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COORDINATION

Introduction

All animals except sponges use a network of nerve cells to gather information about the body’s condition and the external environment, to process and integrate that information, and to issue commands to the body’s muscles and glands. The nervous system, composed of neurons and is a fast communication system and plays a key part in the many feedback systems that maintain the constancy of the body’s internal environment. All input from sensory neurons to the central nervous system are transmitted as electrical signals. Sensory neurons receive input from many different kinds of receptor cells, such as the rod and cone cells found in the vertebrate eye. Different sensory neurons send information to different brain regions and, so, are associated with the different senses. The brain distinguishes among a sunset, a symphony, and searing pain only in terms of the identity of the sensory neuron carrying the action potentials and the frequency of these impulses.

 

Coordination is the process of linking together various physiological activities of an organism. Irritability or sensitivity refers to the ability to respond to a stimulus or is the ability of an organism to detect changes and be able to make appropriate responses. Stimulus is the change in the internal or external environment which is detected by nerve cells or stimulus is a detectable change in the physical or chemical structure of an organism's internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction. These sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanoreceptors. An internal stimulus is often the first component of a homeostatic control system. The stimulus is received by a receptor, it is transmitted by means of nerves or hormones, and effector brings about a response.

Animals, unlike plants, have two different but related systems of coordination: the nervous system and the endocrine system. The two systems have developed in parallel in animals. A good case study about the difference between nervous and hormonal control, is the control of digestive secretions in the gut. Plants also has a chemical coordination system equivalent to hormones, so the extra possession of nervous system in animals is probably related to their need to seek food. This requires sense organs and locomotion, which are controlled by nervous system.

NERVOUS COORDINATION IN ANIMALS

NERVOUS SYSTEM

The ability to respond to external stimuli is the fundamental characteristic of all living organisms. While all cells of multicellular organisms are able to perceive stimuli, the cells of the nervous system are specifically adapted to this purpose. Nervous system is a system of the body that in vertebrates includes the brain, spinal cord, nerves, and sense organs and receives, interprets, and responds to stimuli from inside and outside the body or is the organ system consisting of the brain, spinal cord and associated nerves that coordinates other organ system of the body.

Function of the nervous system

The nervous system is made up of highly specialised cells whose function is to;

·         Receive stimuli from the environment. In multicellular organisms this is done by modified nerve cells called receptors.

·         Converts the stimuli into the form of electrical impulses, a process called transduction.

·         Transmit them, often over considerable distances, to others specialised cells called effectors which are capable of producing an appropriate response.

Thus in a nervous system, the information about changes in the internal or external environment (stimuli) are collected by receptors and then transduced into the electrical signal called nerve impulse which is transmitted to the central nervous system (CNS) that is brain and spinal cord for processing and integration, often in relation to previous experience. After processing is done by CNS, then response is taken from the CNS to the effectors. The series of events will be as shown in figure below. One remarkable feature of the way in which these functions are performed is the speed with which the information is transmitted from one part of the body to another. In contrast to endocrine system, the nervous system responds virtually instantaneously to a stimuli. Between the receptors and effectors are conducting cells of the nervous system, the neurons. These are basic structural and functional units of nervous system and spread throughout the organism forming a complex communication network.

Sequence of events during the process of coordination

 

NERVOUS TISSUES

Nervous tissue is one of four major classes of tissues. It is specialized tissue found in the central nervous system and the peripheral nervous system. It consists of neurons and supporting cells called neuroglia. The nervous system is responsible for the control of the body and the communication among its parts.

NEURONS

Neurons are the basic structural and functional units of the nervous system. Neurons are highly specialized nerve cells that generate and conduct nerve impulses. The neuron is the basic building block of the brain and central nervous system. Neurons are specialized cells that transmit chemical and electrical signals. The brain is made up entirely of neurons and glial cells, which are non-neuronal cells that provide structure and support for the neurons. Nearly 86 billion neurons work together within the nervous system to communicate with the rest of the body. They are responsible for everything from consciousness and thought to pain and hunger. The primary components of the neuron are the soma (cell body), the axon (a long slender projection that conducts electrical impulses away from the cell body), dendrites (tree-like structures that receive messages from other neurons), and synapses (specialized junctions between neurons).

 

Structure of neuron

In addition to having all the normal components of a cell (nucleus, organelles, etc.) neurons also contain unique structures for receiving and sending the electrical signals that make neuronal communication possible. A typical neuron consists of dendrites, the cell body, and an axon.

Dendrite

Dendrites are branch-like structures extending away from the cell body, and their job is to receive messages from other neurons and allow those messages to travel to the cell body. In other words dendrites are responsible for responding to stimuli; they receive incoming signals towards the cell body.  Although some neurons do not have any dendrites, other types of neurons have multiple dendrites. Motor and association neurons possess a profusion of highly branched dendrites, enabling those cells to receive information from many different sources simultaneously. Some neurons have extensions from the dendrites, called dendritic spines, which increase the surface area available to receive stimuli.

 

Cell Body

Like other cells, each neuron has a cell body (or soma) which is an enlarged region that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. The cell body is like a factory for the neuron. It produces all the proteins and contains specialized organelles such as nucleus, granules and Nissl bodies. Extending from the cell body are one or more cytoplasmic extensions called dendrites. The surface of the cell body integrates the information arriving at its dendrites. If the resulting membrane excitation is sufficient, it triggers the conduction of impulses away from the cell body along an axon.

Axon

An axon, at its most basic, is a tube-like structure that carries an electrical impulse from the cell body (or from another cell’s dendrites) to the structures at opposite end of the neuron—axon terminals, which can then pass the impulse to another neuron. The cell body contains a specialized structure, the axon hillock, which serves as a junction between the cell body and the axon. Each neuron has a single axon leaving its cell body, although an axon may also branch to stimulate a number of cells. An axon can be quite long: The axons controlling the muscles in a person’s feet can be more than a meter long, and the axons that extend from the skull to the pelvis in a giraffe are about 3 m long. The axons are responsible for transmitting impulses over long distances from cell body. Some axons are covered with myelin, a fatty material that acts as an insulator and conductor to speed up the process of communication, while other they do not have.

Synapse

The synapse is the chemical junction between the axon terminals of one neuron and the dendrites of the next. It is a gap where specialized chemical interactions can occur, rather than an actual structure.

Myelin sheath

The axon is surrounded by a whitish, fatty layer called the myelin sheath. Outside the myelin sheath there is a cellular layer called the neurilemma (schwann cell). Schwann cells produce myelin in the PNS, while oligodendrocytes produce myelin in the CNS. During development, these cells wrap themselves around each axon several times to form the myelin sheath, an insulating covering consisting of multiple layers of membrane. Axons that have myelin sheaths are said to be myelinated, and those that don’t are unmyelinated. In the CNS, myelinated axons form the white matter, and the unmyelinated dendrites and cell bodies form the gray matter. In the PNS, both myelinated and unmyelinated axons are bundled together, much like wires in a cable, to form nerves. The myelin sheath is interrupted at intervals of 1 to 2 mm by small gaps known as nodes of Ranvier. In the peripheral nervous system, Schwann cells are neuroglia cells that support neuronal function by increasing the speed of  impulse propagation. The Schwann cells are underlain by the medullary sheath. The medullary sheath is interrupted at intervals by the nodes of Ranvier.

Functions of myelin sheath

·         It increase the speed of conduct of the nerve impulse by saltatory conduction

·         It insulate the axon of the neurone. Myelin Sheath together with neurilemma is known as the medullary sheath.

·         It protect the axon against physical damage 

Structure of a typical vertebrate neuron. Extending from the cell body are many dendrites, which receive information and carry it to the cell body. A single axon transmits impulses away from the cell body. Many axons are encased by a myelin sheath that insulates the axon. Small gaps, called nodes of Ranvier, interrupt the sheath at regular intervals.

 

Function of a Neuron

The specialized structure and organization of neurons allows them to transmit signals in the form of electric impulses from the brain to the body and back. Individually, neurons can pass a signal all the way from their own dendrites to their own axon terminals; but at a higher level neurons are organized in long chains, allowing them to pass signals very quickly from one to the other. One neuron’s axon will connect chemically to another neuron’s dendrite at the synapse between them. Electrically charged chemicals flow from the first neuron’s axon to the second neuron’s dendrite, and that signal will then flow from the second neuron’s dendrite, down its axon, across a synapse,  into a third neuron’s dendrites, and so on.

This is the basic chain of neural signal transmission, which is how the brain sends signals to the muscles to make them move, and how sensory organs send signals to the brain. It is important that these signals can happen quickly, and they do. Think of how fast you drop a hot potato—before you even realize it is hot. This is because the sense organ (in this case, the skin) sends the signal “This is hot!” to neurons with very long axons that travel up the spine to the brain. If this didn’t happen quickly, people would burn themselves.

Information flow in a neuron, from dendrites to cell body then to the axon which pass them to the next neuron.

 

Types of neurons

Neurons can be classified depending to functions and shape.

 

Classification of neurons according to functions

According to functions, neurons are classified into three major categories which are: sensory neurons, motor neurons, and interneurons. All three have different functions, but the brain needs all of them to communicate effectively with the rest of the body (and vice versa).

Sensory neurons (afferent neurons).

Sensory neurons are neurons responsible for converting external stimuli from the environment into corresponding internal stimuli. They are activated by sensory input, and send projections to other elements of the nervous system, ultimately conveying sensory information to the brain or spinal cord. Unlike the motor neurons of the central nervous system (CNS), whose inputs come from other neurons, sensory neurons are activated by physical modalities (such as visible light, sound, heat, physical contact, etc.) or by chemical signals (such as smell and taste).

Most sensory neurons are pseudounipolar, meaning they have an axon that branches into two extensions—one connected to dendrites that receive sensory information and another that transmits this information to the spinal cord.

Motor Neurons (efferent neurons)

Motor neurons are neurons located in the central nervous system, and they project their axons outside of the CNS to directly or indirectly control muscles. The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. The structure of motor neurons is multipolar, meaning each cell contains a single axon and multiple dendrites. This is the most common type of neuron.

Interneurons (relay/association neurons)

Interneurons are neither sensory nor motor; rather, they act as the “middle men” that form connections between the other two types. Located in the CNS, they operate locally, meaning their axons connect only with nearby sensory or motor neurons. Interneurons can save time and therefore prevent injury by sending messages to the spinal cord and back instead of all the way to the brain. Like motor neurons, they are multipolar in structure.

Three types of neurons. Sensory neurons carry information about the environment to the brain and spinal cord. Association neurons are found in the brain and spinal cord and often provide links between sensory and motor neurons. Motor neurons carry impulses or “commands” to muscles and glands (effectors).

 

The three basic types of neurons.

a. Sensory neurons receive information both internally and externally. 

b. Motor neurons stimulate muscles and glands. 

c. Interneurons integrate information by conducting signals between neurons.

Classification of neurons according to shape

There are four major types of neurons based on their shape. Which are unipolar neurons, bipolar neurons, pseudounipolar neurons and multipolar neurons.

Ø  Unipolar neurons are the most common neurons in invertebrates. These neurons are characterized by one primary projection that serves as both the axon and the dendrites.

Ø  Bipolar neurons, each having an axon that transmits signals from the cell body going to the brain and the spinal cord, and dendrites that send signals from the body organs to the cell body. These bipolar neurons are usually found in sensory organs such as the eyes, nose and ears.

Ø  Pseudo-unipolar neurons resemble unipolar neurons because each of them has an axon, but no true dendrites. However, pseudo-unipolar neurons are actually variants of bipolar neurons. The reason for this is that the single axon attached to the cell body proceeds to two opposite “poles" or directions – one towards the muscle, joints and skin, and the other towards the spinal cord. Pseudo-unipolar neurons are responsible for the sense of touch, pain and pressure.

Ø  Multipolar neurons are the dominating neurons in vertebrates in terms of number. These neurons are the ones that are the closest to the model neuron that we usually see in neuron structure diagrams. Each of them has a cell body, a long axon, and short dendrites.

Classification of neurons according to shape; This diagram shows the difference between: 1) a unipolar neuron; 2) a bipolar neuron; 3) a multipolar neuron; 4) a pseudounipolar neuron.

 

Unique Cells

There are dozens of neurons that possess very unique structures. Thus, researchers explain that there could be a hundred or more types of neurons in the central nervous system. These unique brain cells include the pyramidal neuron that has a cell body featuring a triangular pyramid shape. Pyramidal neurons are found in the prefrontal cortex. Some other unique neurons include basket cells (cortical interneurons), double bouquet cells (inhibitory interneurons), spiny neurons (found in the striatum and cortex input receivers), chandelier neurons (inhibitory interneuron) and Purkinje cells (tree-like neurons located in the cerebellum)

 

ADAPTATIONS OF NEURONS TO THEIR FUNCTIONS

Nerve cells are called neurons. They are adapted to carry electrical impulses from one place to another. A bundle of neurons is called a nerve. The features of neurons help them to carry out their function efficiently are;

(i)     They have a long fiber (axon) so they can carry messages up and down the body over long distances

(ii)    In a stimulated neuron, an electrical nerve impulse passes along the axon

(iii)  The axon is insulated by a fatty (myelin) sheath - the fatty sheath increases the speed of the nerve impulses along the neuron

(iv)   At each end of the neuron are tiny branches (dendrons), which branch even further into dendrites - the dendrites receive incoming nerve impulses from other neurons

(v)    Presence of mitochondria in the cell body for production of energy required during transmission of nerve impulse

(vi)   Presence of schwann cell which produce myelin sheath around neurons in PNS

(vii)  Presence of synapse which allow the conduction of nerve impulse from one neuron to the next

(viii)       Presence of cell body which control all the metabolic activities of the neuron

 

NEUROGLIA

Neuroglia are supporting cells of the nervous system. Neuroglia are also called “glial cells” or glia (from Greek word means ‘glue’). These cells are ten times more numerous than neurons. These glial cells are involved in many specialized functions apart from support of the neurons, including supplying the neurons with nutrients, removing wastes from neurons, guiding axon migration, and providing immune functions.

Types of neuroglia

There are six types of neuroglia—four in the central nervous system and two in the PNS. Neuroglia in the CNS include astrocytes, microglial cells, ependymal cells and oligodendrocytes. In the PNS, satellite cells and Schwann cells are the two kinds of neuroglia.

Astrocytes

Astrocytes are shaped like a star and are the most abundant glial cell in the CNS. They have many radiating processes which help in clinging to the neurons and capillaries. They support and brace the neurons and anchor them to the nutrient supply lines. They also help in the guiding the migration of young neurons. Astrocytes control the chemical environment around the neurons.

Microglial Cells

Microglial cells are small and ovoid in shape with thorny processes. They are found in the CNS. When invading microorganism or dead neurons are present, the microglial cells can transform into a phagocytic macrophage and help in cleaning the neuronal debris.

Ependymal Cells

Ependymal cells are ciliated and line the central cavities of the brain and spinal cord where they form a fairly permeable barrier between the cerebrospinal fluid that fills these cavities and the tissue cells of the CNS.

Oligodendrocytes

Oligodendrocytes line up along the nerves and produce an insulating cover called myelin sheath. They are found in the CNS.

Satellite Cells

Satellite cells surround neuron cell bodies in the peripheral nervous system (PNS). They are analogous to the astrocytes in the CNS.

Schwann Cells

Schwann cells surround all nerve fibers in the peripheral nervous system and form myelin sheaths around the nerve fibers. They are found in the PNS. Their function is similar to oligodendrocytes.

 

FUNCTION OF NEUROGLIA TISSUES

(i)           Act as parking cells within the CNS

(ii)         Absorbs oxygen and food nutrients from blood capillaries which surround the brain and transport it throughout the brain

(iii)      Play an essential role in a body immunity system since they contain numerous number of lysosomes, peroxisomes, microglia which are microphage like cells

(iv)       They are concerned with transportation of waste and pathogenic materials from the brain to the surrounding blood capillaries for eventually excretion

(v)         Glial cells surround neurons in brain where they confer structural support

(vi)       They have oligodendrocytes cells which are responsible for myelination of the CNS axons. Also they participate in repair of axons.

(vii)     Form a barrier around capillaries, they so called blood brain barrier, to prevent toxins and other substances from the brain

(viii)  They may take part in informational signaling in the brain

(ix)       They stimulate neural growth by secreting growth factors

(x)         They regulate the composition of the extracellular fluid in the CNS by removing potassium ions and neurotransmitters around synapses (Astrocytes).

(xi)       Assist in the production, circulation of cerebral spinal fluid (CSF)

(xii)     They dissolve gas concentration (Astrocytes)

(xiii)  They perform myelination of axons in the PNS and surround their neuron cell bodies.

(xiv)   Concern with storage and recall memories

Generalized structure of glia cell in the CNS

 

ADAPTATIONS OF NEUROGLIA CELLS TO THEIR FUNCTIONS

(i)                They have peroxisomes with catalase enzyme which is concerned with detoxification process

(ii)              They have finger like projections called dendrites which increases the surface area for receiving and recalling of memory

(iii)           They have specialized centre with a fluid within cytoplasm called GABA  concerned with storing memory

(iv)            They have lysosome for digestion of unwanted materials (cell parts) within CNS

(v)              They have schwann cell which is responsible for myelination

(vi)            Ependymal cell have a central canal ventricles for parking the produced cerebrospinal fluid 

 

GANGLIA

Ae group of cells found in the peripheral nervous system. They are composed of cell bodies, dendrites and glia cells. The major function of the peripheral ganglia is to connect the central nervous system to the different parts of the body. There are somatic and autonomic ganglia. Glia cells together with extracellular membranes they form neuroglia. Neuroglia encompasses the non-neural cells in the nervous tissue that provide various crucial support functions for their neurones. They are smaller than neurones, and vary in structure according to their function.

Function of ganglia

   It connect the central nervous system to the different parts of the body

   Contain neuroglia cells which support and protect the cell bodies of neurones

 

ADAPTIVE FEATURES OF NERVE TISSUES TO THEIR FUNCTION

(i)                The cell body is responsible for controlling metabolic activities such as synthesis of energy and is in charge of neuron growth and repair

(ii)              The axon is covered with a white fatty layer called myelin sheath which protect and insulate the axon also it accelerate the electrical signal during transmission

(iii)           They have neurilemma (schwann cell) which secrete the myelin sheath

(iv)            Nodes of Ranvier  play a very important role in salutatory conduction of impulses

(v)              The axon branches out at the end with small bulb like ending also known as terminal buttons which produce neurotransmitter essential in transmission of nerve impulse at the synapse.

(vi)            Neuroglia cells which are protective and supportive structures of the neurons tissues. They are found in branches surrounding the neurons and have ability to regenerate in case of injury.

(vii)          Presence of the periphery nervous system which is responsible in collecting signals from the organs and transmitting them to the central nervous system.

 

SPECIAL PROPERTIES OF NERVOUS SYSTEM

The nervous tissue has two special properties-excitability and conductivity.

1.      Excitability. It is the ability of nerve cells to generate an electrical impulse (excitations) in response to stimulus by altering the normal potential difference across their plasma membrane. Excitation arises at the receptors due to various types of stimuli such as light, temperature, chemical, electrical or pressure which constantly act on the organisms.

2.     Conductivity. It is the ability of nerve cells to rapidly transmit the electrical impulse as a wave from the site of its origin along their length in a particular direction.

 

DIVISIONS OF THE HUMAN NERVOUS SYSTEM

The nervous system can be divided into the central nervous system, CNS (brain and spinal cord) and the peripheral nervous system.

Central nervous system (CNS)

The CNS includes the grey matter and white matter. The grey matter is composed of cell bodies, dendrites, unmyelinated axons and very few myelinated axons while the white matter is composed of myelinated axons. The main function of the central nervous system is integration of information from the various sources. The collection both from internal and external environment is done by receptors. They usually form a sensory system along with neurons which transmit the collected information from different parts. The collected information is processed and integrated in the central nervous system and finally the information is transmitted to effectors (muscle and glands) which act upon it.

Peripheral nervous system (PNS)

The PNS includes the sensory neurons, motor neurons and ganglia. Ganglia are composed of cell bodies, dendrites and satellite glial cells whereby the nerves are composed of myelinated, unmyelinated axons and Schwann cells which are surrounded by connective tissue. The main function of the peripheral nervous system is to collect information from different parts of the body to the central nervous system for processing. After processing of information by the CNS, again motor neurons function to send the response to the effector which are always gland or muscle.

 

Divisions of the peripheral nervous system

The peripheral nervous system can be divided in two; the voluntary nervous system, which is under voluntary control from the brain and the autonomic nervous system which operates automatically (involuntary).

The autonomic nervous system is divided into the sympathetic nervous system (SNS), which has a mainly excitatory effect on the body and parasympathetic nervous system (PNS), which acts antagonistically (oppositely) to the SNS and has a mainly calming influence. The divisions of the nervous system are summarized in the figure below.

THE PERIPHERAL NERVOUS SYSTEM

All the nerves of the body together make up the peripheral nervous system. They all enter or leave the central nervous system, either spinal cord in case of spinal nerves, or the brain in case of cranial nerves. Spinal nerves arise from the spinal cord between the vertebrae along most of the length of the spinal cord. They all carry both sensory and motor neurons and are described as mixed nerves. Cranial nerves arise from the brain and with one exception (the vagus nerve), supply receptors and effectors of the head and neck.  There are 12 pairs of vagus nerves in mammals, numbered I-XII in Roman numerals. Not all cranial nerves are mixed. Three examples of cranial nerves are.

·         Cranial nerve II-optic nerve-this is the sensory nerve running from the retina to the brain

·         Cranial nerve III-oculomotor nerve-this is the  nerve running from the brain to the four eye muscles and helps control eye movements

·         Cranial nerve X-vagus nerve- this is the mixed nerve. It runs between the brain and the heart, gut and parts of respiratory tract and decreases heart rate, stimulates peristalsis and is concerned with speech and swallowing. It includes important motor nerves of the autonomic nervous system supplying the heart, bronchi and the gut.

 

THE AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system (autos, self; nomos, governing) is that part of the peripheral nervous system which controls activities inside the body that are normally involuntary, such as heart rate, peristalsis and sweating. It consists of motor neurones passing to the smooth muscles of internal organs. Smooth muscles are involuntary muscles. Most of the activity of the autonomic nervous system is controlled within the spinal cord or brain by reflexes known as visceral reflexes and does not involve the conscious control of higher centres of the brain. However, some activities, such as the control of the anal sphincter muscles which control defaecation, and bladder sphincter muscles which control urination (micturition), are also under the conscious control of the brain and control of these has to be learned. It is thought that many other autonomic activities may be able to be controlled by conscious effort and learning: many forms of meditation and relaxation have their roots in the control of autonomic activities, and considerable success has already been achieved in regulating heart rate and reducing blood pressure by conscious control or 'will power'. The overall control of the autonomic nervous system is maintained, however, by centres in the medulla (a part of the hind brain) and hypothalamus (also in the brain). These receive and integrate sensory information and coordinate this with information from other parts of the nervous system to produce the appropriate response.

The autonomic nervous system is composed of two types of neurone, a preganglionic neurone, which leaves the central nervous system in the ventral root before synapsing with several postganglionic neurones leading to effectors (figure below).

There are two divisions of the autonomic nervous system: the sympathetic (SNS) and the parasympathetic nervous systems (PNS). The structure of the two systems differs mainly in the organisation of their neurones and he these differences are shown in figure below.

In the sympathetic nervous system the synapses and cell bodies of the postganglionic neurones in the trunk region are situated in ganglia (swellings) close to the spinal cord. Each sympathetic ganglion is connected to the spinal cord by a white ramus communicans and to the spinal nerve by a grey ramus communicans as shown in figure below. A chain of connected sympathetic ganglia runs alongside the spinal cord forming sympathetic trunk. The ganglia of the parasympathetic nervous system are situated close to, or within, the effector organ. Other differences between the two systems include the nature of the chemical transmitter substance released at the postganglionic effector synapse, their general effects on the body and the conditions under which they are active. These differences are summarised in table below.

Connection of sympathetic ganglion to the spinal cord

Three pathway of sympathetic innervation

Summary of the differences between the sympathetic and parasympathetic nervous system

Feature	Sympathetic	Parasympathetic
· Origin of neurones   · Position of ganglion · Length of fibres   · Number of fibres · Distribution of fibres   · Area of influence · Transmitter substance   · General effects             · Overall effect     · Conditions when active	Ø Emerge from cranial, thoracic and lumbar regions of CNS Ø Close to spinal cord Ø Short preganglionic fibres Ø Long postganglionic fibres Ø Numerous postganglionic fibres Ø Preganglionic fibres cover a wide area Ø Effects diffuse Ø Noradrenaline released at effector Ø Increases metabolite levels, e.g. blood sugar Ø Increases metabolic rate Ø Increases rhythmic activities, e.g. heart rate Ø Raises sensory awareness   Ø Excitatory homeostatic effect Ø Dominant during danger, stress and activity; Ø Controls reactions to stress	Ø Emerge from cranial and sacral regions of CNS Ø Close to effector Ø Long preganglionic fibres Ø Short postganglionic fibres Ø Few postganglionic fibres Ø Preganglionic fibres cover a restricted region Ø Effects localized Ø Acetylcholine released at effector   Ø Decreases metabolite levels, e.g. blood sugar Ø None Ø Decreases rhythmic activities, e.g., heart rate Ø Restores sensory awareness to normal levels Ø Inhibitory homeostatic effect Ø Dominant during rest reactions to stress Ø Controls routine body activities

The sympathetic and parasympathetic nervous systems generally have opposing (antagonistic) effects on organs they supply, and this enables the body to make rapid and precise adjustments of involuntary activities in order to maintain a steady state. For example, an increase in heart rate due to the release of noradrenaline by sympathetic neurones is compensated for by the release of acetylcholine by parasympathetic neurones. This action prevents heart rate becoming excessive and will eventually restore it to its normal level when secretion from both systems balances out. A summary of the antagonistic effects of these systems is shown in table below. A careful study of this table will give you a good understanding of the functions of the two systems.

Summary of the effects of sympathetic and parasympathetic nervous system on the body

Region .	Sympathetic	Parasympathetic
· Head       · Heart   · Lungs     · Gut       · Blood             · Skin         · Kidney · Bladder · Penis · Glands	Ø Dilates pupils Ø None Ø Inhibits secretion of saliva   Ø Increases strength and rate of heart beat   Ø Dilates bronchi and bronchioles Ø Increases ventilation rate   Ø Inhibits peristalsis Ø Inhibits secretion of alimentary juices Ø Inhibits contraction of anal sphincter muscle Ø Constricts arterioles to gut and smooth muscle Ø Dilates arterioles to brain and skeletal muscle Ø Increases blood pressure Ø Increases blood volume by contraction of spleen Ø Contracts hair erector muscles (hair 'stands on end') Ø Constricts arterioles in skin of limbs (skin whitens) Ø Increases secretion of sweat Ø Decreases output of urine Ø Contracts bladder sphincter muscle Ø Induces ejaculation Ø Releases adrenaline from adrenal medulla	Ø Constricts pupils Ø Stimulates secretion of tears Ø Stimulates secretion of saliva   Ø Decreases strength and rate of heart beat   Ø Constricts bronchi and bronchioles Ø Decreases ventilation rate   Ø Stimulates peristalsis Ø Stimulates secretion of alimentary juices Ø Contracts anal sphincter muscle   Ø Maintains steady muscle tone in arterioles to gut, smooth muscle, brain and skeletal muscle, allowing normal blood flow Ø Reduces blood pressure Ø None     Ø None   Ø Dilates arterioles in skin of face (skin reddens) Ø None Ø None Ø Inhibits contraction of bladder sphincter muscles Ø Stimulates erection Ø None