheaths in the PNS are formed by Schwann cells

Myelin sheaths develop during the fetal period and continue to develop during the first year of postnatal life

Myelin Sheaths in the PNS

In forming, the cells indent to receive the axon and then wrap themselves around the axon repeatedly in a jellyroll fashion

Initially loose, the wrapping eventually squeeze the cytoplasm outward between cell membrane layers

Myelin Sheaths in the PNS

When the process is complete many concentric layers of Schwann cell plasma membrane wrap the axon in tightly packed coil of membranes

The nucleus and most of the cytoplasm of the Schwann cell end up just external to the myelin layers

This external material is called the neurilemma

Myelin Sheaths - PNS

Because the adjacent Schwann cells along a myelinated axon do not touch one another, there are gaps in the myelin sheath

These gaps called the Nodes of Ranvier, occur at regular intervals about 1mm apart

Myelin Sheaths - PNS

In myelinated axons, nerve impulses do not travel along the myelin-covered regions of the axonal membrane, but instead jumps from the membrane of one Node of Ranvier to the next greatly increasing impulse conduction

Myelin Sheaths in the PNS

Only thick, rapidly conducting axons are sheathed in myelin

Thin, slowly conducting axons lack a myelin sheath and are called unmyelinated axons

Myelin Sheaths in the PNS

In unmyelinated axons the Schwann cells surround the axons but do not wrap around them in concentric layers of membrane

A single Schwann cell can partly enclose 15 or more unmyelinated axons with each in a separate tubular recess on the surface of the cell

Myelin Sheath Myelin increases the speed of transmission of nerve impulses

Myelinated axons transmit nerve impulses rapidly; 150 meters/second

Unmyelinated axons transmit quite slowly; 1 meter/second

Myelin Sheaths in the PNS

Unmyelinated axons are found in portions of the autonomic nervous system as well as in some sensory fibers

Myelin Sheaths of the PNS

Electron micrograph of an unmyelinated axon

Note the tubular tunnels that separate the axons

Myelin Sheaths in the CNS

Oligodendrocytes form the myelin sheaths in the brain and spinal cord

Each oligodendrocyte has multiple processes that coil around several different axons

Myelin Sheaths - PNS The nucleus

of the cell and most of the cytoplasm end up just external to the myelin layers

Myelin Processes - PNS 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

Myelin Sheaths - PNS 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

Myelin Sheaths - PNS Because the adjacent Schwann cells do not touch one another there are gaps in the myelin sheath

These gaps, called nodes of Ranvier, occur at regular intervals about 1 mm apart

Myelin Sheaths - PNS 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 impulse conduction

Myelin Sheaths - PNS Schwann cells that surround but do not coil around peripheral fibers are considered unmyelinated

A single Schwann cell can partly enclose 15 or more axons

Each ends occupying a separate tubular recess

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

Graded Potential In humans, natural stimuli are not applied directly to axons, but to dendrites and the cell body which constitute the receptive zone of the neuron

When the membrane of this receptive zone is stimulated it does not undergo a polarity reversal

Instead it undergoes a local depolarization in which the inner surface of the membrane merely becomes less negative

Graded Potential This local depolarization is called a graded potential which spreads from the receptive zone to the axon hillock (trigger zone) decreasing in strength as it travels

If this depolarizing signal is strong enough when it reaches the initial segment of the axon, it acts as the trigger that initiates an action potential in the axon

Signals from the receptive zone determine if the axon will fire an impulse

Synaptic Potential Most neurons in the body do not receive stimuli directly from the environment but are stimulated only by signals received at synapses from other neurons

Synaptic input influences impulse generation through either excitatory or inhibitory synapses

Synaptic Potential In excitatory synapses, neurotransmitters released by presynaptic neurons alter the permeability of the postsysnaptic membrane to certain ions, this depolarizes the postsynapatic membrane and drives the postsynaptic neuron toward impulse generation

Synaptic Potential Inhibitory synapses cause the external surface of the postsynaptic membrane to become even more positive, thereby reducing the ability of the postsynaptic neuron to generate an action potential

Thousands of excitatory and inhibitory synapses act on every neuron, competing to determine whether or not that neuron will generate an impulse

Neural Integration 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 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

Neuronal Pools 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 withSeveral different neurons in the pool. WhenIncoming fiber is excited it will excite somePostsynaptic neurons and facilitate others.

Neuronal Pools 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 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

Facilitatedzone

Types of Circuits 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 Diverging, converging, reverberating, and parallel discharge circuits

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

Reverberating (oscillating) Circuits

In reverberating circuits the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in the previous part of the pathway

As a result of this positive feedback, the impulses reverberates through the circuit again and again

Reverberating (oscillating) Circuits

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 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

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

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 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

Parallel Processing 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 Barking dog

The same stimulus can hold common or unique meaning to different individuals

Parallel Processing 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 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 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

Reflexes 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

Reflex Arcs Reflex arcs are simple chains of neurons that explain our simplest, reflective behaviors and determine the basic structural plan of the nervous system

Reflex arcs are responsible for reflexes, which are defined as rapid, automatic motor responses to stimuli

Reflex Arcs Reflexes that involve the contraction of skeletal muscle are referred to as somatic reflexes

Reflexes that involve the contraction of smooth muscle, cardiac muscle, or glands are referred to as visceral reflexes

Serial Processing: A Reflex Arc

Reflexes occurs over neural pathways called reflex arcs that contain five essential components

Receptor Sensory neuron CNS integration center Motor neuron Effector

Reflex Arcs The receptor, sensory neuron, motor neuron, and effector are all relatively straightforward components

When considering the integration center associated with reflex arcs, it is important to understand that the number of synapses involved can vary

The simplest reflex arcs involve only one synapse in the CNS while others involve multiple synapses and interneurons

Reflex Arcs

At the top is a reflex arc, at the left is a monosynaptic reflex and on the right is a poly synaptic reflex

Reflex Arcs

The monosynaptic reflex has only one synapse and no interneuron, while the polysynaptic has multiple synapses and an interneuron

Reflex Arcs - Monosynaptic

This is the simple knee-jerk reflex

The impact of the hammer on the patellar tendon stretches the quadriceps muscles

Reflex Arcs - Monosynaptic

Stretching activates a sensory neuron that directly activates a motor neuron in the spinal cord, which then signals the quadriceps muscle to contract

This contraction counteracts the original stretching caused by the hammer

Reflex Arcs - Monosynaptic

Many skeletal muscles of the body can be activated by monosynaptic stretch reflexes

These reflexes help maintain equilibrium and upright posture

In these postural muscles, sensory neurons sense the stretching of muscles that occurs when the body begins to sway

Motor neurons activate muscles that adjust the body’s position to prevent a fall

Reflex Arcs - Monosynaptic

Because stretch reflexes contain just one synapse monosynaptic reflexes are the fastest of all reflexes

They are used in the body to maintain balance and equilibrium where speed of adjustment is essential to keep from falling

Reflex Arcs - Polysynaptic

Polysynaptic reflexes are the more common reflexes in the body

In these reflexes, one or more interneurons are part of a reflex pathway between the sensory and motor neurons

Reflex Arcs - Polysynaptic

Most of the simple reflex arcs in the body contain a single interneuron and therefore have a total of three neurons

Since there are two synapses joining the three neurons they are referred to as polysynaptic

Reflex Arcs - Polysynaptic

Withdrawal reflexes by which we pull away from danger are three-neuron reflexes

Pricking a finger with a tack initiates an impulse in the sensory neuron, which activates the interneuron in the CNS

Reflex Arcs - Polysynaptic

The interneuron signals the motor neuron to contract the muscle that withdraws the hand from the negative stimulus

Reflex Arcs - Polysynaptic

The three neuron reflex arc are of special importance in the science of neuroanatomy

Three neuron reflex arcs reveal the fundamental design of the entire nervous system

Design of the Nervous System

Three neuron reflex arcs from the basis of the structural plan of the nervous system

Design of the Nervous System

Note that the cell bodies of the sensory neurons lie outside the CNS in sensory ganglia and that their central processes enter the dorsal aspect of the cord

Design of the Nervous System

In the CNS the cell bodies of most interneurons lie dorsal to those of the motor neurons and the long axons exit the ventral aspect of the spinal cord

Design of the Nervous System

The nerves of the PNS consist of the motor axons plus the long peripheral process of the sensory neurons

Design of the Nervous System

These motor and sensory nerve fibers extend throughout the body to reach the peripheral effectors and receptors

Design of the Nervous System

Even though reflex arcs determine its basic organization, the human nervous system is obviously more complex than a series of simple reflex arcs

To appreciate its complexity, we must expand our conception of interneurons

Interneurons include not only the inter- mediate neurons of reflex arcs, but also all the neurons that are entirely confined within the CNS

Design of the Nervous System

The complexity of the CNS arises from the organization of the vast numbers of interneurons in the spinal cord and brain into complex neural circuits that process information

The complexity of the CNS results from long chains of interneurons that are interposed between each sensory and motor neuron

Design of the Nervous System

Although tremendously oversimplified, the infor-mation depicted is a useful way to conceptualize the organization of neurons in the CNS

Design of the Nervous System

The CNS has distinct regions of gray and white matter that reflect the arrangement of its neurons

The gray matter is a gray colored zone that surrounds the hollow cavity of the CNS

It is H-shaped in the spinal cord, where its dorsal half contains cell bodies of interneurons and its ventral half contains cell bodies of motor neurons

Design of the Nervous System

Gray matter is a site where neuron cell bodies are clustered

Specifically, gray matter is a mixture of neuron cell bodies, dendrites, and short unmyelinated axons

Design of the Nervous System

White matter which contains no neuron cell bodies but millions of axons

Its white color comes from the myelin sheaths around many of the axons

Most of these axons ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these two regions of the CNS to communicate with each other

Design of the Nervous System

White matter consists of axons running between different parts of the CNS

Within the white matter, axons traveling to similar destinations form axon bundles called tracts

Nervous Tissue Development

During the embryonic period, which spans 8 weeks, the embryo goes from zygote to blastocyst, to two layer embryo, to three layer embryo

The embryo upon reaching three layers begins to form the neural tube from which will differentiate the brain and spinal cord

Nervous Tissue Development

The nervous system develops from the dorsal section of the ectoderm, which invaginates to form the neural tube and the neural crest

Nervous System Development

The walls of the neural tube begin as a layer of neuroepithelial cells become the CNS

These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide

Nervous System Development

These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide

They cluster as future interneurons and motor neurons

Nervous System Development

Just external to the neuroepithelium, the neuroblasts cluster into alar and basal plates

Nervous System Development

Dorsally, the neurons of the alar plate become interneurons

Ventrally, the neuroblasts of the basal plate become motor neurons and sprout axons that grow out to the effector organs

Nervous System Development

Axons that sprout from the young interneurons form the white matter by growing outward the length of the CNS

These events occur in both the spinal cord and the brain

Nervous System Development

Most of the events described take place in the second month of development, but neurons continue to form rapidly until the about the sixth month

At the sixth month neuron formation slows markedly, although it may continue at a reduced rate into childhood

Nervous System Development

Just before neuron formation slows, the neuroepithelium begins to produce astrocytes and oligiodendrocytes

The earliest of these glial cells extend outward from the neuroepithelium and provide pathways along which young neurons migrate to reach their final destination

As the division of its cells slows, the neuroepithelium becomes the ependymal layer

Nervous System Development

Sensory neurons do not arise from the neural tube but from the neural crest

This explains why the cell bodies of the sensory neurons lie outside the CNS

Sensory neurons also stop dividing during the fetal period

Nervous System Development

Sensory neurons cell bodies develop outside the CNS in the neural crest

Sensory neurons also stop dividing during the fetal period

Nervous System Development

Neuroscientists are actively investigating how forming neurons “hook up” with each other during development

As the growing axons elongate at growth cones, they are attached by chemical signals from other neurons called neurotrophins

At the same time, the receiving dendites send out thin, extensions to reach the approaching axons to form synapses

Nervous System Development

Which synaptic connections are made, and which persist, are determined by two factors; The amount of neurotrophin initially received

The degree to which a synapse is used after being established

Nervous System Development

Neurons that make “bad” connections are signaled to die via apoptosis

Of the neurons formed during the embryonic period, about two-thirds die before birth

This initial overproduction of neurons ensures that all necessary neural connections will be made and that mistaken connections will be eliminated