Nervous System Brain Spinal Cord Nerves Neurons. Communication between cells through nerve signals.
General Education Programfptcu.com/Gep Files/Biology/WEEK 9, nerve signals... · The signals of the...
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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).