Biology

Key Unit Competence 

Describe the structure of neurons and explain the mechanisms of impulse transmission

Learning Objectives 

By the end of this unit, I should be able to:

– Describe the arrangement of neurons in a reflex arc. 

– Describe the structure neurones. 

– Explain how a resting potential is maintained. 

– Explain how an action potential is generated. 

– Explain how a nerve impulse is propagated along a neurone. 

– Explain the factors affecting the speed of impulse transmission. 

– Describe the properties of a nerve impulse limited to: saltatory conduction, all or nothing law, and refractory period. 

 – Describe the functions of neurones in a reflex arc. 

– Explain how information passes across a synapse from one neurone to another or from a neurone to its effector.  

– Outline the roles of synapses.   

– Describe the roles of neuromuscular junctions, transverse system tubules and sarcoplasmic reticulum in stimulating contraction in striated muscle. 

 – Relate the structure of a cholinergic synapse to its functions. 

– Interpret graphs for all or nothing law and refractory period. 

– Investigate the nature of a nerve impulse in a nerve tissue of a frog 

– Appreciate the importance of a coordinated behaviour in organisms. 

– Show concern about the need to have reflexes as rapid responses 

9.1 Overview of control and co-ordination in mammals

Activity 9.1 

– Use charts showing the parts of human brain and watch the movies on you tube showing the different parts of human brain.  

– Use the school library and search additional information on the internet. Read the information related to human brain, and take    short notes on human brain. 

1. Illustrate with diagram the main parts of human brain 

2. Write down the relative functions of each identified part of the human brain

Coordination:It  is the process in which body coordinate, ordinate and control different activities.

The nervous system plays the main functions such as: 

(i) Sensory input: Sensory receptors present in skin and organs respond to external and internal stimuli by generating nerve impulses that travel to the brain and spinal cord, 

(ii) Integration: The brain and spinal cord sum up the data received from all over the body and send out nerve impulses 

(iii) Motor output: The nerve impulses from the brain and spinal cord go to the effectors, which are muscles and glands.

The Nervous system is divided into two main divisions: The central nervous system (CNS) and the peripheral nervous system (PNS) The central nervous system (CNS) consist of the brain and spinal cord, which are located in the midline of the body. The peripheral nervous system (PNS), which is further divided into the somatic division and the au- tonomic division, includes all the cranial and spinal nerves. 

9.1.1 Some key word definitions 

– Irritability or sensitivity. This is the ability of living organisms to respond to a stimulus 

– A stimulus: This is any change in the external or internal environment which provokes a response 

– Receptors: These are specialized cells that detect a stimulus 

– Neurons: These are cells which transmit nerve impulses 

– Effectors: are organs that respond to the stimuli and bring about a response. 

– A nervous system: This is a system which involved in the detection of stimuli (sensory inputs) integration and response (motor output)  

– The response may be to both the external and internal environments. 

– Neurone or nerve cell: It is the basic functional unit of the nervous system. Neurones are cells specialized to generate and transmit nerve impulses (action potentials) are cells which transmit nerve impulses (action potentials).

9.1.2 The division of nervous system

The nervous system of a mammal comprises of the central nervous system (CNS) consisting of the brain and the spinal cord, and the peripheral nervous system (PNS) consisting of the cranial nerves from the brain, the spinal nerves from the spinal cord and the sensory organs (Figure 9.1).

1. The human brain 

The brain is the enlarged end of the spinal cord. It is enclosed in the skull and is divided into three main parts namely: the fore brain, the mid brain, the hind brain. 

a.  The fore brain 

This consist of: cerebrum, thalamus, hypothalamus and pituitary gland 

• The cerebrum:

This is the largest part of the brain made up of two hemispheres called the right and the left cerebral hemispheres. The left cerebral hemisphere controls those activities of the right side of the body while the right cerebral hemisphere controls those of the left side of the body. 

The functions of the cerebral hemisphere 

– It is the centre of the judgment, memory, reasoning and imagination. 

– It receives the impulses from the sensory organs: sight, taste, sound and touch. 

– It controls all the body’s voluntary activities, e.g. walking, eating, singing,

• The thalamus: 

This is a relay centre. It relays sensory information towards higher centre. It is the centre for the perception of pain and pleasure. 

• The hypothalamus 

It performs many functions such as; regulates and monitors the temperature of blood, monitors and regulates the water content of blood, a co-ordinating centre for activities of the internal organs, e.g. rate of heart beat, blood pressure. It is a centre of for feelings such as; hunger, thirst, sex drive, satisfaction, sleep, speech, etc. As an endocrine gland, it produces hormones i.e. anti-diuretic hormone (ADH) and oxytocin.

• The pituitary gland:

It produces hormones such as: Follicle-stimulating hormone (FSH), Thyroidstimulating hormone (TSH), Adreno-cortico trophic hormone (ACTH), Prolactin hormone and Luteinizing hormone (LH)  

b.  The mid brain 

This acts as an association centre between the fore and the hind brain. It is a relay centre for audio and visual information. It is also responsible for movement of the head and the trunk. 

c.  The hind brain 

This receives the impulse from the ear, the skin and the semi-circular canals. It consists of: The cerebellum and the medulla oblongata The cerebellum:  It lies behind the optic lobes. It receives impulses simultaneously from the eyes and the ears. It regulates and co-ordinate muscular movement, especially those concerned with maintaining body equilibrium and controls all the unconscious activities of the body. 

The medulla oblongata: This control all the involuntary movements of the body especially those concerned with respiration, digestion, heartbeat, breathing rate and sneezing.

2. The spinal cord 

The spinal cord is a dorso -ventrally flattened cylinder of nervous tissue running from the base of the brain down the lumbar region. It is protected by the vertebrate of the backbone and the meninges.

Functions of the spinal cord include;  

– It is a coordinating centre for simple reflex such as the knee-jerk response and the autonomic reflexes such as contraction of the bladder. 

– Providing a means of communication between peripheral nerves and the brain. 

– It sends messages to the effectors

A transverse section of the spiral cord shows an H-shaped central core of grey matter. Grey matter is composed of nerve cell bodies, dendrites and synapses surrounding a central canal which contains cerebrospinal fluid. White matter: around the grey matter, is an outer layer containing nerve fibres whose fatty myelin sheaths give it its characteristics colour.

The spinal cord acts as a coordinating centre for simple reflex such as knee jerk response and autonomic reflexes. The spinal   cord acts as means of communication between spinal nerves and the brain. It sends impulses to the brain through sensory neurons from the body and returns the motor impulses to the effectors which are muscles and glands.

Self-assessment 9.1 

Describe the form in which the information is conveyed in the nervous system

9.2 Structure, types and functions of neurons

Activity 9.2 

 – Use charts describing the neuron and watch the movies showing the types of neuron. 

– Using textbooks or searching additional information on the internet, read the information related to the structure, types and functions of neurone.

a. Draw and label the structure of a neurone 

b. Make a table compering different types of neurons

Aneuron also called nerve cell is the basic functional unit of the nervous system. Neurons are cells specialized to generate and transmit nerve impulses (action potentials) are cells which transmit nerve impulses (action potentials). 

9.2.1 Types of neurons 

Nerve cells may be grouped according to the number of processes they possess so that their types include: 

– Unipolar neurons: those with one process only, found mainly in invertebrates. 

– Bipolar neurons: those with two separate processes such as neurons in the retina of the vertebrate eye. 

– Multipolar neurons: those with more than two processes such as most of the vertebrate neurons.

9.2.2 Classification of neurons by their functions 

In vertebrates, it is also common to group neurons according to their functions. They include: 

– Sensory or afferent neurons: transmit impulses from the receptors to the central nervous system. In addition to sensory or afferent neurons. 

– Motor or efferent neurons: that transmits impulses from the central nervous system to effectors motor organs such as muscles or glands that carry out the response. Most motor neurones are stimulated by impulses conducted by interneurons. However, there are some others that are stimulated directly by sensory neurons. 

– Interneurons also known as intermediate or association, or relay or interneuron connect the pathways of sensory and motor impulses, and are found mainly in the central nervous system.

9.2.3 Parts of a neuron and their functions 

Each motor neuron possesses a cell body and cytoplasm with many mitochondria, endoplasmic reticulum, golgi apparatus and ribosomes. The Nissl granules which consist of endoplasmic reticulum and ribosomes function in protein synthesis. The table below (Table 9.1) shows all parts of neuron and their functions. 

Self-assessment 9.

2 Explain what would happen when a neuron is damaged

9.3 Nature and generation of a nerve impulse

Activity 9.3 

– Watch the movies showing the generation of a nerve impulse.  

– Use the school library and search additional information on the internet. 

– Read the information related to the generation of the nerve impulse and take short notes on generation of the nerve impulse.  

– Answer the following questions: 

a. Draw, label and interpret the graph showing the action potential 

b. What do you understand by action potential?

All cells in animal body tissues are electrically polarized—in other words, they maintain a voltage difference across the cell’s plasma membrane, known as the membrane potential. This electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels. Each excitable patch of membrane has two important levels of membranepotential: the resting potential, which is the value the membrane potential maintains as long as nothing passes along the cell, and a higher value called the threshold potential.  

9.3.1. Resting potential in a neuron 

A neuron is said to be in the resting state when it is not conducting an impulse. The membrane potential of an unstimulated excitable cell is called the resting potential. A resting potential is the difference in charge (electrical potential difference) which exists between the inside and the outside of the cell membrane. In excitable cells, the resting potential is about -70 millivolts (mV) and the threshold potential is around -55 mV. The negative sign indicates the interior of the cell is negative with respect to the exterior environment.

The resting potential difference across the neuron membrane is maintained by: 

– The sodium –potassium pump (Na+ /K+). This is always working. Three sodium ions (Na+) are actively transported out of the cell for every two potassium ions (K+) pump into the cell. Energy supplied by ATP is used for the transport of ions against their electrochemical gradients. 

– The axon membrane: It is more permeable to potassium ions than the sodium ions. This is due to the presence of more potassium ion non-gated, voltageindependent channels and few sodium ion non-gated channels. More K+ ions can diffuse out back again faster than Na+ ions which can diffuse back in. The resting membrane potential is mainly determined by sodium-potassium pump, facilitated diffusion and electrochemical gradient of K+ ions across the membrane. 

9.3.2. Action potential 

Action potential is the technical term for impulse. An action potential is rapid temporary reversal in the electrical potential difference of an excitable cell e.g. a neuron or a muscle cell. It is caused by changes in the permeability of the membrane following the application of a threshold stimulus. The action potential has a depolarization phase and a repolarization phase. There may be a short hyperpolarized phase after the repolarization phase. The time taken for an action potential is 2 to 3 milliseconds.

9.3.3. Depolarization 

When a stimulus such as electric current reaches a resting neuron, some sodium voltage gated channels in the stimulated region of the axon membrane open. Sodium ions (Na+) move into the axon by facilitated diffusion down an electrochemical gradient. The initial influx of sodium ions is slow. The axon membrane becomes slightly depolarized and the sodium voltage gates are sensitive to voltage changes. More gates open allowing more Na+ ions to diffuse into the cell leading to further depolarization. 

When the potential difference across the membrane reaches a threshold value (-50mV), many more sodium voltage gated channels open. This is an example of positive feedback. The rapid diffusion of Na+ ions leads to a sudden increase in the cell’s potential difference which becomes positive (+ 40mV). This reversal in the potential difference is known as depolarization and lasts for about 1 millisecond.

9.3.4. Repolarization

The reversal in polarity to + 40 mV causes the voltage gated sodium channel to close. At the same time the voltage gated potassium channels open. The potassium ions K+ diffuse out of the cell down their electrochemical gradient to the tissue fluid outside. The axon membrane is repolarized. The action potential alters from + 40 mV   to -70mV.

9.3.5. Hyperpolarization

The potassium voltage-gated channels are slow to close. An excess of K+ ions leave the axon. The inside of the membrane becomes more negative. The voltage falls slightly below -70mV and causes hyperpolarization. However, within a few milliseconds, the potassium voltage-gated channels close. The resting potential of -70mV is reestablished by the Na+ /K+ pump and different rates of facilitated diffusion of K+ and Na+ ions through the non-gated ion channels.

9.3.6. Frequency of action potentials 

Information in axons is coded in the frequency of the action potentials. A weak stimulus above threshold produces fewer action potentials. A stronger stimulus produces a greater frequency of action potentials. As the intensity of stimulation increases, more action occurs.

9.4 Transmission of nerve impulses

Activity 9.4 

The dissection of a frog sciatic nerve

Materials required 

Laptop computer, projector, nerve chamber, cable and nerve chamber leads (red and black), glass hooks, Stimulator cable, grounding adapter or cable, forceps, scalpel, frog Ringer’s solution at two temperatures.

Procedure  

– To begin dissection, retrieve a frog from your teacher and place it in a dissecting tray. 

– Remove the skin from the legs by making an incision through the skin and around the entire lower abdomen.  

– Cut the connections between the skin and the body especially around the base of the pelvic girdle. 

– Use stout forceps to pull the skin off the frog in one piece (like a pair of pants).  – Place the frog with its dorsal side up.  

– Moisten the exposed tissue (legs) with Ringer’s solution and place a wet paper towel (saturated with Ringers solution) over one of the legs of the frog so that it is completely covered and wet.   

– Use forceps to separate the muscles of the thigh (the leg not covered with the paper towel).  

– Pin the muscles apart so that more underlying muscle is visible.  

 – This should also expose the cream-colored Sciatic nerve lying deeply between the muscles. 

 – Use a glass hook to separate the nerve from the fascia and the vessels. If possible, avoid cutting the blood vessels. If bleeding does occur, rinse away the blood with lots of Ringer’s solution. Free the nerve from the knee joint to the pelvis.  

– Use the glass hook to place a suture thread under the nerve. Move the thread as close to the knee joint as possible.  

– Ligate (tie off) the nerve; you may observe calf muscle fibrillation or foot movement as the knot is tied off.   

– Be sure the knot is tied tightly.  Cut the nerve between the knot and the knee joint. Keep the exposed nerve moist at all times with Ringer’s solution.  

– Carefully separate the muscles of the pelvis to expose the sciatic nerve. Remember to rinse any blood away with Ringer’s solution.  

– Carefully expose the remainder of the nerve through an opening along the lateral side of the urostyle. To avoid cutting the nerve, lift the end of the urostyle with forceps as you cut the muscle away from the urostyle with blunt scissors.  

– Cut along the urostyle from its tip to the vertebral column.  

– Deflect the muscle away from the urostyle to expose the Sciatic nerve.  

– Use a glass hook to separate connective tissue from the nerve and to place a piece of suture thread under the nerve.  

– Move the thread as high as possible on the nerve to obtain  as large a section as possible.  

– Ligate (tie off) the nerve; the leg may jump again as the knot is tied tightly.  

– Cut the nerve between the knot and the vertebral column and keep the exposed nerve moist at all times.  

– Use forceps to grasp the suture thread at the proximal end (end closest to head) and lift the nerve out of the body cavity.  

– Do not pinch or stretch the nerve.   

– Remove any connective tissues, blood vessels, or nerve branches that may still keep the nerve attached to the frog.  

– Continue to grasp the suture to lift the nerve until it is clear of the abdomen, the pelvis, and the thigh. 

– Grasp the suture at either end to remove the nerve from the body entirely.   

– Place the nerve across the gold-coloured electrode pins in the nerve bath.   

– Add a small quantity of Cold Frog Ringers to the bottom of the chamber.   

– The Frog ringers should not touch the gold-plated electrode pins. 

– Cover the chamber with a glass slide.

Questions 

1. Draw a picture of the laboratory setup used for this exercise. 

2. Find and dissect the frog sciatic nerve for placement in a nerve chamber.

9.4.1 Mechanism of transmission of nerve impulses along an axon

– The neurons, like other cells, are positively charged outside and negatively charged inside. The membrane of the axon is said to be polarized. The potential difference (voltage) across their membranes is of – 70mV and is called resting membrane potential (RMP).  

– A stimulus (heat, pain, bite, sound …) creates an action potential (AP) or an impulse that is transmitted along an axon by electro-chemical change.  

– During an action potential, the membrane potential falls until the inside becomes positively charged with respect to the exterior. The membrane at this point is said to be depolarized. It takes few milliseconds to happen. In fact, the potential changes from – 75 mV to + 40 mV at the point of stimulation. That is an electrical change that runs along the axon. 

– As the impulse is transmitted along the axon, the Na+/K+ pumps of the axolemma are re-established. Sodium channels open first, allowing a large number of Na+ ions to flow in.  

– The axoplasm becomes progressively more positive with respect to the outside of the axolemma. Then, almost instantly, the permeability of the membrane to Na+ ions ceases, and the net flow of Na+ ions stop.  At the same time K+ ion channels start to open and K+ ions flow out from axoplasm where they are in high concentration. The counter-flow is of 3Na+ ions against 2K+ ions.  

– The axoplasm now starts to become less positive again. This begins the process of re-establishing the resting potential difference of the membrane. That is an electro-chemical change.

a. Factors that affect the transmission of nerve impulses along the axon membrane 

Along the axon membrane, the transmissions of nerve impulses are affected as follows: 

– The diameter of the axon: the greater the diameter the faster the speed of transmission of nerve impulses. 

– The myelin   sheath: myelinated neurones conduct impulses faster than non- myelinated neurones. 

– The presence of nodes of Ranvier: speeds up the movement of impulses in myelinated neurones. 

b.  Structure of a synapse 

Information from one neuron flows to another neuron across a synapse. The synapse is a small gap separating two adjacent neurons. The synapse consists of: 

– A presynaptic ending that contains neurotransmitters, mitochondria and other cell organelles, 

– A postsynaptic ending that contains receptor sites for neurotransmitters and,

– A synaptic cleft or space between the presynaptic and postsynaptic endings.  It is about 20nm wide. 

– The swollen tip of the axon of the presynaptic neuron, called synaptic knob or synaptic bulb contains many membrane 

– bounded synaptic vesicles, mitochondria and microfilaments. 

– The synaptic   vesicles contains neuro transmitter molecules such as acetylcholine or noradrenaline.

c. Neurotransmitter 

A neurotransmitter is a relatively small chemical found in the synaptic vesicle. It helps to transmit an impulse across a synapse or neuromuscular junction. There are about 50 different types of neurotransmitters in the human body. Examples are acetylcholine released by cholinergic neurons, noradrenaline (norepinephrine) released by adrenergic neurons, dopamine and serotonin including amino acids glutamate and glycine. 

9.4.2 Mechanism of nerve impulse transmission across a synapse 

– The arrival of an impulse on the synaptic knob causes the opening of Ca+2 ion channels on the presynaptic membrane, and Ca+2 ions flow in the presynaptic region from the synaptic cleft.  

– The Ca+2 ions induce a few presynaptic vesicles to fuse with presynaptic membrane and to secrete their neurotransmitters (e.g. acetylcholine) by exocytosis into the synaptic cleft  

– The neurotransmitter then binds with the receptor protein on the postsynaptic membrane. This causes the opening of Na+ channels on the postsynaptic neuron which in turn becomes depolarized.  

– This causes a depolarization of the post-synaptic cell membrane, which may initiate an action potential, if the threshold is reached 

– The action of the neurotransmitter does not persist because an enzyme cholinesterase catalyses the hydrolysis of acetylcholine into choline and acetate. The breakdown products (choline) are absorbed by the pre-synaptic neuron by endocytosis and used to re-synthesize more neurotransmitter, using energy from the mitochondria.

9.4.3 Properties of a nerve impulse 

a. All or nothing law 

An action potential can only be generated after the threshold value is exceeded. After the threshold is reached, the size of the action potential produced remains constant and is independent of the intensity of the stimulus. This is the all or nothing response. All action potentials are of the same amplitude.

b. Refractory period 

This is a brief period when an axon is unable to transmit an impulse following transmission of the same. It lasts about 5-10 milliseconds.  It is divided into two; absolute and relative periods. During the absolute refractory period which lasts about 1ms, the axon membrane is unable to respond to another stimulus, no matter how strong it is. An action potential cannot be produced. This is because there is conformational change in voltage-gated sodium channels which are still in a closed, inactive state. This also prevents the action potential from moving backwards.

Following   the absolute refractory period, there is a relative refractory period which lasts around 5ms.  During this period, the resting potential is gradually restored by Na+ /K+ pump and the relative permeability of membrane to facilitated diffusion of ions is also restored. A new action potential can then be produced if the stimulus is greater than the usual one. The refractory period therefore allows impulses to move only in one direction and limits the frequency at which successive impulses can pass along axon.

c. Salutatory conduction 

It is movement or jump of nerve impulses from one node of Ranvier to another along the axon membrane of neurone.

9.5 Structure and function of a cholinergic synapse.

Activity 9.5

 Use textbooks from school library and other additional information using internet, read the information related to the cholinergic synapse and take    short notes on cholinergic synapse.

a. Draw and label a diagram showing a cholinergic synapse 

b. Make a table of different functions of a cholinergic synapse

The cholinergic synapse is a  synapse  which uses acetylcholine (Ach) as neurotransmitter.  Calcium and vesicles are involved in the release of neurotransmitter across the synaptic cleft in the mechanism of synaptic transmission to generate an excitatory post-synaptic potential.

9.5.1. Functions of synapses 

Synapses have a number of functions which include: 

a. Transmit information between neurones 

The main function of synapses is to convey information between neurons. It is from this basic function that the others arise. 

b. Pass impulses in one direction only

 As the neurotransmitter substance can only be released from one side of a synapse, it ensures that nerve impulses only pass in one direction along a given pathway 

c. Act as junctions Neurons may converge at synapse.

 In this way a number of impulses passing along different neurons may release sufficient neurotransmitter to generate a new action potential in a single postsynaptic neuron whereas individually they would not.  This is known as spatial summation. In this way responses to a single stimulus may be coordinated. 

d. Filter out low level stimuli 

Background stimuli at a constantly low level, e.g. the drone of machinery, produce a low frequency of impulses and so cause the release of only small amounts of neurotransmitter at the synapse. This is insufficient to create a new impulse in the postsynaptic neuron and so these impulses are carried no further than the synapse. Such low level stimuli are of little importance and the absence of a response to them is rarely, if ever harmful. Any change in the level stimulus will be responded to in the usual way.  

e. Allow adaptation to intense stimulation:

 In response to a powerful stimulus, the high frequency of impulses in the presynaptic neuron causes considerable release of neurotransmitter into the synaptic cleft. Continued high-level stimulation may result in the rate of release of neurotransmitter exceeding the rate at which it can be formed. In these circumstances the release of neurotransmitter ceases and hence also any response to the stimulus. The synapse is said to be fatigued.

9.5.2. Effects of drugs on synapses 

Several types of chemicals such as drugs interfere at synapses, either amplifying or inhibiting the transmission o of impulses. For example, 

 – Caffeine and nicotine amplify the transmission of impulses by mimicking the action of natural neurotransmitters.  

– Insecticides that prolong the effect of neurotransmitters by blocking the enzymatic breakdown of transmitters. Other drugs such as  

– Anaesthesia including atropines inhibit the transmission of impulses across the synaptic membranes. Atropine acts to prevent an action potential being generated by acetylcholine when it attaches to its receptor protein on the postsynaptic membrane

9.5.3. The neuromuscular junction 

A special kind of synapse is the nerve-muscle known as neuromuscular junction, the point where the terminal dendrite of a motor nerve cell makes contact with a muscle fibre. The region of the sarcolemma (cell surface membrane) of muscle fibre that lies directly under the terminal portion of the motor neuron is known as the motor end plate. At the nerve-muscle junction the membrane of the muscle fibre is modified to form an end-plate to which the dendrite is attached.

When an impulse arrives at the nerve-muscle junction, acetylcholine is discharged from synaptic vesicles into the synaptic cleft. The acetylcholine diffuses across the gap and depolarizes the muscle end plate. End-plate potentials can be recorded and it has been shown that if these build up sufficiently an action potential is fired off in the muscle fibre.

9.6 Functions of sensory, relay and motor neurons in a reflex arc

9.6.1. Reflex actions

A reflex action is a quick and involuntary response of the central nervous system to a stimulus. Example: The quick withdrawal of the hand from a hot object. When the spinal cord alone is involved, the reflex action is called spinal reflex and when the brain alone is involved, it is a cranial reflex e.g. blinking of eyes.

Reflex actions are described as involuntary actions and the same stimuli produce the same responses every time. Reflexes are useful because they make autonomic involuntary adjustments to changes in the external environment, such as the irispupil reflex and the balance during locomotion. They also control the internal environment, such as breathing rate and blood pressure, and prevent damage to body as in cuts and burns. These help to maintain constant conditions, in other word they are involved in homeostasis.

The sequences of changes that occur during a spinal reflex are:

 – A sensory receptor receives a stimulus and impulse is generated in it 

– The impulse is transmitted along a sensory neuron towards the spinal cord via the dorsal root 

– Once the impulse reaches the grey matter inside the spinal cord, it is passed on to the relay neuron across a synapse 

– The relay neuron then transfers the impulse to a motor neuron across another synapse. 

– The motor neuron conveys the impulse to an effector such as a muscle where a response takes place. 

The pathway that is followed by an impulse along the sensory neurons relay and motor neurone, during a reflex action is called reflex arc.

The components of reflex arc are:

– Stimulus 

– Receptors 

– The sensory receptor that detects the stimulus  

– The sensory (or afferent) neurone along which the sensory impulse is transmitted; 

– The relay neurone in the central nervous system to which the sensory impulse is passed on. 

– The motor (or efferent) neurone along which the motor impulse is transmitted; and 

– The effector (Muscle or gland) which the motor impulse triggers to bring about an appropriate response.

 – CNS (Brain or spinal cord)

9.6.2 Conditioned reflex actions 

This type of reflex involves the brain but it is also as fast as the simple reflex. Salivation on smelling one’s favourite food is an example of conditional reflex. The individual recognizes and based on the previous experience, the response (salivation) occurs. The recognition of the previous experience involves the association centres of the brain. 

A series of experiments were conducted by   Ivan PAVLOV, a Russian biologist who demonstrated conditioned reflex. He found that when a bell rung every time a dog was given food, the dog showed salivation only at the sound of the bell. The ringing of the bell is called stimulus. The dog had, thus, learnt to associate the sound of the bell to the food and this made it salivate at the sound of the bell.