CBSE - Control and Coordination

A reflex action is a rapid, automatic response to a stimulus. It does not involve any thinking. For example, we close our eyes immediately when the bright light is focused. Walking, on the other hand, is a voluntary action. It is under our conscious control.

A very small gap that occurs between the last portion of axon of one neuron and the dendron of the other neuron is known as a synapse. It acts as a one way valve to transmit impulses in one direction only. This uni-direction transfer of impulses occurs as the chemicals are produced in only one side of the neuron i.e., the axon's side. From axon, the impulses travel across the synapse to the dendron of the other neuron.

Cerebellum, a part of hindbrain is responsible for maintaining posture and equilibrium of the body.

The thinking part of our brain is the forebrain. It has separate areas that are specialized for hearing, smelling, sight, taste, touch, etc. The forebrain also has regions that collect information or impulses from the various receptors. When the smell of an incense stick reaches us, our forebrain detects it. Then, the forebrain interprets it by putting it together with the information received from other receptors and also with the information already stored in the brain.

Reflex actions are sudden responses, which do not involve any thinking. For example, when we touch a hot object, we withdraw our hand immediately without thinking as thinking may take time which would be enough to get us burnt.

The sensory nerves that detect the heat are connected to the nerves that move the muscles of the hand. Such a connection of detecting the signal from the nerves (input) and responding to it quickly (output) is called a reflex arc. The reflex arcs - connections present between the input and output nerves - meet in a bundle in the spinal cord.

Reflex arc

Reflex arcs are formed in the spinal cord and the information (input) reaches the brain. The brain is only aware of the signal and the response that has taken place. However, the brain has no role to play in the creation of the response.

Plant hormones or phytohormones are naturally-occurring organic substances. These are synthesized in one part of the plant body (in minute quantities) and are translocated to other parts when required. The five major types of phytohormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

The movement of leaves of the sensitive plant, Mimosa pudica or “touch me not”, occurs in response to touch or contact stimuli. This movement is independent of growth. The movement of shoot towards light is known as phototropism. This type of movement is directional and is growth dependent.

Auxin is an example of growth-promoting plant hormone.

Auxin is synthesized at the shoot tip. It helps the cell grow longer. When a tendril comes in contact with a support, auxin stimulates faster growth of the cells on the opposite side, so that the tendril forms a coil around the support. This makes the tendrils appear as a watch spring.

Take two small beakers and label them as A and B. Fill beaker A with water. Now make a cylindrical-shaped roll from a filter paper and keep it as a bridge between beaker A and beaker B, as shown in the figure. Attach few germinating seeds in the middle of the filter paper bridge. Now, cover the entire set-up with a transparent plastic container so that the moisture is retained.



Observation:

The roots of the germinating seeds will grow towards beaker A.



This experiment demonstrates the phenomenon of hydrotropism.

Chemical coordination takes place in animals with the help of hormones.Hormone is the chemical messenger that regulates the physiological processes in living organisms. It is secreted by glands. The regulation of physiological processes, and control and coordination by hormones comes under the endocrine system. The nervous system along with the endocrine system in our body controls and coordinates the physiological processes.

Which of the following is a plant hormone?

(a) Insulin

(b) Thyroxin

(c) Oestrogen

(d) Cytokinin

Answer :

(d) Cytokinin is a plant hormone.

Iodine stimulates the thyroid gland to produce thyroxin hormone. It regulates carbohydrate, fat, and protein metabolism in our body. Deficiency of this hormone results in the enlargement of the thyroid gland. This can lead to goitre, a disease characterized by swollen neck. Therefore, iodised salt is advised for normal functioning of the thyroid gland.

The gap between two neurons is called a

(a) dendrite.

(b) synapse.

(c) axon.

(d) impulse.

Answer :

(b) The gap between two neurons is called a synapse

Adrenalin is a hormone secreted by the adrenal glands in case of any danger or emergency or any kinds of stress. It is secreted directly into the blood and is transported to different parts of the body.

When secreted in large amounts, it speeds up the heartbeat and hence supplies more oxygen to the muscles. The breathing rate also increases due to contractions of diaphragm and rib muscles. It also increases the blood pressure. All these responses enable the body to deal with any stress or emergency.

Diabetes is a disease in which the level of sugar in the blood is too high. Insulin, a hormone secreted by the pancreas, helps in regulating the blood sugar levels. This is the reason why diabetic patients are treated by giving injections of insulin.

The brain is responsible for

(a) thinking.

(b) regulating the heart beat.

(c) balancing the body.

(d) all of the above.

 Answer :

(d) The brain is responsible for thinking, regulating the heart beat and balancing the body.

Receptors are sensory structures (organs/tissues or cells) present all over the body. The receptors are either grouped in case of eye or ear, or scattered in case of skin.

Functions of receptors:

(i) They sense the external stimuli such as heat or pain.

(ii) They also trigger an impulse in the sensory neuron which sends message to the spinal cord.

When the receptors are damaged, the external stimuli transferring signals to the brain are not felt. For example, in the case of damaged receptors, if we accidentally touch any hot object, then our hands might get burnt as damaged receptors cannot perceive the external stimuli of heat and pain.

Neurons are the functional units of the nervous system. The three main parts of a neuron are axon, dendrite, and cell body.



Structure of a neuron

Functions of the three parts of a neuron:

Axon: It conducts messages away from the cell body.

Dendrite: It receives information from axon of another cell and conducts the messages towards the cell body.

Cell body: It contains nucleus, mitochondria, and other organelles. It is mainly concerned with the maintenance and growth.

The growth movement in plants in response to light stimulus is known as phototropism.The shoots show positive phototropism and the roots show negative phototropism. This means that the shoots bend towards the source of light whereas the roots bend away from the light source.

Some examples of phototropism are as follows:

(a) The flower head of sunflower is positively phototropic and hence it moves from east to west along with the sun.

(b) The ovary stalk of groundnut is positively phototropic before fertilization and becomes negatively phototropic after fertilization, so that the fruit is formed underground.

The reflex arc connections between the input and output nerves meet in a bundle in the spinal cord. In fact, nerves from all over the body meet in a bundle in the spinal cord on their way to the brain. In case of any injury to the spinal cord, the signals coming from the nerves as well as the signals coming to the receptors will be disrupted.

In animals, control and coordination occur with the help of nervous system. However, plants do not have a nervous system.

Plants respond to stimuli by showing movements. The growth, development, and responses to the environment in plants is controlled and coordinated by a special class of chemical substances known as hormones. These hormones are produced in one part of the plant body and are translocated to other needy parts. For example, a hormone produced in roots is translocated to other parts when required. The five major types of phytohormone are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. These phytohormones are either growth promoters (such as auxins, gibberellins, cytokinins, and ethylene) or growth inhibitors such as abscisic acid.

The maintenance of the body functions in response to changes in the body by working together of various integrated body systems is known as coordination. All the movements that occur in response to stimuli are carefully coordinated and controlled. In animals, the control and coordination movements are provided by nervous and muscular systems. The nervous system sends messages to and away from the brain. The spinal cord plays an important role in the relay of messages. In the absence of this system of control and coordination, our body will not be able to function properly. For example, when we accidentally touch a hot utensil, we immediately withdraw our hand. In the absence of nerve transmission, we will not withdraw our hand and may get burnt.

Involuntary actions cannot be consciously controlled. For example, we cannot consciously control the movement of food in the alimentary canal. These actions are however directly under the control of the brain. On the other hand, the reflex actions such as closing of eyes immediately when bright light is focused show sudden response and do not involve any thinking. This means that unlike involuntary actions, the reflex actions are not under the control of brain.

Nervous system mechanism	Hormonal system mechanism
1. The information is conveyed in the form of electric impulse.	1. The information is conveyed in the form of chemical messengers.
2. The axons and dendrites transmit the information through a coordinated effort.	2. The information is transmitted or transported through blood.
3. The flow of information is rapid and the response is quick.	3. The information travels slowly and the response is slow.
4. Its effects are short lived.	4. It has prolonged effects.

Movement in sensitive plants	Movement in our legs
1. The movementthat takes place in a sensitive plant such as Mimosa pudica occurs in response to touch (stimulus).	1. Movement in our legs is an example of voluntary actions..
2. For this movement, the information is transmitted from cell to cell by electro-chemical signals as plants do not have any specialised tissue for conduction of impulses.	2. The signal or messages for these actions are passed to the brain and hence are consciously controlled.
3. For this movement to occur, the plant cells change shape by changing the amount of water in them.	3. In animal muscle cells, some proteins are found which allow the movement to occur.

1. Which of the following statements is correct about receptors?
(a) Gustatory receptors detect taste while olfactory receptors detect smell
(b) Both gustatory and olfactory receptors detect smell
(c) Auditory receptors detect smell and olfactory receptors detect taste
(d) Olfactory receptors detect taste and gustatory receptors smell
Ans. (a) Gustatory receptors detect taste while olfactory receptors detect smell
2. Electrical impulse travels in a neuron from
(a) Dendrite→ axon →axonal end →cell body
(b) Cell body →dendrite →axon →axonal end
(c) Dendrite →cell body →axon →axonal end
(d) Axonal end →axon →cell body →dendrite
Ans. (c) Dendrite→ cell body→ axon →axonal end
Explanation: Dendrites of a neuron receive electrical impulse from axonal end of another neuron. After that, electrical impulse travels through cell body, axon; to the axonal end.

3. In a synapse, chemical signal is transmitted from
(a) dendritic end of one neuron to axonal end of another neuron
(b) axon to cell body of the same neuron
(c) cell body to axonal end of the same neuron
(d) axonal end of one neuron to dendritic end of another neuron
Ans. (d) axonal end of one neuron to dendritic end of another neuron 
Explanation: Refer to previous question’s explanation.

4. In a neuron, conversion of electrical signal to a chemical signal occurs at/in
(a) cell body
(b) axonal end
(c) dendritic end
(d) axon
Ans. (b) Axonal end
Explanation: At the axonal end, electrical impulse sets off the release of some chemicals; called neurotransmitters. These chemicals enter the dendrite of another neuron to transmit the signal.

5. Which is the correct sequence of the components of a reflex arc?
(a) Receptors→ Muscles→ Sensory neuron→ Motor neuron→ Spinal cord
(b) Receptors→ Motor neuron →Spinal cord →Sensory neuron →Muscle
(c) Receptors →Spinal cord →Sensory neuron →Motor neuron →Muscle
(d) Receptors →Sensory neuron →Spinal cord →Motor neuron →Muscle
Ans. (d) Receptors →Sensory neuron →Spinal cord →Motor neuron →Muscle  

Explanation: In a reflex arc, sensory neurons pick signals from receptors. These signals are then sent to the spinal cord; from where they go to muscle via motor neuron.

6. Which of the following statements are true?
(i) Sudden action in response to something in the environment is called reflex action
(ii) Sensory neurons carry signals from spinal cord to muscles
(iii) Motor neurons carry signals from receptors to spinal cord
(iv) The path through which signals are transmitted from a receptor to a muscle or a
gland is called reflex arc
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (ii) and (iii)
Ans. (c) (i) and (iv)
Explanation: Sensory neurons carry signals from muscles to spinal cord, hence statements (ii) and (iii) are incorrect.

7. Which of the following statements are true about the brain?
(i) The main thinking part of brain is hind brain
(ii) Centres of hearing, smell, memory, sight etc are located in fore brain.
(iii) Involuntary actions like salivation, vomiting, blood pressure are controlled by the medulla in the hind brain
(iv) Cerebellum does not control posture and balance of the body
(a) (i) and (ii)
(b) (i), (ii) and (iii)
(c) (ii) and (iii)
(d) (iii) and (iv)
Ans. (c) (ii) and (iii)
Explanation: The main thinking part of the brain is forebrain and hence statement (i) is incorrect. Cerebellum controls posture and balance of the body and hence statement (iv) is incorrect.

8. Posture and balance of the body is controlled by
(a) cerebrum
(b) cerebellum
(c) medulla
(d) pons
Ans. (b) Cerebrum
Explanation: Refer to previous question’s explanation.
9. Spinal cord originates from
(a) cerebrum
(b) medulla
(c) pons
(d) cerebellum 
Ans. (b) medulla 
10. The movement of shoot towards light is
(a) geotropism
(b) hydrotropism
(c) chemotropism
(d) phototropism
Ans. (d) phototropism
Explanation: The term ‘phototropism’ is composed of two terms. ‘Photo’ means light and ‘tropism’ means growth.

11. The main function of abscisic acid in plants is to
(a) increase the length of cells
(b) promote cell division
(c) inhibit growth
(d) promote growth of stem
Ans. (c) Inhibit growth

12. Which of the following is not associated with growth of plant?
(a) Auxin
(b) Gibberellins
(c) Cytokinins
(d) Abscisic acid
Ans. (d) Abscisic acid
Explanation: It is an inhibitory hormone and hence is not associated with growth of plant.

13. Iodine is necessary for the synthesis of which hormone?
(a) Adrenaline
(b) Thyroxin
(c) Auxin
(d) Insulin
Ans. (b) Thyroxin
Explanation: This is the reason, deficiency of iodine results in hypothyroidism.

14. Choose the incorrect statement about insulin
(a) It is produced from pancreas
(b) It regulates growth and development of the body
(c) It regulates blood sugar level
(d) Insufficient secretion of insulin will cause diabetes
Ans. (b) It regulates growth and development of the body
Explanation: Insulin has no role to play in growth and development of the body.

15. Select the mis-matched pair
(a) Adrenaline: Pituitary gland
(b) Testosterone: Testes
(c) Estrogen: Ovary
(d) Thyroxin: Thyroid gland
Ans. (a) Adrenaline: Pituitary gland
Explanation: Adrenaline is secreted by adrenal gland.

16. The shape of guard cells changes due to change in the
(a) protein composition of cells
(b) temperature of cells
(c) amount of water in cells
(d) position of nucleus in the cells
Ans. (c) Amount of water in cells
Explanation: Excess amount of water results in guard cells becoming turgid, while lack of water results in flaccidity of guard cells.

17. The growth of tendril in pea plants is due to
(a) effect of light
(b) effect of gravity
(c) rapid cell divisions in tendrillar cells that are away from the support
(d) rapid cell divisions in tendrillar cells in contact with the support
Ans. (c) rapid cell divisions in tendrillar cells that are away from the support
Explanation: Due to this, there is more growth on portion away from support. This results in tendrils getting twined around the support.

18. The growth of pollen tubes towards ovules is due to
(a) hydrotropism
(b) chemotropism
(c) geotropism
(d) phototropism
Ans. (b) chemotropism
Explanation: Ovules release certain chemicals which stimulate the growth of pollen tubes towards ovules.

19. The movement of sunflower in accordance with the path of sun is due to
(a) phototropism
(b) geotropism
(c) chemotropism
(d) hydrotropism
Ans. (a) phototropism
Explanation: Refer to explanation for question 10.

20. The substance that triggers the fall of mature leaves and fruits from plants is due to
(a) auxin
(b) gibberellin
(c) abscisic acid
(d) cytokinin
Ans. (c) abscisic acid

21. Which of the following statements about transmission of nerve impulse is
incorrect?
(a) Nerve impulse travels from dendritic end towards axonal end
(b) At the dendritic end electrical impulses bring about the release of some chemicals
which generate an electrical impulse at the axonal end of another neuron
(c) The chemicals released from the axonal end of one neuron cross the synapse and
generate a similar electrical impulse in a dendrite of another neuron
(d) A neuron transmits electrical impulses not only to another neuron but also to muscle
and gland cells
Ans. (b) At the dendritic end electrical impulses bring about the releases of some chemicals
which generate an electrical impulse at the axonal end of another neuron.
Explanation: Chemicals are released from the axonal end and not from dendritic end.
22. Involuntary actions in the body are controlled by
(a) medulla in fore brain
(b) medulla in mid brain
(c) medulla in hind brain
(d) medulla in spinal cord
Ans. (c) Medulla in hind brain
Explanation: Medulla is a part of hind brain.
23. Which of the following is not an involuntary action?
(a) Vomiting
(b) Salivation
(c) Heart beat
(d) Chewing
Ans. (d) Chewing

24. When a person is suffering from severe cold, he or she cannot
(a) differentiate the taste of an apple from that of an ice cream
(b) differentiate the smell of a perfume from that of an agarbatti
(c) differentiate red light from green light
(d) differentiate a hot object from a cold object
Ans. (b) differentiate the smell of a perfume from that of an agarbatti
Explanation: When a person is suffering from severe cold, his olfactory receptors are blocked. Due to this, sense of smell is compromised.

25. What is the correct direction of flow of electrical impulses?

Ans. (c) Explanation: Dendrites of a neuron receive electrical impulse from axonal end of
another neuron. After that, electrical impulse travels through cell body, axon; to the
axonal end.
26. Which statement is not true about thyroxin?
(a) Iron is essential for the synthesis of thyroxin
(b) It regulates carbohydrates, protein and fat metabolism in the body
(c) Thyroid gland requires iodine to synthesise thyroxin
(d) Thyroxin is also called thyroid hormone
Ans. (a) Iron is essential for the synthesis of thyroxin
Explanation: It is iodine which is essential for the synthesis of thyroxin.

27. Dwarfism results due to
(a) Excess secretion of thyroxin
(b) Less secretion of growth hormone
(c) Less secretion of adrenaline
(d) Excess secretion of growth hormone
Ans. (b) Less secretion of growth hormone
Explanation: As the name suggests, growth hormone is responsible for proper growth in a person. Hence, lack of growth hormone would result in dwarfism.

28. Dramatic changes of body features associated with puberty are mainly because of secretion of
(a) oestrogen from testes and testosterone from ovary
(b) estrogen from adrenal gland and testosterone from pituitary gland
(c) testosterone from testes and estrogen from ovary
(d) testosterone from thyroid gland and estrogen from pituitary gland
Ans. (c) testosterone from testes and estrogen from ovary
Explanation: These are called sex hormones and are secreted from gonads. Testosterone is secreted in males and oestrogen is secreted in females.

29. A doctor advised a person to take an injection of insulin because
(a) his blood pressure was low
(b) his heart was beating slowly
(c) he was suffering from goiter
(d) his sugar level in blood was high
Ans. (d) his sugar level in blood was high
Explanation: if a person is suffering from diabetes then his blood sugar level may
become high. This disease is managed in some patients by administering insulin injection.
30. The hormone which increases the fertility in males is called
(a) oestrogen
(b) testosterone
(c) insulin
(d) growth hormone
Ans. (b) Testosterone

31. Which of the following endocrine glands is unpaired?
(a) Adrenal
(b) Testes
(c) Pituitary
(d) Ovary
Ans. (c) Pituitary
32. Junction between two neurons is called
(a) cell junction
(b) neuro muscular junction
(c) neural joint
(d) synapse
Ans. (d) synapse
33. In humans, the life processes are controlled and regulated by
(a) reproductive and endocrine systems
(b) respiratory and nervous systems
(c) endocrine and digestive systems
(d) nervous and endocrine systems
Ans. (d) nervous and endocrine systems
Explanation: Reproductive or respiratory systems have no role to play in control and coordination.

Ans. (a) Sensory neuron
 (b) Spinal cord
 (c) Motor neuron
 (d) Muscle

(a) elongation of cells
 Ans. Auxin
(b) growth of stem
 Ans. Giberellins
(c) promotion of cell division
 Ans. Cytokinin
(d) falling of senescent leaves.
 Ans. Abscisic acid 
 

Ans. (a) Pineal gland
 (b) Pituitary gland
 (d) Thyroid gland
 (d) Thymus

Ans. Figure ‘a’ appears more accurate. In this figure roots are showing positive geotropism, while shoot is showing negative geotropism. 

Ans. (a) Dendrite
 (b) Cell body
 (c) Axon
 (d) Axon terminal 

Column (A)	Column (B)
(a) Olfactory receptors	(i) Tongue
(b) Thermo receptors (temperature receptors)	(ii) Eye
(c) Gustatoreceptors	(iii) Nose
(d) Photoreceptors	(iv) Skin

Ans. (a)-iii
 (b)-iv
 (c)-i
 (d)-ii

Directional growth or movement in response to a stimulus is called tropic movement. For example; roots generally show positive geotropic movement, which means roots usually grow towards gravity.

Iodine is essential for manufacture of thyroxin in the body. If intake of iodine in our diet is low, it will reduce the production of thyroxin. This can result in hypothyroidism. It can even result in goiter.

The junction between two neurons is called synapse. When electrical impulse reaches the axonal end of a neuron, it sets off the release of neurotransmitters in the synapse. These neurotransmitters enter the dendrite of another neuron to set off electrical signal in that neuron. That is how electrical impulse travels from one neuron to another.

(a) Which hormone is responsible for the changes noticed in females at puberty?
 Ans. Oestrogen
(b) Dwarfism results due to deficiency of which hormone?
 Ans. Growth hormone
(c) Blood sugar level rises due to deficiency of which hormone?
 Ans. Insulin
(d) Iodine is necessary for the synthesis of which hormone?
 Ans. Thyroxin 

(a) Name the endocrine gland associated with brain?
 Ans. Pituitary
(b) Which gland secretes digestive enzymes as well as hormones?
 Ans. pancreas
(c) Name the endocrine gland associated with kidneys?
 Ans. Adrenal gland
(d) Which endocrine gland is present in males but not in females?
 Ans. Testis 

Function of Neuron: Neuron is the functional unit of nervous system. It carries electrical impulse from brain to different organs and vice-versa

Parts Of Brain	Function
Cerebrum	The cerebrum controls the voluntary motor actions. It is the site of sensory perceptions; like tactile and auditory perceptions. It is the seat of learning and memory.
Cerebellum	It coordinates the motor functions.
Hypothalmus	It controls sleep and wake cycle (circadian rhythm) of the body. It also controls the urges for eating and drinking.
Medulla	Medulla controls various involuntary functions; like hear beat, respiration, etc.

Central nervous system is composed of the brain and spinal cord. Peripheral nervous system is composed of nerves which are outside the spinal cord. Central nervous system has a well-developed system for protection. Brain is enclosed in skull for protection. Spinal cord is enclosed in vertebral column for protection. Additionally, the cerebrospinal fluid provides a cushion against mechanical shocks.

(a) Thyroxin
 Ans. General metabolism
(b) Insulin
 Ans. Glucose metabolism
(c) Adrenaline
 Ans. Preparing the body for emergencies
(d) Growth hormone
 Ans. Growth
(e) Testosterone. 
Ans. Secondary sexual characters

Various plant hormones and their effects are as follows:

• Auxin: Promotes cell elongation and thus promotes growth of a plant part.
• Gibberellins: Promotes growth.
• Cytokinin: Promotes cell division and thus promotes growth.
• Abscissic acid: Inhibits growth and is responsible for shedding or older parts.

The sudden involuntary movement in a voluntary organ; in response to a stimulus; is called reflex action.
 Examples of reflex action:
 (a) Moving your hand away from a hot iron plate
 (b) Blinking of eyes

Reflex Arc: The path of electrical impulse during a reflex action is called reflex arc. A reflex arc is composed of a sensory neuron, spinal cord, motor neuron and muscle. It involves following steps:
• The sensory neuron picks signals from the stimulus and carries the signals to the spinal cord.
• Spinal cord process the signals and sends message through the motor neuron.
• Motor neuron transmits the signals to the effector muscle so that the muscle can take immediate action. 

Control and coordination of functioning of various systems is under the direct control of nervous system. It is the nervous system which governs the way a particular organ or organ system has to work. This control is achieved by a complex network of neurons which carry signals in the form of electric impulses; to and from the brain.

 The hormonal system, on the other hand, coordinates the functioning of nervous system. The hormonal system has somewhat indirect control on various functions. It tells a system to either slow down or pace; according to the situation. 
 
 Nervous and hormonal systems are complementary to each other. Thus, it can be said that nervous and hormonal system together perform the function of control and coordination in human beings 

Hormones facilitate chemical control in animals. Hormones are chemicals which are directly released in bloodstream. A particular hormone reaches the target site through blood. Hormone tells the target tissue to behave in a particular manner. To understand this, let us takes the example of adrenalin. Adrenalin is secreted by adrenal gland. It reaches the heart and lungs and also to the GI tract. Heart speeds up its pumping action so that more blood could be supplied to the limbs and facial muscles. But activity of the GI tract is slowed down to ensure better blood supply in limbs. Thus, adrenalin prepares the body for a fight or flight situation.

Electrical impulse travels through a neuron. But to be transmitted to another neuron, it need to be passed in the form of neurotransmitters. Neurotransmitters are specialized chemicals. These can enter a neuron only through specialized channels. Such channels are present in dendrites but not in axon. On the other hand, a neurotransmitter can enter a dendrite. Due to this, the flow of signals in a synapse is from axonal end of one neuron to dendritic end of another neuron but not the reverse.