NERVE SIGNALS MAINTAIN

HOMEOSTASIS

General Education Program

Biology

Presented by: Dr. Shaimaa Nasr Amin

Lecturer of Medical Physiology

Nervous system

Nervous systems perform the three overlapping functions of

sensory input, integration, and motor output

A nervous system has three overlapping functions :

sensory : conduction of signals from Sensory input )1(

receptors: such as light detecting cells in the eyes to

integration centers.

: is the process by which the signal from Integration )2(

sensory input is interpreted and associated with appropriate

responses of the body. Integration is carried out in the central

nervous system (CNS) which consists of the brain and the

spinal cord.

: is the conduction of signals from the Motor output )3(

integration center, the CNS, to effector cells, the muscle cells

or the glands.

The signals of the nervous system are

conducted by nerves.

: ropelike bundles of extensions of Nerves

neurons tightly wrapped in connective

tissue.

The nerves that communicate motor and

sensory signals between the CNS and the rest of

the body are collectively called peripheral

nervous system (PNS).

The structural and functional unit of the nervous

system is the neuron (nerve cell)

Networks of neurons with intricate connections

form nervous systems

A neuron has a:

and other nucleus: contains the Cell body) 1

organelles.

: short highly branched Dendrites) 2

processes that receive incoming messages

from other cells and carry this information

toward the cell body.

Neuron structure and synapses

Fig. 48.2

: usually much longer than dendrite. Axon) 3

convey outgoing messages from the neuron to other cells.

: the region of the axon where it joins the cell body Axon hillock

Some axons are enclosed by an insulated layer called the myelin sheath

Axon endings are called synaptic terminals, that contain chemical messengers called neurotransmitters (which conduct a signal across a synapse).

A synapse: is the

junction where one

neuron (presynaptic

neuron) communicate

with another neuron

(postsynaptic neuron)

in a neural pathway or

where a neuron

communicates with a

muscle or gland cell.

The site of contact between a synaptic terminal and a target cell is called a synapse .

Nerve impulses are conducted along a neuron.

Dendrite cell body axon hillock axon

Functional organization of neurons

Functionally there are three classes of neurons:

(1Sensory neurons: convey information about the external and

internal environment from sensory receptors to CNS

3) Motor neurons: convey

impulses from the CNS to

effector cells

2) Interneurons: located within the

CNS, integrate sensory input and

motor output

A Simple Nerve Circuit – the Reflex Arc.

The simplest type of nerve circuit regulates a reflex

(or autonomic response) and is called a reflex arc.

The simplest reflex arc require only two kinds of

nerve cells:

1) Sensory neuron: conveys signals from a

sensory receptor to a motor receptor

2) Motor neuron: which sends signals to an

cell that carries out glandor a muscle cell, a effector

the response.

Human knee-jerk reflex

Knee jerk reflex the one that makes your leg jerk forward when the

doctor hits your knee with a small hammer:

Knee jerk reflex involves more than simple sensory/motor circuit.

Contraction of the quadriceps (the front thigh muscle) is accompanied by

inhibition of the back thigh muscles (flexor muscles) that flex the lower leg

(pull it toward the body).

This inhibition involves a second nerve circuit:

The sensory neurons from the quadriceps form synapses not only with

motor neurons but also with interneurons in the spinal cord.

These interneurons inhibit motor neurons to the flexor muscles,

preventing them from contracting.

The cell body of the sensory neuron is located outside the spinal cord in a

structure called ganglion.

A ganglion is a cluster of nerve cell bodies within the PNS.

Cell bodies of motor neurons and interneurons are located in the gray matter of

the spinal cord.

Supporting Cells (Glia)

There are essential for the structural integrity of the nervous system and for the

normal functioning of the neurons.

Glia outnumber neurons by tenfold to fiftyfold.

There are several types of glia in the brain and in the spinal cord.

(a) Astrocytes are found within the CNS.

Provide structural and metabolic support.

Induce the formation of tight junctions between cells lining the

capillaries in the brain. This results in the formation of the blood-

brain barrier, which restricts the passage of most substances into

the brain.

Like neurons, astrocytes communicate with one another via

chemical signals.

(b) Oligodendrocytes are found within the CNS.

Form a myelin sheath by insulating axons.

(c) Schawnn cells are found within the PNS.

Form a myelin sheath by insulating axons

Neurons differ in terms of both function and shape.

Adapted for different functions, sensory neurons, motor

neurons and interneurons differ in shape (Figure 48.4)

1) Vertebrate motor neurons (Figure 48.2 a): the cell

body is connected to the dendrites and axon

2) Vertebrate sensory neurons: the cell body is

connected only to the axon. The short multibranched

dendrites communicate with sensory receptors cells. A

signal, long axon convey signals from dendrites to synapses

with neurons in the CNS.

3) Vertebrate interneurons: two types found in the

mammalian brain. One has multiple dendrites and a branched

axon, and the other has finely branched meshlike dendrites.

4) Invertebrate motor neuron: the cell body connects only to the

dendrites.

Neurons differ in terms of both function and shape.

Adapted for different functions, sensory neurons, motor

neurons and interneurons differ in shape

1) Vertebrate motor neurons : the cell body is connected

to the dendrites and axon

3) Vertebrate interneurons: two types found in the mammalian brain. One has

multiple dendrites and a branched axon, and the other has finely branched

meshlike dendrites.

2) Vertebrate sensory neurons: the cell body is connected only to the axon.

The short multibranched dendrites communicate with sensory receptors cells. A

signal, long axon convey signals from dendrites to synapses with neurons in the

CNS.

4) Invertebrate motor neuron: the cell

body connects only to the dendrites.

Types of Nerve Circuits

Nerve circuit show three basic patterns of organization:

source, such as singlefrom a informationOne kind of circuit takes )1

an eye, to several parts of the brain:

Single presynaptic neurons several postsynaptic neuron

sources such as vision hearing and touch several Information from )2

to identify an object.

Several presynaptic neurons single postsynaptic neuron

, from one neuron to others and circular pathInformation flows in a )3

then back to its source. Memories may be processed by circular paths.

Vertebrate Nervous Systems

Vertebrate nervous systems have central and peripheral components Figure 48.16 The nervous system of a vertebrate

In all vertebrates the nervous

system is divided into the

Central nervous system

(CNS) and the

Peripheral nervous system

(PNS)

Figure 48.16 The nervous system of a vertebrate

The nervous

system

Central Nervous

System

(CNS)

Peripheral Nervous

System

(PNS)

Brain Spinal

cord

Cranial

Nerve

12 pairs

Spinal

Nerves

31

pairs

(viscera)

(skeletal muscle)

Central nervous system (CNS): consists of the brain and

spinal cord

The brain provides the integrative power that underlies the

complex behavior characteristic of all vertebrates.

The spinal cord runs lengthwise inside the vertebral column,

integrates simple responses (knee-jerk reflex) and conveys

information to and from the brain.

The central canal of the spinal cord is continuous with the fluid

filled spaces, called ventricles, of the brain. These cavities are filled

with cerebrospinal fluid (CSF), which is formed in the brain by

filtration of the blood.

It conveys nutrients, hormones and white blood cells across

the blood-brain barrier, to different parts of the brain.

It acts as a shock absorber, cushioning the brain.

The brain and the spinal cord are surrounded by layers of

connective tissues called meninges.

In mammals cerebrospinal fluid circulates between the

meninges providing an addition cushion for the brain.

Axons in the CNS are located in bundles and their myelin sheaths

give them a white appearance (white matter).

White matter : is composed of bundles of myelinated axons

The white matter is distinguishable from the gray matter which

consists of dendrites, unmyelinated axons and nerve cell bodies.

Gray matter : consists of unmyelinated axons, nuclei, and

dendrites.

The divisions of the peripheral nervous

system interact in maintaining homeostasis

Peripheral nervous system (PNS): everything outside the

CNS. It consists of paired cranial and spinal nerves and associated

ganglia.

The cranial nerves (12 pairs) originate in the brain and

innervate organs of the head and upper body.

The spinal nerves (31 pairs) originate in the spinal and innervate

the entire body.

Most of the cranial nerves and all of the spinal nerves contain

both sensory and motor neurons; a few of the cranial nerves are

sensory only (e.g. the olfactory and optic nerves).

The PNS can also be divided into:

1) The sensory division of the PNS is made up of the sensory

or afferent (incoming) neurons that convey information to the

CNS from sensory receptors that monitor the external and

internal environment.

2) The motor division is composed of the motor or efferent

(outgoing) neurons that convey signals from CNS to effector

cells.

The motor division is divided into:

a) The somatic nervous system: carries signals to skeletal

muscles, mainly in response to external stimuli. It is often

considered voluntary, (but a substantial proportion of skeletal

muscle movement is determined by reflexes).

: conveys signals that autonomic nervous systemb) The

regulate the internal environment by controlling smooth and

cardiac muscles and the organs of the gastrointestinal,

cardiovascular, excretory and endocrine systems. This control

is generally involuntary.

The somatic and autonomic nervous systems often cooperate

in maintaining homeostasis.

e.g. in response to a drop in temperature, the hypothalamus

of the brain signals the autonomic nervous system to constrict

surface blood vessels, which reduces heat loss; at the same

time the hypothalamus signals the somatic nervous system

and causes shivering.

The nervous

system

Central Nervous

System

(CNS)

Peripheral Nervous

System

(PNS)

Brain Spinal

cord

Cranial

Nerve

12 pairs

Spinal

Nerves

31

pairs

(viscera)

(skeletal muscle)

The autonomic Nervous System

ORGANIZATION:

the nervous system can be divided into:

: The central nervous system-A

The central nervous system consists of:

1- Brain 2- spinal cord

: The peripheral nervous system-B

The peripheral nervous system consists

of:

: The somatic nervous system-1

It is further divided into:

1-the cranial nerves

2- the spinal nerves,

: The autonomic nervous system-2

It is further divided into:

1- the sympathetic division

2-the parasympathetic division

The ANS has 2 divisions: (1) The parasympathetic system

(2) The sympathetic system.

The parasympathetic or craniosacral

consists of: system

-Outflows in cranial nerves III, VII, IX and X.

-Outflows from the sacral part of the spinal

cord (S2, 3 and 4).

lumbar -The sympathetic or thoraco

consists of system

-Outflows from the thoraco-lumbar part

of the spinal cord (T1-L3).

has its The first neuron-

cell body in the CNS; it is

and called myelinated

. preganglionic neuronthe

neurons in series to get from the 2 A nerve impulse in the ANS has to travel along -

CNS to an effector cell

unmyelinatedis The second

and called the

.postganglionic neuron

-In the parasympathetic system the

nerve impulse is transmitted from

the postganglionic neuron to the

effector cell by acetylcholine.

-The nerve impulse is transmitted

from the pre- to the postganglionic

neuron by the chemical transmitter

acetyicholine (ACh). Where this

occurs, the nerve cell body forms a

swelling or ganglion.

in the sympathetic system the nerve

impulse is transmitted from the

postganglionic neuron to the effector

cell by nor adrenaline (NA).

- In a few exceptional cases post- ganglionic sympathetic

neurons release acetylcholine e.g. in sweat glands and in

vasodilator fibres in skeletal muscle blood vessels

-The effect of acetylcholine at

mimickedcan be ganglion cells

so ,nicotineby the drug

on acetylcholine receptors

are postganglionic neurons

.nicotinic receptorsclassified as

acetylcholine effects at

parasympathetic postganglionic

by the mimickedcan be junction

acetylcholine , so muscarinedrug

are receptors on the effector cells

muscarinic classified as

. receptors

AUTONOMIC GANGLIA

The sympathetic ganglia:

1-sympathetic chains:

-consist of 22 pairs of ganglia

linked by nerve fibres which form

the sympathetic chains.

-These chains run on either side of

the vertebral column from the base

of the skull down to the coccyx.

2-Collateral ganglion:

are situated at the origin of the

main arteries which come off the

abdominal aorta and take their

names from these arteries

The parasympathetic ganglia:

Terminal ganglia:

Are very near the organ which they innervate .

Intramural ganglion:

Are in the organ which they innervate .

Function of autonomic ganglia - Relay stations.

-Release of chemical transmitters.

-Distributing function.

-Regeneration of postganglionic fibers.

AUTONOMIC REFLEX

The ANS and somatic NS

are organized on the basis

of the reflex arc which

carries the reflex action.

Definition:

The reflex action is the

involuntary response of the

tissue towards a stimulus

The reflex arc:

It is the structural unit of

the reflex action and it

consists of:

a. Receptor: for reception of stimuli.

B.An afferent autonomic nerve fibers:

An afferent autonomic nerve fibers

initiated in visceral receptors (It

transmit impulses from the viscera to

the center which controls the visceral

functions)

spinal (the C.The centre in the C.N.S:

cord and brain stem).

-The centers analyses the afferent

impulses.

-Some of the sensory neurons convey

information about events in the

viscera to higher autonomic centres

which send impulses to modify the

activity of lower autonomic centre in

brain stem and spinal cord.

:D.The autonomic efferent nerve fibers

-The autonomic efferent nerve fibers arise from the centers (mentioned above)

and proceed outside the N.S. to the autonomic ganglia where they relay

(synapse) with their nerve cells (This is called the preganglionic fibers or

neurons).

-From such ganglia a second neuron arises and proceeds to the effectors

organ. (This fiber is called the postganglionic fibers or neurons). It carries

the nerve impulse from the centre to the effectors organ.

The functions of the autonomic system

-The functions of the autonomic system are normally reflexly controlled and are

carried out below the level of consciousness.

-The sympathetic and parasympathetic nervous systems are complementary regulators

of all autonomic functions, usually act in balanced reciprocal fashion.

-However this is not always true, e.g. most blood vessels have only a

sympathetic innervations

PARASYMPATHETIC OUTFLOWS SYMPATHETIC OUTFLOWS

: It In the head and the neck-

supplies the eye, salivary

glands, blood vessels and

sweet glands.

: In cardio vascular systems-

It supplies the heart and

coronary blood

Vessels.

: It In the respiratory system-

supplies the bronchi, lungs

and bronchioles.

: it supplies the eye The III cranial nerve-

and blood vs.

it supplies the The facial nerve (VII):-

salivary (sublingual and sub- mandibular)

and lacrimal glands and blood vs.

the The glossopharyngeal nerve (IX):-

parotid salivary glands and blood vs.

The vagus nerve (X):-

: It supplies In cardio vascular systems-

the heart and coronary blood vessels.

: It supplies the In the respiratory system-

bronchi, lungs and bronchioles.

PARASYMPATHETIC OUTFLOWS SYMPATHETIC OUTFLOWS

: It In the gastrointestinal tract-

supplies all the smooth muscles

of the tract and sphincters, the

liver, pancreas, spleen, gall

bladder and blood vessels

: smooth In the pelvic organs-

muscle of the urinary bladder,

kidney and rectum, internal anal

and urethral sphincter, male

external genital system and

pelvic blood vessels.

: It In the gastrointestinal tract-

supplies all the smooth

muscles of the tract and

sphincters, the liver,

pancreas, spleen, gall

bladder and blood vessels.

It :The pelvic nerve -

supplies smooth muscle of

the urinary bladder, and

rectum, internal anal and

urethral sphincter, kidney,

male external genital system

and pelvic blood vessels.

PARASYMPATHETIC ACTION SYMPATHETIC ACTION

Sympathetic nervous system produces vasoconstriction of all B VS except the

coronary &skeletal muscle BVS.

Acetylcholine

is the chemical transmitter

for

parasympatheticthe

nor adrenaline

is the chemical transmitter

for

the sympathetic

Transmitters of Autonomic N.S.

Acetylcholine Acetylcholine synthesis:

-Synthesis of acetylcholine involves the reaction of choline base

with acetate.

Choline

Choline+ Acetyl-CoA ---------------------- Acetylcholine

Acetyl transferase

-Acetylcholine is hydrolyzed and inactivated by acetyl

cholinesterase.

Acetylcholinesterase

Acetylcholine--------------------------- Choline + Acetate

-In all preganglionic fibers

(symp.and parasymp.).

-In neuromuscular junction

(motor end plate).

-In all postganglionic

parasympathetic fibers.

-In post ganglionic sympathetic

supplying sweat glands, and

skeletal muscle blood vessels.

-In C.N.S. as a chemical

transmitter of synapses and in

the eye.

b. Sites of Secretion of Acetylcholine (Cholinergic Neurons):

Actions of Acetylcholine

It has two actions:

preganglionicwhich exists in all I. Nicotinic like action

and neuromuscular junctions due to presence of

nicotinic receptors.

all postganglionicwhich exists in ,Muscarinic like actionii.

parasympathetic fibers and post-ganglionic sympathetic

fibers supplying sweat glands and skeletal blood vessels. It is

named as muscarinic since it contains muscarinic receptors on

the effectors organ

-The nicotinic action can be blocked by curare. Curare

produces competitive inhibition of nicotinic action of

acetylcholine.

-The muscarinic action can be blocked by atropine which

produces competitive inhibition of acetylcholine on the receptor

site.

Nor epinephrine -11

- It is the primary transmitter for the postganglionic

sympathetic neurons.

-It is synthesized in all post-ganglionic sympathetic fibers.

-It is removed from neurons by:

a) Reuptake.

b) Metabolized by:

-mono-amine oxidase (MAO), and

-catechol-O-methyltransferase (COMT).

Adrenergic Receptors

(Adrenoreceptors):

: receptors1 α-

-Are located on smooth muscle

except that of the bronchial

Tree

-Are excitatory receptors.

-Are equally sensitive to both

norepinephrine and epinephrin

(norepinephrine is dominant).

:receptors2 α-

- Are located in the

presynaptic nerve terminals,

smooth muscles and fat

cells.

-Are inhibitory in action.

-β 1receptors:

-Are located in the heart.

-Are excitatory in action.

-Are sensitive to both

epinephrine and

norepinephrine by

activation of adenylate

cyclase and production of

cyclic AMP.

receptors:2β -

-Are located on

vascular smooth muscle,

bronchial and visceral

smooth muscles,

liver ,eye and GIT.

-Are inhibitory in action

produce relaxation.

- Are more sensitive to

epinephrine.

Higher Brain Centers Regulating (A.N.S.):

-The involuntary controlling actions of A.N.S is under the control of

C.N.S. which control voluntary actions.

-These C.N.S.centers are:

1. Hypothalamus: The hypothalamus contains neurons for

control of A.N.S., stimulation of posterior nuclei of hypothalamus

leads to Sympathetic responses while stimulation of the anterior

nuclei leads to parasympathetic responses. .

2. The medulla oblongata: It contains the centers, which control

the autonomic responses, for example it contains:

-The cardiac center that regulate heart rate.

-The vasomotor center that regulate the blood pressure.

3. The spinal cord: This contains the lateral horn cells of the

sympathetic (thoraco- lumbar segments) and the parasympathetic

(sacral segments 2, 3, 4).