Coordination in Humans Summarized Biology Notes

Learning Objectives

● identity motor, relay and sensory neurones from diagrams
● describe effectors in terms of muscles and glands
● define a synapse as a junction between neurones or nerves and muscle
● describe how nerve impulses are transmitted across the synapse
● describe the effects of alcohol on the release of the neurotransmitter
● distinguish between voluntary and involuntary actions
● describe a simple reflex arc in terms of sensory, relay and motor neurones
● define a reflex action as a means of integrating and coordinating stimuli with responses of effectors
● define sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals

Coordination in humans

There are two systems of coordination in your body, namely the endocrine (hormonal) system and the nervous system. The endocrine system is made of receptors, endocrine glands, and hormones and effectors. The nervous system is  made of receptors, neurones (nerves), the spinal cord, brain and effectors. Basically, both the endocrine system and nervous system coordinate the body, but in different ways. You will learn about the differences between the hormonal system and nervous system later.

 The human nervous system

Your nervous system is made up of two main parts.
● the central nervous system
● the peripheral nervous system.
The central nervous system, or CNS, is made up of:
● the brain, which controls the whole nervous system, receiving messages from, and sending messages to, all parts of the body
● the spinal cord, which consists of a great number of nerve cells grouped together.
The peripheral nervous system is made up of:
● Cranialnerves, which are connected to the brain and carry messages directly to and from the brain 
● spinal nerves, which are connected to the spinal cord and carry messages from the receptors to the spinal cord and brain, and from the spinal cord and brain to the effectors.

Nerve activity begins when the nervous system detects a stimulus. The specialised parts of the body that carry out this function are called receptors. When a receptor detectsa stimulus, it sends a message to the brain or spinal cord. The brain or spinal cord receives the message, and decides which effectors need to react to the stimulus. The brain or spinal cord then sends the messages on, along the appropriate nerve fibres, to the appropriate effector, which is usually a muscle or gland.

Note it, Receptors are sense organs, such as the eye and ear, which detect stimuli. Efectors are muscles and glands. These carry out the response to a stimuli. We will look at both of these later in this unit.

Note it, The job of the CNS is to coordinate the messages travelling through the nervous system

Neurones

Neurones are special cells that are adapted to conduct messages in the form of electrical signals, called nerve impulses, through the nervous system. Each neurone has a nucleus the bulk of cytoplasm from which fine cytoplasmic nerve tibres run, and a cell membrane. Most fibres are very long. These fibres are specialised for the transmission of impulses. The nervous coordination is extremely tast and responses are virtually immediate. In mammals, impulses are transmitted along the fibre at speeds between 30 and 120 metres per second.

Different types of neurones

To understand how a reflex action works, you need to look at three different types of neurones:
● sensory neurones that transmit impulses from the receptors to the CNS
● motor neurones that transmit impulses from the CNS to the effectors, such as muscles and glands
● interneurones, also known as relay neurones, that have many, short fibres, and transmit impulses within the CNS.

Neurones are surrounded by different supporting cells, called neuroglia cells, such as Schwann cells. Dendrons and axons are protected by Schwann cells, which wrap themselves forminga myelin sheath. Between each pair of Schwann cell is a junction in the myelin sheath called a node of Ranvier. The myelin sheath and its junction help to increase the speed at which impulses are transmitted.

Note it, A nerve impulse is an electrical signal that travels along the neurone.

look at Figures above to identity the differences between a sensory and motor neurone. Sensory neurones have a single long dendron, which transmits impulses towards the cell body, and a single axon, which transmits impulses away from the receptors to the CNS. Motor neurones have many fine dendrites, which transmit impulses towards the cell body from the CNS, and a single long axon, which transmits impulses away from the cell body to the effectors, such as muscles and glands.

Synapses and Neurotransmitters

A Synapse is a junction between two neurones or between a neurone and an effector. Communicating neurones do not touch each other; there is a small gap between them called a synaptic gap. Nerve impulses are transmitted from one neurone to another, or from a neurone to an ettector, across this junction or gap. A nerve impulse cannot jump across this gap, so a chemical substance is released, which is called neurotransmitters. The impulse can only be transmitted in one direction.

The neurone on one side of the synapse is described as the presynaptic neurone. The neurone on the other side is the post-synaptic neurone. The presynaptic neurone Communicates with the post-synaptic neurone by releasing a chemical messenger, a neurotransmitter substance, such as acetylcholine, across the synapse. The neurotransmitter Is stored in tiny vesicles at the end of the neurone. Figure below shows a synapse between one axon terminal of one neurone and part of the membrane of a dendrite of another neurone.

The effect of alcohol on the release of the neurotransmitter

Alcohol affects brain chemistry by altering levels of neurotransmitters. Neurotransmitters are chemical messengers that transmit the signals throughout the body that control thought processes, behaviour and emotion. Neurotransmitters are either excitatory, meaning that they stimulate brain electrical activity, or inhibitory, meaning that they decrease brain electrical activity. Alcohol increases the effects of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the brain. GABA causes the sluggish movements and slurred speech that often occur in alcoholics. At the same time, alcohol inhibits the excitatory neurotransmitter glutamate. Suppressing this stimulant results in a similar type of physiological slowdown. In addition to increasing the GABA and decreasing the glutamate in the brain, alcohol increases the amount of the chemical dopamine in the brain's reward centre, which creates the feeling of pleasure that occurs when someone takes a drink.

Voluntary and Involuntary actions

Many of the actions associated with normal activities, such as walking and breathing, are automatic and unconscious. Other examples are coughing, blinking and sneezing. We call these involuntary actions. Some actions can be done deliberately and consciousily. These are voluntary actions. Here the brain has control over the reflex action. As well as nerve impulses going through the route taken by a simple reflex arc, other neurones pass to the brain to check out whether or not a simple reflex action is appropriate. If it is not appropriate, the response will be modified. Decisions, such as whether to pick up a piece of paper, go to the shop or go to the toilet, are examples of voluntary actions. The reflex action shown in Figure which will come in figure titled reflex arc is a very simple one. One receptor is receiving one stimulus and one impulse is being sent to the spinal cord. In reality, though, many receptors are constantly sending many impulses to your brain and spinal cord. Your brain and spinal cord coordinate the information coming in to them. They then coordinate your actions by sending impulses to appropriate effectors. For example, imagine you have picked up a hot plate. Before you picked it up, your eyes send impulses to your brain telling it that your favourite meal was on the plate.
Other impulses told your brain that you were hungry. So, when the impulse from your pain receptors arrived in your central nervous system, there were already other messaqes saying food' and 'hungry'. The central nervous system weighs up all this information. Instead of sending impulses to your muscles telling them to contract quickly, it might tell different muscles to contract carefully, putting the plate down gently, even if you do burn your fingers.

Antagonistic muscles

Have you wondered wnat creates movements of your body, such as walking, bending your arm, even waving to the friend passing by? Antagonistic muscles Create movements, wnen one contracts, the other relaxes. The most well-known antagonistic muscle pair is the biceps and triceps of the arm. The bi and the tri for short, they are what cause movement of the arm. They a work simultaneously to bend or straighten the arm. The biceps is located in front of the humerus bone of the upper arm. The biceps is joined to the radius bone of the lower arm and the triceps is joined to the ulna bone of the lower arm. Muscles are attached to bones by strong fibres called tendons. When you want to bend your arm, the brain sends two electrical impulses, one to the bicep making it contract and one to the tricep telling it to relax. The biceps can be called a flexor because it flexes (bends) the arm. The triceps can be called an extensor because it extends (straightens) the arm. When the bicep contracts, it becomes shorter, pulling the bones to which it is attached close and bending the arm. This causes the fibres of the tricep to stretch while they are relaxed. To straighten your arm, the brain sends electrical impulses to both muscles, making the bicep relax in order to leave the muscle it is attached to free. The tricep contracts and becomes shorter, pulling the muscle it is attached to into place and straightening the arm. To bend your arm, your biceps muscle has to contract. The biceps is attached your shoulder blade (the scapula) at one end, and to the radius bone in your at  one end elbow as the biceps muscle pulls your lower arm upwards. When your biceps contracts, your arm bends at the elbow as the biceps muscle pulls your lower arm upwards.

Note it; About 30 different neurotransmitters have been identified, among them are nor-adrenaline and acetylcholine. Each neurone releases one kind of transmitter

To straighten your arm, the triceps muscle contracts. One end of your triceps muscle is attached to your shoulder blade and the humerus bone in your upper arm, and the other end is attached to the ulna bone in your lower arm. AS the triceps muscle contracts it pulls your lower arm down. These two muscles are antagonistic muscles. By working together, they can make the bones move in both directions. When one of the muscles is contracted, the other one automatically relaxes. Impulses from receptor cells are passed to your brain, which coordinates your responses to stimuli around you. Impulses are sent along neurones to effectors to ensure that the correct responses take place. Muscles and glands are effectors. They carry out a response. For example: When your hand touches a hot plate, the response is for your hand to move rapidly away trom the plate. The effector in this reflex arc is the biceps muscle in the upper arm. Another example: Immediately after a meal your blood glucose level rises and the pancreas beta cells secrete insulin, which is transported in the bloodstream for the liver to stimulate the liver cells to convert excess glucose into glycogen. In this case, the liver is the eftector.

 Reflex Arc

A reflex is a fast, automatic response to a stimulus. A reflex arc is the pathway a nerve impulse travels along to bring about a reflex action. A retlex arc allows a rapid response to a stimulus. Simple reflex arcs involve sensory, relay and motor neurones. The knee-jerk reflex is a good example of a simple reflex arc. Figure below shows the sequence of events in the knee-jerk reflex.

1. The stimulus is the tap on the tendon. 2.Receptor cells in the thigh muscle detect the stimulus.
3. An electrical impulse is carried along the sensory neurone to the spinal cord.
4. In the spinal cord, the impulse travels from the sensory neurone to the relay neurone and then to the motor neurone.
5. The motor neurone carries the impulse from the spinal cord to the effector, which is the thigh muscle in the leg.
6. The thigh muscle responds by Contracting and this causes the lower leg to jerk up.

Most of the pathways that nerve impulses travel along are more complex than this and involve many more neurones. Relay neurones pass the impulses to other pathways, including the brain, so you become aware of what is happening.

Reflex actions

What is a reflex action? A reflex action is a means of integrating and coordinating stimuli With responses of effectors. The following activity should help you to understand what a reflex action is.

Practical investigation:The knee-jerk response

Spend about 5 to 10 minutes on this activity. You will need someone to help you do this activity.
1. Sit on the edge of a table, with both legs hanging free as shown below.
2. Just below your kneecap is a tendon that connects the muscle in your thigh to your kneecap. You can see this below. Ask your friend to give your tendon a sharp tap with the edge of a ruler, or the edge of his or her hand. The diagram below shows how this should be done.

3. Your lower leg should jerk up. Your friend may need to repeat this it you do not respond the first time.
4. Now repeat the investigation, but try to stop your knee from jerking. Could you stop your lower leg from moving up?

Note it, Although the knee--jerk is a reflex action, you know your knee has been hit! Reflex actions are very useful. The message gets trom the receptor to the effector as quickly as possible. You do not waste any time thinking about what to do!

The knee-jerk response and the pupil reflex are both examples of reflex actions. Each reflex follows the same pattern, giving the same response. Reflex actions are:
● simple responses to stimuli
● rapid, automatic and involuntany responses
● present at birth and not learnt.
Doctors test the reflexes of newborn babies to check, for example, that they have no damage to their brain.

Sense Organs and The Eye

Sense organs are groups of receptor cells responding to specitic stimuli. These stimuli can be light, sound, temperature and chemicals. Receptors send intormation about these changes to the CNS as nerve impulses. The flow chart summarises the links between stimuli in your environment and your body. Each type of receptor is sensitive to a specific stimulus. For example, temperature receptors only respond to changes in temperature
● Receptors in humans can be simple nerve endings of sensory neurones, for example, temperature receptors and touch receptors in your skin
● Groups of receptor cells combined with other tissues to form complex sense organs, for example, the eyes, ears, nose and mouth.

Stimuli --->Receptors ---> nervous and hormonal systems ---> Effectors

Summary

● The nervous system consists of the central nervous system and the peripheral nervous system.
● The central nervous system consists of the brain and spinal cord.
● The peripheral nervous system consists of the
receptors and neurones.
● Synapses are functional junctions between neurones. They integrate incoming signals from many neurones. Neurotransmitters are special chemicals that transmit nerve impulses across synapses.
● Voluntary actions are deliberate and conscious. Involuntary actions are automatic and unconscious.
● The nervous system provides a link between
receptors and effectors.
● Nerves carry impulses to and from the brain and spinal cord.
● Sense organs respond to stimuli.

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