Download Hearing - 港九潮州公會中學

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Response and coordination
Section A : The detection of environmental conditions
1. The skin
There are several types of sense organs in the skin. Some of them are simply the free nerve
endings, while the others are in capsular forms, the corpuscles. They are responsible for
the perception of pain, touch, pressure and temperature.
2. The eye
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a) Structure and function of the eye
Structure
Special features
Functions
Sclerotic coat 鞏膜
Tough and fibrous.
To give shape to the eye.
To protect the eye.
Cornea 角膜
The front part of the
sclerotic coat.
Transparent.
To allow light to pass through.
To protect the front part of the eye.
Pigmented.
To prevent the reflection of light inside the
eye.
Choroids 脈絡膜
With rich blood supply.
To supply oxygen and nutrients to the eye.
Retina 視網膜
Contains two types of
light-sensitive cells :
rods and cones.
Rods for vision in dim light; cones for vision
in bright light and for color vision.
Lens 晶狀體
Convex.
To focus the image on the retina.
Suspensory ligament
懸韌帶
To hold the lens in position.
Ciliary bodies 睫狀肌
For accommodation.
To control the size of the pupil and hence the
Iris 虹膜
amount of light entering the eye.
Pupil 瞳孔
An aperture for light to pass through.
Aqueous and vitreous
humour 水狀液及玻璃
狀液
To maintain the shape of the eye.
To help to refract light on the retina.
To transport oxygen and nutrients to the lens
and cornea.
Eye muscles 眼肌
To move the eyeball in the orbit.
Conjunctiva 結膜
To protect the front part of the eye.
Fovea 黃點
Blind spot 盲點
This region contains
the highest density of
cones
This is the region which gives the clearest
vision.
This is the region where nerves from the rods
and cones leave the eye.
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b) Accommodation (focusing)
for distant object :
The radial ciliary muscle contracts
(the circular ciliary muscle relaxes)
Suspensory ligament is stretched
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for close object :
The circular ciliary muscle contracts
(the radial ciliary muscle relaxes)
Suspensory ligament becomes slackened
The lens is pulled and becomes less convex
The lens becomes more convex under
its own elasticity.
Focal length of the lens increases
Focal length of the lens decreases
Image of distant object falls on the retina
Image of close object falls on the retina
The changes which take place in the eyes when watching a near object.
The circular ciliary muscle contracts, thus suspensory ligament become slackened. The lens
becomes more convex under its own elasticity. Focal length of the lens decreases.
Image of close object falls on the retina. The above changes involve reflex actions.
c) Rods 視桿 and cones 視錐
There are two types of light sensitive cells, the rods and the cones. The rods are stimulated
by dim light while the cones are stimulated by bright light. Cones can detect colours while
rods cannot. Cones have high visual acuity while rods have not.
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Compare and contrast the structures and functions of a cone and a rod cell.
Similarities :
1. Both are light-sensitive cell / photoreceptors located on the retina.
2. Both have photosensitive pigments.
3. Both synapse with bipolar neurons to the optic nerve connected to the retina.
4. Both have mitochondria in the inner segment.
5. Both have outer segments containing lamellae.
Differences between rods and cones
Rods 視桿
Cones 視錐
Outer segment is rod-shaped
Outer segment is cone-shaped.
More numerous
Less numerous
Distributed more or less evenly over the
retina
Much more concentrated in and around the
fovea centralis
Arranged in functional units, each unit
being served by one bipolar neurone.
Each cone is served by its own bipolar
neurone.
Give poor visual acuity because many rods Give good visual acuity because each cone
share a single neurone connection to the
has its own neurone connection to the brain
brain.
Sensitive to low-intensity light, therefore
mostly used for night vision
Sensitive to high-intensity light, therefore
mostly used for day vision
One type of rod, stimulated by most
wavelengths of visible light except red;
therefore provide black and white vision.
Three types of cone, each selectively
responsible to different wavelengths of
visible light; therefore provide colour vision
Contain the visual pigment rhodopsin
視紫紅質 which has a single form
Contain the visual pigment iodopsin
視紫藍質 which occurs in three forms
Rapid regeneration of the visual pigment
rhodopsin
Slow regeneration of the visual pigment
iodopsin
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Cone cells are better than rod cells at distinguishing objects close together.
The closely packed cones in the fovea show no convergence because each cone is connected to
its own optic neuron. Therefore more cones are exposed to the focused image and the eye is
able to see objects clearly which are close together.
Rod cells are more sensitive than cone at very low light intensity.
Rods show convergence.
About 300 rods synapse with one optic neuron. This provides increased collective sensitivity
under conditions of very low light intensity through the process of summation.
The mechanism of photoreception in rods
Rods contain many photosensitive pigment rhodopsin 視紫紅質(visual purple). When
exposed to light, rhodopsin will break down to produce a generator potential.
This will
generate an action potential along the neurons leading from the cell to the brain to produce a
sensation of sight. Rhodopsin is reformed immediately in the absence of further light
stimulation. This resynthesis is carried out by the mitochondria inside the rod cells.
Resynthesis takes longer time than the splitting and is more rapid at low light intensity.
A similar process occurs in cone cells except that the pigment here is iodopsin 視紫藍質. This
is less sensitive to light and so a greater intensity is required to cause its breakdown and so
initiate a nerve impulse.
Rhodopsin 視紫紅質 is a light-sensitive pigment in the rod cells of the retina.
pigment which is sensitive to low levels of illumination.
Iodopsin 視紫藍質 is a light-sensitive pigment in the cone cells of the retina
It is a violet
It occurs in three
forms and is sensitive to high levels of illumination.
Colour vision
There are 3 types of photoreceptors 光感受器 for red, green and blue colours. The various
colours are detected by different combinations of the 3 types of photoreceptors.
The changes which take place in the eyes when you walk into bright daylight after working
in a dark room.
1. In daylight, the pupils become smaller to reduce the amount of light incident on the retina.
It is under autonomic nervous control and is a reflex action. The sensor is retinal
photoreceptors and the response is the relaxation of radial iris muscle and contraction of
circular iris muscle.
2. Eyelid closes in response to bright light.
3. After staying in a dark room for an hour, almost all visual pigments in photoreceptor cells
of the retina are regenerated. Visual pigment is sensitive to light. When acted upon by light,
visual pigment is bleached. This breakdown process of visual pigment triggers the excitation
of photoreceptor cells. This will produce an action potential then goes to the brain to produce
a sensation of sight.
4. Exposure of a dark-adapted retina to bright light will over-run the above change. Thus a
brief period of poor vision will be experienced.
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The structure and the function of the human retina.
Retina is composed of three layers.
The outermost layer is the photoreceptor layer containing rods and cones. Rods are for night
vision while cones are for colour vision and day vision.
The next layer is the bipolar neurone layer.
The inner most layer contains ganglion 神經結 cells.
The mechanism of photoreception :
Rod and cone cells contain much visual pigment. When exposed to light, this visual pigment
will break down to produce a generator potential. This will generate an action potential along the
neurones leading from the cell to the brain to produce a sensation of light. The visual pigment is
reformed immediately in the absence of further light stimulation to maintain its ability to
respond to light.
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3. The ear
Structures
Pinna 耳殼
Functions
To collect and direct the sound waves from the
environment into the external auditory canal.
External auditory canal 外耳道 To transmit sound wave to the tympanum.
Tympanum 耳膜
Middle ear ossicles 中耳骨
(malleus, incus, stapes)
To convert the sound wave into the vibration of the
ossicles.
To amplify the vibration.
To conduct sound wave to the inner ear.
Eustachian tube 耳咽管
To equalize the pressure on either side of the eardrum
to avoid bursting of eardrum.
oval window 卵圓窗
To transmit the vibration from the stapes to the
cochlea.
To damp the vibration of the perilymph 外淋巴 in the
round window 圓窗
cochlea.
To prevent excess pressure in the perilymph.
Cochlea 耳蝸
For hearing, converting sound waves to nerve
impulses which will be interpreted as sound at the
cerebrum 大腦.
Cochlear nerve 耳蝸神經
Carry impulses from the cochlea to the cerebrum.
Semi-circular canal 半規管
For balance.
Vestibular nerve 前庭神經
Carry impulses from the semi-circular canals to the
cerebellum.
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Hearing
The outer, middle and inner ear are all involved in the hearing process.
Pinna helps in collecting and directing the sound waves-from the environment into the external
auditory canal which is just a passage for sound. Tympanic membrane then changes the sound
waves into mechanical vibrations which are then transmitted and amplified by the 3 ear ossicles
into the inner ear. An air-filled canal, called the Eustachian tube, connects the middle ear with
the pharynx. This tube equalizes the pressure on either side of the eardrum to avoid the
bursting of eardrum. The innermost ossicle, the stapes, is connected to another membrane
called oval window which is part of the inner ear. The oval window transmits the vibrations
into a coiled, fluid-filled tube, cochlea which contain 3 canals separated by 2 membranes. The
upper vestibular canal 前庭管 is connected to the oval window. Between the vestibular canal
and the median canal is Reissner’s membrane.
canal from the lower tympanic canal 耳蝸管.
The basilar membrane separates the median
Vibrations of the oval window generate pressure waves in the fluid of vestibular canal. The
waves bring about vibration of Reissner’s membrane, then pressure waves in fluid of median
canal, then vibration of basilar membrane and finally pressure waves in fluid of tympanic canal.
The sensory region of cochlea is the organ of Corti 哥蒂氏器 in the median canal. Vibrations
of basilar membrane cause hair cells to move against the tectorial membrane 覆膜, deflecting the
sensory hairs. As a result, nerve impulses are generated and transmitted to the brain along
auditory nerve 聽覺神經. Sound with high frequency stimulates hair cells near the oval
window. Hair cells at the tip are stimulated by low frequency sound.
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The flow chart of hearing
Sound (air vibration)
pinna
ear drum – vibration
peri-lymph and endolymph – vibration
basilar membrane – vibration
stimulation of sensory hair cells of Organ of Corti when touching the tectorial membrane
auditory nerve
cerebrum
The discrimination of different sound frequencies by the ear.
The cochlea of the inner ear contains sensory cells for the appreciation of sound. The cochlea
is a spiral tube divided into three canals. The organ of Corti is the structure where the sensory
cells for perception of sound are located.
The stiffness of the basilar membrane gradually decreases from the base to the apex of the
cochlea. Vibrations initiated by the ear ossicles pass along the basilar membrane for a certain
distance and then die out. High frequency waves travel only a short distance; low frequency
waves travel much further. Furthermore, the sensory cells in different parts of the organ of
Corti respond to different frequencies. Those near the base respond to high frequencies, while
those towards the apex respond to low frequency vibrations.
Different parts of the cochlea therefore respond to different frequencies. A given frequency
moves a specific distance along the basilar membrane and stimulates a specific part of the organ
of Corti, and so can be discriminated.
More about hearing
The most sensitive frequency range of the ear is 3000-4000 HZ. Higher or lower sound
frequencies are more difficult to detect. That means auditory threshold intensity (dB) is
affected by the value of sound frequency.
high sound frequencies.
Aged adults have less sensitive sense of hearing at
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Maintenance of balance
The inner ear is concerned with balance. Two parts of the inner eat are responsible for body
balance, vestibular apparatus 前庭器管 and semicircular canals 半規管.
Semicircular canals consists of 3 fluid-filled canals which are oriented at right angles to one
another. They detect the head movements. The ends of the canals are enlarged to form an
ampulla 壺腹 inside which are ampullary hairs projecting into a gelatinous mass called cupula
蝸頂. The cupula is suspended in the endolymph 內淋巴. Any movement of the head moves
semicircular canals in the same direction. The endolymph inside the canals lags behind due to
its inertia and push the cupulae in the opposite direction. As a result, hairs are bent and nerve
impulses are sent to the cerebellum along the vestibular nerve.
Vestibular apparatus 前庭器管 consists of 2 lymph-filled sacs called saccule 球囊 and utricle 橢
圓囊. They tell us about the head position with reference to gravity when we are stationary.
The horizontal utricle and vertical saccule contain receptors called masculae which are sensitive
to gravity. Small hairs project from the receptor cells into the lymph. The hairs are attached
to calcium carbonate granules called otoliths 耳石. Gravity causes otoliths to distort the
sensory hairs in a direction dictated by the position of the head. In response to distortion, nerve
impulses pass along the vestibular nerve to the brain. If the head is tilted to a different position,
the otoliths distort the sensory hairs in a different direction.
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Section B : Nervous coordination 神經協調
The functions of the nervous system
1. The nervous system senses changes within the body and in the external environment.
2. It interprets the changes.
3. The nervous system responds to the interpretation by initiating action in the form of
muscular contraction or glandular secretions.
4. It integrates body activities in multicellular animals.
5. It acts as a storehouse of information.
There are three types of neurons :
1) Sensory neurons : carry impulses from the receptors to the central nervous system.
2) Motor neurons : carry impulses from the central nervous system to the effectors.
3) Association neurons (intermediate or relay neurons, interon) : link up sensory neurons with
the motor neurons.
Myelin sheath : This is a fatty tissue acting as an insulating layer to prevent the nervous
impulses from leaking out.
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Why the rate of conduction of a nerve impulse is greater in a myelinated axon 有鞘神經
than in an unmyelinated axon?
Nerve impulse in unmyelinated axon is propagated as a continuous spread forming a smooth
progressive movement.
But in myelinated axon, due to the high membrane resistance in the myelin sheath, nerve
impulses can occur only at the node of Ranvier 郎飛結.
Thus action potential propagates as discontinuous spread hopping along the fibre from one Node
of Ranvier to another.
1. Basic arrangement of the 3 types of neurons
In the nervous system, the 3 types of neurons are arranged in a basic pattern : the sensory
neuron carries impulses from the receptors to the association neurons inside the central
nervous system while the motor neuron carries impulses from the association neurons to
the effectors.
The neural pathway involved when a man voluntarily bends his right arm.
Motor nerve impulses are initiated from the somatic motor area of the left cerebral cortex,
a region of grey matter in the cerebrum. Motor nerve impulses are transmitted down the
axons which pass directly to the spinal cord through two large pyramidal tracts via the
medulla (brain stem) to synapse with the motor neurones at the grey matter of the spinal
cord. Motor nerve impulses are relayed to the motor neurones which emerge from the right
ventral root of the spinal cord and innervate the biceps muscle of the right arm by motor
end-plate / neuromuscular junctions.
2. The properties of the membrane in the excited and resting regions of a neuron
The cell membrane of a resting neuron is more permeable to potassium ions than to sodium.
However, the permeability to sodium is greater than that of potassium in an excited neuron.
The resting neuron has negative membrane potential inside while the excited one has
positive inside.
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3. The nature and mechanism of nerve impulse conduction along a nerve fibre
In its resting state, the membrane of a neuron is negatively charged internally with respect to
the outside. This potential difference is known as resting potential 靜止電位 and in this
condition, the membrane is said to be polarized 極化.
The concentration of potassium ion is much higher inside the neuron and thus rapidly diffuse
out. The concentration is maintained by the sodium-potassium pump which actively
transporting in potassium ions and removing sodium ions. This is a kind of active transport
and requires ATP.
The sodium-potassium pump 鈉-鉀泵 of neuron breaks down when excited. The
permeability of the membrane to sodium at the point of stimulation increased. As a result,
sodium ions pour into the neuron and potassium ions move out of the neuron. This brings
about a reversal of potential – the inside of neuron changes from negative to positive. This
is called depolarization 去極化 and the resulting potential is called action potential 動作電位
because it can cause depolarization of other region and can therefore move along the neuron.
Some time later, at the point originally stimulated there is a decrease in the membrane’s
permeability to sodium and an increased permeability to potassium. Potassium ions rapidly
move outward, again making the outside of the membrane positive in relation to the inside
(repolarization). Sodium and potassium pumps transport Na+ back out of and K+ back into
the cell. The area would repolarize to resume the resting potential.
4. The structural characteristics of neurons and the conduction speed of a nerve impulse
The greater the diameter of the neuron, the faster would be the speed of transmission.
Further, presence of myelin sheath 髓磷脂鞘 would increase the speed of transmission.
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5. The synaptic transmission 突觸傳遞
axon
When an action potential arrives at the synaptic knob, the presynaptic membrane 突觸前膜
would be depolarized. As a result, calcium ions enters the synaptic knob 突觸小結. Then,
vesicles containing neurotransmitter 神經遞質 will fuse with the presynaptic membrane and
release the neurotransmitter (acetylcholine). Consequently, the postsynaptic membrane 突
觸後膜 will be depolarize to produce an action potential in the neuron on the other side of
the synaptic cleft 突觸間隙 if the depolarization is above a certain threshold 臨界. The
neurotransmitter will be destroyed immediately.
Acetylcholine 乙醯膽鹼 is produced by the presynaptic knob of a synapes.
It depolarizaes
the post-synpatic membrane by affecting membrane permeability, which in turn generates
and action potential.
The unidirectional (one way) transmission of nerve impulse between neurons.
The unidirectional transmission is mainly determined at synapse. Since only the presynaptic
portion (the axon terminal) can produce neurotransmitter, and the post-synaptic membrane
possesses receptors to specifically combine with the neurotransmitter, leading to
post-synaptic depolarization and propagation of nerve impulse. Impulses can therefore travel
in one way only, i.e. from axon to dendrite, but not the reverse.
The unidirectional conduction of nervous impulses along the axon.
There is a short refractory period 不應期 after the action potential. At this period the axon
will not respond to another stimulus. The axon has to recover first. The membrane has to be
repolarized and the normal distribution of ions restored before another action potential can
be transmitted. It means that the action potential can only be propagated in the region which
is not refractory, ie. in a forward direction. The action potential is thus prevented from
spreading out in both directions while travelling along the axon.
The function of synapses 突觸.
1. Ensuring unidirectional transmission of impulses from receptors to CNS and from CNS
to effectors .
2. Allowing great flexibility in the integrative function of the nervous system.
3. Involved in the mechanism of learning and memory.
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6. The effect of the strength of the stimulus on the size of the action potential.
The size of action potential is not related to the strength of the stimulus. Nerve and muscle
cells obey the all-or-nothing law, which states that a threshold stimulus evokes a maximal
response and a subthreshold 低於臨界 stimulus evokes no response.
7. Spinal reflex 脊髓反射
A reflex is an automatic response which follows a sensory stimulus. It is not under
conscious control and is therefore involuntary.
Any reflex arc 反射弧 which is localized within the spinal cord and does not involve the
brain is called a spinal reflex.
eg. 1 : Knee jerk reflex 膝躍反射
The sequence of events in knee jerk reflex :
1. Hit the tendon of thigh muscle at a point just below the knee cap (patella).
2. The stretch receptor in thigh muscle is stimulated to generate an action potential.
The action potential propagate along the sensory nerve fibre to spinal cord.
3. In the grey matter of spinal cord, action potential goes across the synapse to motor neuron.
4. Motor neuron then generates an action potential. This goes across motor end plates to thigh
muscle. The muscle contracts causing the lower leg to jerk. The cerebrum is not involved
in this response.
eg. 2 : The withdrawal reflex 退縮反射 when a barefooted person steps on a sharp nail
Pain receptors are stimulated to produce a generator potential. If above a threshold, an action
potential will be produced in the sensory neuron. This potential is a propagative wave of
depolarization. It travels along the neuron towards the central nervous system or spinal cord.
Here, it has synapses with a number of intermediate neurons.
A postsynaptic potential will be produced in the intermediate neurons, which then synapse with
motor neurons. The motor neurons for the flexors of the ankle, knee and hip will be excited to
produce action potentials. These potentials travel towards the flexors where they generate
end-plate potentials to produce contractions. On the other hand, motor neurons for extensors
will be inhibited. As a result, the extensors relax. Finally, the leg will withdraw.
The above response is inborn, stereotyped and independent of cerebrum for initiation.
However, the sensory neuron may also send off nerve impulse through ascending tract to the
cerebrum for sensation.
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8. Division of the nervous system
It is divided into three parts.
1. Central nervous system 中央神經系統 (CNS) : brain and spinal cord.
2. Peripheral nervous system 外圍神經系統 (PNS) : spinal nerve and cranial nerve
3. Autonomic nervous system 自主神經系統 (ANS) : sympathetic and parasympathetic
system.
9. The brain
Structure of the brain
Functions of the brain :
a) Medulla oblongata 延腦
It contains many important centers of the autonomic nervous system. These centers
control reflex activities like breathing rate, heart rate and blood pressure.
b) Cerebellum 小腦
It coordinates muscular movement and control of the body posture.
c) The cerebral cortex 大腦皮層 is folded. This increase its surface area for containing
more neurons to increase its efficiency. It control the voluntary muscular movements.
It receive and interprets sensory impulses from various parts of the body.
For higher mental activities such as memory, learning, imagination and reasoning.
10. Spinal Cord
1. Continuous with the medulla of the brain.
2. Protected by the vertebrae.
3. Consists of an inner layer, the grey matter (the cell bodies of the neurons) and an outer
layer, the white matter (the nerve fibres of the neurons).
4. The central canal is filled with a cerebrospinal fluid.
5. Fibres of the sensory neurons enter the spinal cord as the dorsal root and the fibres of the
motor neurons leave the spinal cord as the ventral root. A short distance from the
spinal cord, the two roots join up to form a spinal nerve.
6. The spinal white matter contains many nerve tracts, some descending to convey
information from the brain to the spinal cord, other ascending to transmit information
from the spinal cord to the brain.
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Functions of the spinal cord :
1) Responsible for reflex actions.
2) For the transmission of impulses to and from the brain.
11. Comparison of the functions of the cerebrum and the spinal cord
Cerebrum
Spinal cord
1. Responsible for voluntary response
2. The organism is immediately conscious of
the response initiated in the cerebrum.
3. The voluntary response is not stereotyped.
4. Voluntary responses are generally slower
and acquired after birth.
Responsible mainly for reflex action
Not immediately conscious of the
response initiated in the spinal cord.
The spinal reflexes is stereotyped.
Reflex actions are quick and inborn.
5. Contains many sensation areas.
None.
6. Has many areas for association of neurons None.
responsible for higher mental activities
like learning and imagination.
12. Hypothalamus 下丘腦
This is the main regulatory center of the body.
A regulatory system is composed of many components shown in the following diagram.
Input
(stimulus)
detector
regulator
effector
output
(response)
(corrective mechanism)
This generally involves a negative feedback system.
The hypothalamus can act as detector itself in addition to being a regulator. Take some
examples, change in core temperature, osmotic pressure, metabolite and hormone level in
the blood.
On the other hand, hypothalamus may need input from other detectors, eg. cutaneous
thermoreceptors, tactile sensors in nipples 乳頭, and almost all parts of the brain, eg. taste,
smell and visceral 內臟 receptors.
After receiving input, hypothalamus will initiate corrective mechanisms if necessary. The
information may be sent to the effectors through the following pathways:
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1. Sending nervous impulses to appropriate effector organs : The effectors may be
sweat glands, skin arterioles, pili erector muscles 豎毛肌, skeletal muscles and adrenal
gland.
2. Secretion of hormones to target effector organs :
(impulses are sent to pituitary 腦下垂體, pituitary then secrete hormones)
Hormones
Effector organs
anti-diuretic hormone
Kidney tubules
oxytocin
Uterus, milk gland
follicle stimulating
hormone
ovary
3. Release of trophic hormones to target endocrine glands:
e.g. Release of adrenocortico trophic hormone 促腎上腺皮質激素(to adrenal cortex.
(Adrenocortico trophic hormone initiate the secretion of aldosterone 醛固酮)
Conclusion :
The hypothalamus is a very important regulator as shown above. It regulates a wide variety
of parameters of the body. It receives inputs itself and from many types of receptors. It
also gives information to many effectors through both the nervous system and the hormone
system.
13. Autonomic nervous system 自主神經系統 (ANS)
The general functions of autonomic nervous system (SNS & PNS).
The autonomic nervous system takes an important role in regulating the internal
environment of the body. By increasing or decreasing its activities, it helps to maintain a
steady internal environment in response to the changing physiological conditions due to the
change of demands at a particular time. The autonomic nervous system is a motor system
innervating the viscera (internal organs) which are under involuntary control.
Sympathetic 交感神經(SNS) and parasympathetic 副交感神經(PNS) nervous system
Sympathetic nervous system
parasympathetic nervous system
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Comparison of some effects of sympathetic and parasympathetic nervous systems
Sympathetic nervous system
Parasympathetic nervous system
Increases cardiac output
Decreases cardiac output
Increases blood pressure
Decreases blood pressure
Dilates bronchioles
Constricts bronchioles
Increases ventilation rate
Decreases ventilation rate
Dilates pupils of the eyes
Constricts pupils of the eyes
Contracts anal and bladder sphincters
Relaxes anal and bladder sphincters
Contracts erector pili muscles, so raising hair
No comparable effect
Increases sweat production
No comparable effect
No comparable effect
Increases secretion of tears
Organs that are innervated by both the SNS and PNS: heart, blood vessels, lung and eye.
Comparison of the structure and functions of the SNS and PNS
Similarities :
Both systems are autonomic, emerging from the CNS.
Both are efferent system consisting of motor neurons.
Differences :
1) Location
The SNS emerges from thoracic and lumbar region of the spinal cord while PNS from
cranial nerves and sacral region of spinal cord. Numerous post-ganglionic and
pre-ganglionic nerves are present in SNS over a wide area. The condition is opposite in
PNS.
The ganglia of SNS lie alongside the vertebrae close to the spinal cord while the
ganglia of PNS embedded in the effector.
2) The ganglia of SNS are joined while that of the PNS are not.
3) In SNS, the pre-ganglionic nerve fibre is short and the post-ganglionic fibre is long.
The situation is opposite in PNS.
4) Neuro transmitter
Both pre-ganglionic fibres produce acetylcholine. Post-ganglionic fibre of SNS produces
nor-adrenaline while that of PNS produces acetylcholine.
5) Functions
The two systems are antagonistic to each other. SNS usually prepares the body for
emergency while PNS for relaxed state. The effect of SNS is usually diffuse while that of
PNS is localized.
Chapter 16
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An example of coordination by the autonomic nervous system
(the control of heart beat)
The heart rate is coordinated by the autonomic nervous system. The sympathetic and
parasympathetic nervous systems have opposing effects on organs, and this enables the body to
make rapid and precise adjustment of heart beat in order to maintain a steady state. An
increase in heart rate due to release of noradrenaline by sympathetic neurons is compensated for
the release of acetylcholine by parasympathetic neurons. This action prevents heart rate
becoming excessive and will eventually restore it to its normal level when secretion from both
systems balances out.
Emergency
Inhibit the activities of
Parasympathetic nerve
stimulate the activities
of sympathetic nerve
Heart rate increases
Calm down from emergency
Stimulate the activities
Of parasympathetic nerve
inhibit the activities
of sympathetic nerve
Heart rate decreases
The response of sympathetic nervous system during emergency (stress)
At emergency, cerebrum sends off nerve impulse to sympathetic nervous system. The
sympathetic nerve endings then release noradrenaline. Adrenal medulla will then secrete
adrenaline.
The effects of these hormones are :
1. It increases heart beat and stroke volume.
2. It increases breathing rate.
3. It causes vasodilation in skeletal muscles and vasoconstriction in muscles of the skin and
alimentary canal.
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Section C: Hormonal coordination
The nature of endocrine gland :
The endocrine glands are ductless glands which secrete hormones directly into the blood stream.
The characteristics of a hormone :
Hormones are chemical messenger produced from endocrine glands in minute amount,
transported by blood stream to a specific target organ where it exerts its effect.
Undersecretion and oversecretion of a particular hormone results in a specific disease.
Chemically, hormones can be amino acid derivatives (eg. thyroxin, adrenaline), short peptides
(eg. oxytocin, anti-diuretic hormone), long peptides (eg. insulin, glucagons), proteins (eg.
growth hormones) or steroids (eg. sex hormones).
Position of the major endocrine glands 內分泌腺
1. Pituitary gland 腦下垂體
Most endocrine glands work under the influence of a single master gland, the pituitary gland.
In this way the actions of individual glands can be coordinated.
The pituitary depends upon information received from the hypothalamus. The
hypothalamus plays a dominant role in collecting information from other regions of the
brain and from blood vessels passing through it. This information passes to the pituitary
gland which, by its secretions, directly or indirectly regulates the activity of all other glands
to bring about coordination and homeostasis.
The anterior pituitary secretes somatotrophin (1)or growth hormone which regulates growth.
It also secretes TSH(2) which controls the secretion of thyroxin from thyroid gland.
Thyroxin also regulates growth and metabolism.
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The anterior pituitary also secretes gonadotrophins, FSH(3) and LH(4). FSH stimulate the
ovary to secrete estrogen which helps the development of female secondary sexual
characteristics. LH also induces ovulation and initial progesterone secretion.
Progesterone helps to maintain the thickness of uterus for pregnancy. In male testosterone
雄激素(5) stimulates spermatogenesis and development of secondary sexual characteristics.
Prolactin (6) is also secreted by anterior pituitary. Prolactin 促乳素 initiated the production
of milk from mammary glands.
The posterior pituitary releases 2 hormones ADH 抗利尿激素(7) and oxytocin 催產素(8).
Oxytocin causes contraction of uterus and ejection of milk. ADH leads to an increase in
permeability to water of collecting duct so that water is reabsorbed into plasma.
What will happen if the posterior pituitary of a man is removed ?
The posterior pituitary is source of anti-diuretic hormone (ADH). The hormone will
increase the permeability of the collecting ducts in kidneys to reabsorb water from urine
before it leaves the medulla of the kidney. Removal of posterior pituitary will eliminate
ADH supply. Kidneys would fail to reabsorb water at the collecting duct. As a result, a
large volume of dilute urine produced at a high rate.
2. Thyroid gland 甲狀腺
It is a H-shaped gland lying in front of the trachea at the neck region.
hormone, thyroxin 甲狀腺素.
It secretes a
Functions of thyroxin :
1) It controls the basal metabolic rate, especially the rate of respiration.
2) It promotes growth in young mammals.
Hyposecretion 過少分泌 :
a) In children, causes physical and mental retardation.
b) In adults, decrease of metabolic rate, thickening of skin, physical and mental
sluggishness.
Hypersecretion 過多分泌 :
This leads to goitre 甲狀腺腫, ie. Swelling of the thyroid gland and the protrusion of the
eye balls. There is a great increase in body metabolism, loss of weight, nervousness,
physical and mental restlessness.
Feedback control of the thyroid gland (endocrine feedback) :
The secretion of thyroxin from thyroid gland is stimulated by a hormone, thyrotrophin (thyroid
stimulating hormone), secreted from the anterior lobe of the pituitary. However, the secretion
of thyrotrophin (TSH) is in turn regulated by the thyroxin level in blood: an increase of
thyroxin level in blood will inhibit the anterior lobe of the pituitary to secrete thyrotrophin
(TSH), and this will then reduce the activity of the thyroid gland, leading to a drop in thyroxin
level.
This method of control is known as feedback control (endocrine feedback, not *negative
feedback) which keeps the amount of thyroxin in blood constant.
The secretion of thyrotrophin by pituitary is also influenced by environmental factors, eg. a low
temperature will stimulate the secretion : thus the constant level of thyroxin can vary to meet the
need of the environment.
Chapter 16
*negative
港九潮州公會中學生物科主任楊祐添編
Response
feedback : see the control of blood glucose level by insulin.
Note : this example can be used to explain the inter-relationship between the nervous system
and the endocrine system.
The interrelationship between the nervous system and the endocrine system
The inter-relationship between the nervous system and the endocrine system in
controlling the release of thyroxin is illustrated by the following diagram.
Hypothalamus of brain (nervous system)
TSH releasing factor
+
_
anterior lobe of pituitary gland (endocrine system)
thyroxin
(thyrotrophin)
thyroid stimulating hormone (TSH)
thyroid gland
+
“ + ” means stimulating effect
“ - “ means negative feedback e.g. if concentration of thyroxin is too high, the
hypothalamus will be inhibited to initiate the increase of thyroxin concentration.
The explanation of the five basic components of the coordinating system by thyroxin
secretion
Thyroxin secretion in response to cold environment is hormonal co-ordination.
Stimulus : prolonged period of cold
Receptor : thermo-receptors of the skin and hypothalamus.
Controller : hypothalamus will detect the drop in blood temperature and inform the effector
thyroid gland with thyroid-stimulating hormone.
Effector : thyroid gland
Response :
increased secretion of thyroxin will increase basal metabolic rate.
3. Adrenal gland 腎上腺
They are a pair of glands located anterior to the kidney. Each gland has two components
distinct in their functions but closely fused together :
a) The outer part is called adrenal cortex which secretes aldosterone 醛固酮
(increase reabsorption of water at kidney tubules 腎小管) that control salt and water
balance of the body.
b) The inner part is called adrenal medulla which secretes a hormone called adrenalin.
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Chapter 16
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Adrenalin 腎上腺素
Adrenalin is secreted as a result of the stimulation from the sympathetic nerves of the
autonomic nervous system under the condition of fright or anger.
Functions of adrenalin :
a) It increases blood pressure, rate of breathing and heart beat.
b) It converts glycogen to glucose and hence increases the blood glucose level.
c) It causes the capillaries of the skin and gut to contract so that more blood will flow to
the brain and the skeletal muscle for faster response.
d) It causes the pupil to dilate.
The overall effects of adrenalin is to make the animal become powerful and alert so as to
deal with the emergency. For this reason, adrenalin is also known as the hormone of
freight, flight and fight.
4. Insulin 胰島素 and glucagons 高血糖激素
They are secreted from islet of Langerhans 胰島. Each islet is made up of two types of
cells : α cells secretes glucagons and β cells secrete insulin 胰島素.
Action of the hormones :
Functions of insulin :
a) It converts glucose to glycogen which is stored in muscles and liver.
b) It increases the uptake of glucose by the cells especially the muscle cells.
c) It facilitates the oxidation of glucose to carbon dioxide and water.
Consequently, it lowers the blood glucose level.
The functions of glucagons are opposite to those of insulin :
a) It decreases glucose oxidation.
b) It stimulates glycogenolysis (breakdown of glycogen) in the liver.
As a result, it elevates the blood glucose level.
Relation between insulin and glucagons:
Insulin and glucagons are a pair of antagonistic hormones having opposite effects on the
inter conversion of glucose and glycogen :
a) When energy is needed during starvation, the insulin-glucagons ratio is low, favouring
glycogen breakdown and glucose utilization.
b) When the need for energy is low, the insulin-glucagons ratio is high, favouring the
deposition of glycogen.
If glucose level is too high
Insulin secretion increases
Increase utilization of glucose
Drop in glucose level
Chapter 16
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If glucose level is too low
Sympathetic nervous system secretes
more adrenalin and glucagons
insulin secretion decreases
Glycogenolysis increases
Increase in blood glucose level
Diabetes 糖尿病 :
Diabetes is a result of insulin deficiency. It is characterized by loss of large amount of water
and loss of weight. These symptoms are
a) A reduced entry of glucose into various tissues.
b) An increased liberation of glucose into circulation from the liver.
As a result, the extracellular glucose is in excess but the intracellular glucose is deficient.
When the blood glucose level has exceeded the renal capacity for glucose reabsorption, glucose
is excreted in the urine. Since glucose is osmotically active, excretion of it results in the loss of
large amount of water.
Positive feedback and negative feedback.
Positive feedback intensifies the stimulus and therefore the response, that is, an increase in one
factor reinforces and increases the first factor. In negative feedback an increase in one factor
decreases the production of the first factor, this is important in the maintenance of homeostasis.
Example of negative feedback : the control of blood glucose level by insulin.
1. The pancreas detects the concentration of glucose in the blood.
2. Increased concentration of glucose in blood, eg. after a meal stimulates the beta cells of the
Islets of Langerhans in the pancreas to secrete more insulin.
3. Insulin increases oxidation of glucose and conversion of glucose into glycogen and fat,
especially in liver and muscle cells.
4. Glucose level in the blood falls.
5. This decreases the secretion of insulin from the pancreas.
Chapter 16
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Question : The concentrations of glucose and two pancreatic hormones (A and B) in the blood
of a mammal were monitored before, during and after a normal carbohydrate rich
meal. The following table shows the typical changes in the levels of blood
glucose and these two hormones during the experiment :
Concentration in blood
Time (min)
Glucose
hormone A
Hormone B
(10 g/ml)
(10 international units/ml)
(10-12g/ml)
89
16
127
0
87
17
125
30
60
125
134
100
140
103
92
120
180
240
300
105
94
89
88
89
38
19
18
77
114
126
125
-5
60 minutes
before meal
Meal
After
meal
-6
a) Plot the results in the form of a graph.
b) How do the two hormones differ in their response to changes in blood glucose level?
Name the two hormones.
Hormone A is ___________. Its level increases and decreases with the _____________
___________. Hormone B is _______________. Its level _______________when
glucose level decreases and vice versa.
Chapter 16
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c) Draw a flow diagram to indicate the relationship between the increase in blood glucose level
and the secretion of hormones A and B. Indicate in your diagram, for each hormone, its site
of secretion and one major effect on the target tissue concerned.
Increase in blood glucose level
__________ secretion (hormone ____ )
will increase.
Site of secretion : ____________
______ cells
effect :
__________ will change
to __________
___________ secretion (hormone ____ )
will decrease.
____________
______ cells
___________ will change
to ____________
d) if, instead of the carbohydrate meal, an equivalent amount of glucose is administered
directly into the bloodstream, how would this affect the observed changes in blood glucose
level? Give a reason for your answer.
All the curves would shift to the ________ for _______ minutes. It is because the blood
glucose level will be (give value)__________________ immediately and cause subsequent
changes in the __________ levels. Diet carbohydrates take time in _____________
___________________ to get into the blood.
e) In a similar experiment, the blood glucose was monitored as in the above experiment.
Three hours after the meal, the adrenal medulla of the animal was briefly stimulated to
secrete a large amount of an adrenomedullary hormone, the effect of which disappeared
within an hour. Sketch a curve in your answer book based on the curve plotted in (a), but
modified to show the possible changes in blood glucose level after the stimulation.
f) Name this adrenomedullary hormone.
What is the biological significance of the change in blood glucose level caused by the
secretion of this adrenomedullary hormone ?
Chapter 16
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This hormone is ________________.
During emergency, this hormone will be secreted. This hormone will bring about an
increase in __________________________ level. Glucose is an important raw material for
release of energy for quicker muscular response, increased rate of _____________________
______________________________.
5. Sex hormones
a) Testosterone 雄激素
Source : testis
Its function is to stimulate the development of male secondary sexual characteristics,
such as development of bread, muscle and deepening of voice. It also controls sperm
production.
b) Progesterone 孕酮
Source : ovary
It prepares the female for gestation by stimulating the development of uterus.
It also maintains the thickened uterus and foetal development. On the other hand,
it also inhibits ovulation.
c) Oestrogen 雌激素
Source : ovary
It is responsible for sexual characters and repairing of uterine lining.
Section D : Miscellaneous
(1) The control of lactation by endocrine system and nervous system
Diagram to show the endocrine and nervous control of lactation (milk production and milk
secretion).
(Endocrine control)
(Both nervous and endocrine control)
Milk production
Low level of oestrogen and
progesterone after birth
milk secretion
suckling by baby
Hypothalamus
nipple receptor
Pituitary gland
sensory neurone
Secretion of prolactin
Development of mammary gland increases
Milk production
hypothalamus
posterior pituitary
secretion of oxytocin
milk secretion
Chapter 16
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(2) The various ways in the regulation of the secretion of hormones
There are several mechanisms in the body which regulate the secretion of hormones :
1. Negative feedback mechanisms, eg. insulin and thyroxin.
2. Releasing factors and inhibiting factors, produced by the neurosecretory cells of the
hypothalamus, e.g. growth releasing factor. These factors pass through the vein to the
anterior pituitary, where they control the secretion of specific anterior pituitary hormones,
such as luteinizing hormone, growth hormone and adrenocorticotrophic hormone.
3. Inhibitory action of certain hormones, e.g. progesterone inhibits the secretion of follicle
releasing factor from the hypothalamus. As a result the secretion of FSH from the
anterior pituitary is inhibited. This favours foetal development in the uterus and
prevents unnecessary production of eggs.
4. Chemical stimulation, e.g. acidic chyme from the stomach stimulates the release of
secretin and cholecystokinin 膽囊收縮素 from cells of the duodenal mucosa 中黏層.
Secretin 胰泌素 causes secretion of pancreatic juice from the pancreas, cholecystokinin
stimulates the gall bladder to contract and release bile.
(3) Comparison of nervous and hormonal control
Nervous control
Hormonal control
1. The message is nervous impulse which
travels along nerve fibre.
The message is hormones which travel
by blood.
2. The nervous impulse is electrical in nature.
The hormone is chemical in nature.
3. The effect is localized.
The effect is more generalized.
4. Faster in action.
Slower in action.
5. The effect is comparatively short-termed.
The effect is comparatively long-termed.
(4) Comparison of animal hormone and phytohormone 植物激素
Similarity : Both influence growth and metabolic activities.
Differences :
Phytohormone
Animal hormone
Transported by diffusion and active transport
Diffuse into the blood
Target organ not very specific
Target organ very specific
Produced at tips of shoots and roots
Produced by specific glands (endocrine
glands)
Secretion triggered by environmental factors
Secretion can be triggered by nervous
stimulation