The Nervous System

Life SciencesMatric Syllabus

Mind Action Series: Life Sciences Textbook and Workbook

Module 2: Life Processes

2.2a) The Nervous System

• Co-ordination is the way in which information is communicated between the receptors of the body, the central nervous system and the effectors to bring about appropriate responses to any change in the environment.

• There are essentially two co-ordinating systems – the nervous system and the endocrine system, which work together to respond to external changes and to control conditions inside the body.

• Co-ordination is brought about in two ways: – The fastest method is by means of impulses travelling along nerves and is

called nervous co-ordination. – The slower method is by means of chemicals called hormones which are

carried in the blood. This is known as chemical co-ordination.


• The nervous system is the body’s control and communication centre.

• It provides a continuous flow of information between the brain and the parts of the body.

• This enables the body to function in an orderly and effective way and to perform everyday tasks such as eating, writing, running, solving a problem and controlling emotions such as fear.


The nervous system has three overall functions:– Sensory function – sensory receptors detect the changes in the

external environment and the environment within the organism.– Integrative function – the central nervous system integrates this

information.– Motor function – effectors (muscles and glands) bring about a

response.

For example: you are riding a bicycle and see that the traffic light has turned red (sensory function). Your central nervous system integrates this information (red light means ‘stop’ – integrative function), and you use your muscles to apply your brakes to stop the bicycle (motor function).


• We live in an environment where external factors are constantly changing. Inside our bodies, conditions are also changing, e.g. changes in blood pressure, tension in muscles, blood sugar levels etc.

• These internal and external changes are called stimuli.• A stimulus is any physical or chemical change in our internal

or external environment that is capable of causing a response in an organism.


stimulus

Receptors respond to stimuli

CNS processes and integrates the sensory input

sensory

motor

Effectors e.g. muscles and glands make the correct response

response

Sensory Function

Integrative Function

Motor Function


The nervous system is made up of millions of special cells called neurons that form a continuous network all over the body. •Neurons can function for over a lifetime, given the correct conditions. •They cannot undergo mitosis, and thus cannot be replaced if destroyed.•They have a very high metabolic rate and need continuous supplies of oxygen and glucose to survive.In addition to neurons, there are supporting cells called neuroglia which bind neurons together, giving them support and helping them with their nutrition.


• A neuron has the same basic parts as most animal cells: they have a cell body, containing a cell membrane, cytoplasm, nucleus and cell organelles.

• In addition, neurons have long, thin fibres of cytoplasm extending from the cell body which make it possible for nerve impulses to be carried long distances.

• The fibres may be:– Dendrites, which carry impulses towards the cell body.– An axon, which carries impulses away from the cell body.


• Neurons can be classified according to their structure or functions.

• Functionally, neurons are classified according to the direction in which nerve impulses travel:– Sensory neurons– Connector neurons– Motor neurons

• Structurally, they are divided according to the number of fibres stretching out from the cell body:– Multipolar neurons– Bipolar neurons– Unipolar neurons


• These carry impulses from receptors towards the central nervous system (CNS), where the sensation is interpreted. They are afferent neurons, i.e. they take the sensation TOWARDS the CNS.

• They are usually unipolar, i.e. they have one fibre attached to the cell body, which then divides.


• They are also known as relay neurons, association neurons or interneurons.

• They are multipolar neurons with many dendrites.

• They are found in the CNS and they make up 99% of the neurons in the body.

• They receive impulses from sensory neurons and pass them on to other neurons.


• Motor neurons carry impulses away from the CNS to effectors (such as muscles) to bring about the appropriate response.

• They are therefore efferent neurons. ‘Efferent’ means away from.

• They are multipolar neurons, and are located in the grey matter in the CNS.


Functions of parts:Dendrites collect information from other cells and pass it on to the cell body.The cell body controls the metabolism of the cell.The node of Ranvier allows rapid conduction of impulses by forcing them to ‘jump’ from one node to the next.Schwann cells are wrapped around the axon several times. Their inner layers of fatty tissue fuse to form the myelin sheath.The axon is extended to carry impulses away from the cell body over long distances.The myelin sheath forms electrical insulation around the axon, preventing distortion of impulses from activity in neighbouring cells.Terminal branches carry impulses to the synaptic knobs/end plates.Synaptic knobs form a synapse with another neuron, a muscle cell or a gland cell. 2.2a) The Nervous System

• Adjacent neurons do not touch each other, there is a small gap separating them. This gap or synaptic cleft plus the adjacent membranes, form a synapse.

• A synapse is thus the junction across which a nerve impulse passes from an axon terminal to a neuron, a muscle cell or a gland cell.

• In the cytoplasm of the synaptic knobs there are hundreds of small vesicles (sacs) filled with molecules of a neurotransmitter such as the chemical acetylcholine.


As neurons do not touch, impulses have to cross the gap (synapse) to continue their path through the nervous system.•When nerve impulses reach the terminal branches of axons, they cause vesicles in the synaptic knob to fuse with the presynaptic membrane, using energy from the mitochondria.•The membranes then burst, releasing neurotransmitters which diffuse across the synaptic cleft.•The neurotransmitters become attached to neuroreceptors in the post-synaptic membrane, which they either excite or inhibit.


• The receptors release the neurotransmitters back to the synaptic cleft which can then be inactivated by enzymes and reabsorbed by the pre-synaptic membrane. This is very important as, unless the transmitter is removed from the synaptic cleft, subsequent impulses would have no effect as the receptors on the postsynaptic membrane would all be bound.

In this way, impulses are carried across the gap chemically, i.e. by way of a chemical neurotransmitter. An impulse carried along a nerve fibre is an electric impulse.

Synapses are important because: 1) they make sure that the flow of impulses travel in one direction only, 2) they allow the dispersal of impulses, 3) they allow unnecessary and continual stimuli to be filtered out.


The human nervous system is divide into the:•Central Nervous System• Brain• Spinal cord

•Peripheral Nervous System• Cranial nerves• Spinal nerves


• The CNS is made up of a concentrated mass of interconnected neurons, grouped together to form the brain and spinal cord.

• The CNS tissues are protected and nourished by three connective tissue membranes called meninges. The space between the 2nd and 3rd layer of membranes is filled with cerebro-spinal fluid which acts as a cushion, protecting the CNS against shock and damage.


• The whole CNS and its meninges are enclosed within a bony case made up of two separate structures.– The cranium is a strong, dome-shaped structure that protects the

brain against mechanical injuries. It is made up of eight, curved skull bones, immovably dove-tailed together by fibrous joints called sutures.

– The vertebral column is made up of thirty-three, irregular shaped vertebrae that surround the spinal cord and protect it against mechanical injury.


• The brain is the enlarged, upper part of the spinal cord. It is shaped like a large mushroom. The cap of the mushroom is the cerebrum, and the stalk is the brain stem.


• The large cerebrum is divided into two cerebral hemispheres by a longitudinal fissure. The hemispheres are held together by a mass of white tissue called the corpus callosum that provides a communication link between the cerebral hemispheres.

• Its surface is made up of a large number of folds and grooves that enlarge the surface area so that a large number of brain cells can fit into a small cranial cavity.

• There are four cavities known as ventricles in the centre of the brain. They are filled with cerebro-spinal fluid.

• The outer 3mm of the cerebrum forms the cerebral cortex, made up of grey matter. The grey matter is made up of a collection of cell bodies of neurons. Below the grey matter is an area of white matter, made up of a collection of myelinated nerve fibres.


Functions of the cerebrumThe cerebral cortex is divided into three functional areas:•The motor area in the frontal lobe is where all the voluntary movements of the body originate.•The sensory areas receive and interpret impulses from the sense organs, e.g:

– Hearing, tasting and smelling in the temporal lobe– Sight in the occipital lobe– Skin sensations in the parietal lobe, e.g. touch, cold and hot

temperatures and pain.

•The association cortex is not involved with motor or sensory functions and is involved in higher mental activities such as intelligence, memory, perception, language and consciousness.



The cerebellum forms the upper part of the brain stem. It is made up of two hemispheres and has shallow surface folds.FunctionsThe cerebellum receives sensory input from the motor region of the cerebrum, the eyes, muscle spindles & organs of balance in inner ears.•It processes this information and uses it to co-ordinate the actions of the voluntary muscles so that complicated physical actions can be performed in a smooth, controlled way.•It is partly responsible for controlling muscle tone (correct tension in the muscles).•Using information from the inner ear, it helps maintain posture and balance.


The hypothalamus is situated above the pituitary.Functions•Controls homeostasis, e.g. temperature, water levels, pH levels, food intake, etc.•Controls and drives certain behaviours, e.g. aggression, self-defence and reproductive behaviours.•Controls functioning of the pituitary gland.


Forms the lowest part of the brain stem.When it leaves through the foramen magnum at the base of the cranium, it is known as the spinal cord.Functions•Serves as the pathway for impulses to and from the brain.•Acts as a reflex centre, controlling important reflexes such as breathing, heartbeat, blood pressure, swallowing and peristalsis.•Also controls less important reflexes such as sneezing, coughing, hiccupping and salivating.


• The spinal cord is an elongated rod of nervous tissue that extends from the medulla oblongata through the foramen magnum.

• It is situated in the vertebral canal of the vertebral column and is approximately 45cm long.


• A spinal cord has an H-shaped central area of grey matter made up of nerve cell bodies, dendrites and synapses.

• White matter surrounds the grey matter and is made up of axons with myelin sheaths.

• The central canal occurs in the centre of the grey matter. It is filled with cerebro-spinal fluid. The central canal runs down from the ventricles in the brain.


• The cord is partly divided into right and left sides by two grooves: dorsal groove and ventral groove.

• A pair of spinal nerves enters and leaves the spinal cord between successive vertebrae. Each spinal nerve has a dorsal root that enters the grey matter, and a ventral root that leaves the grey matter.

• A collection of neuron cell bodies forms a ganglion in the dorsal root. 2.2a) The Nervous System

The PNS consists of all the nervous tissue outside the CNS. It is divided into two main divisions:•Somatic nervous system that controls all voluntary muscular movements.•Autonomic nervous system, that controls the functioning of involuntary muscles and glands.The PNS is made up of nerves that link the CNS to the receptors and effectors of the body, i.e. 43 pairs of nerves that include:•12 pairs of cranial nerves connected to the brain. These may be afferent (sensory) nerves or efferent (motor) nerves or mixed nerves made up of both sensory and motor nerves. •31 pairs of spinal nerves which enter and leave spinal cord between the vertebrae (all mixed nerves).


Functions of the PNS:•The PNS collects information from receptors and transmits this information by way of impulses along sensory neurons to the CNS – sensory function.•It transmits impulses from the CNS by way of motor neurons to effectors to bring about an appropriate response – motor function.


This system involves receiving information from receptors and bringing about appropriate responses by voluntary, skeletal muscles.


A reflex action is a fast, automatic response by an effector organ, i.e. a muscle (whether voluntary or involuntary) or a gland, to a stimulus received by a receptor organ.Reflex centres may be situated in the:•Brain – controlling reflexes such as sneezing, breathing, blinking etc.•Spinal cord – controlling primitive reflexes such as the knee jerk when tapped below the knee-cap.•A reflex arc is the pathway along which nerve impulses are carried from a receptor to an effector to bring about a reflex action.•A reflex arc is thus the functional unit of the nervous system, while a neuron is the structural unit of the nervous system.


Consider what happens when you prick your finger.•The stimulus of the pin prick generates impulses in sensory nerve endings of pain in the skin of your finger – the receptor.•These impulses are conducted along sensory neurons to the grey matter in the spinal cord. Here the impulses synapse with connector neurons which serve as reflex centres.•From the connector neurons, impulses make synaptic contact with the motor neurons in the grey matter and travel along these neurons to the effectors.•The effector, the flexor muscle in the finger, brings about a response by contracting very quickly to pull the finger away from the pin to protect the finger.•Impulses are also carried up the spinal cord to the cerebral cortex (parietal lobe) where the stimulus is interpreted as pain.


• The ANS is involved in keeping a balanced internal environment i.e. maintaining homeostasis. It responds to sensory input from all body organs and makes the necessary adjustments.

• The effectors in the ANS are smooth, involuntary muscle, cardiac muscles and glands.

• There are two divisions in the ANS:– Sympathetic nervous system – gets you ‘ready for action’– Parasympathetic nervous system – calms you down


Body part Sympathetic system Parasympathetic system

Heart beat Strengthens and accelerates Weakens, slows down

Blood vessels of skin and digestive organs

Constricts arteries and raises blood pressure

Dilates arteries, lowers blood pressure

Blood vessels of heart, lungs, skeletal muscles

Dilates arteries, allowing the flow of more blood – can run faster

Constricts arteries, reduces flow of blood

Digestive tract Slows down peristalsis Speeds up peristalsis

Urinary bladder Relaxes bladder wall Contracts bladder wall

Bladder and anal sphincters Contracts Relaxes

Muscles in bronchial tubes Muscles relax, dilating air passages for easier breathing

Muscles contract, causing air passages to constrict


Body part Sympathetic system Parasympathetic system

Muscles of iris Radial muscles contract, dilating pupil – see better

Circular iris muscles contract, constricting pupil

Sweat glands Increases sweat to cool one down No effect

Liver Stimulates the breakdown of glycogen into glucose – use for energy

No effect

Salivary glands Decreases secretion of saliva Increases secretion of saliva

Tear glands Little effect Increases secretion of tears

Adrenal medulla Stimulates secretion of adrenalin No effect

Kidney Decreased excretion of urine increased excretion of urine

Brain Increased concentration No effect2.2a) The Nervous System

• The sympathetic nervous system works together with adrenalin in emergency situations that cause stress and require the ‘fight or flight’ state. Stimulation by these nerves results in more oxygen, glucose and blood being sent to the skeletal muscles, speeding up cellular respiration to release more energy for action.

• The parasympathetic nervous system enables the body to ‘rest and digest’, i.e. to recover from sympathetic stimulation and return to normal.

• Most organs are innervated by both the sympathetic and parasympathetic fibres (double innervation).

• Effects produced by each system are generally antagonistic.


Alzheimer’s disease is a progressive and degenerative disease of the brain, which causes the deterioration of memory and thinking skills. CausesAlthough the cause is not fully understood, it is known that Alzheimer’s disease develops because of a complex series of events that happen in the brain over a long period of time. Genetic, environmental and lifestyle factors may be at fault.PrevalenceAlzheimer’s is prevalent in older people and affects both genders. 15% of people over the age of 65, and 50% of people over the age of 85 develop Alzheimer’s.


SymptomsPeople with Alzheimer’s show symptoms of memory loss and changes in their thinking skills. Problems may include:

– Getting lost– Taking a long time to do daily tasks– Mood and personality changes– Poor judgement– In some cases, hallucinations.

In severe cases, the people affected by Alzheimer’s cannot communicate properly and are dependent on others to care for them. They may be in bed for most of the time as their body stops functioning.


TreatmentThere is no cure for Alzheimer’s. Some medicines, however, can help maintain thinking skills and can control or improve certain kinds of behaviour for a short period of time. These medicines, however, cannot delay the progression of the disease.Certain factors may reduce the risk of developing Alzheimer’s:

– Regular exercise– Healthy diet– Limiting alcohol consumption– Avoiding tobacco– Staying socially active– Engaging in intellectually stimulating activities


Co-ordination = the way in which information is communicated between the receptors of the body, the central nervous system and the effectors to bring about appropriate responses to any change in the environment.Nervous system = the body’s control and communication centre. Synapse = the junction across which a nerve impulse passes from an axon terminal to a neuron, a muscle cell or a gland cell.Neurotransmitter = a chemical substance released from axon terminals at a synapse to either stimulate or inhibit the post-synaptic membrane.Stimulus = any physical or chemical change in our internal or external environment that is capable of causing a response in an organism.


Cerebro-spinal fluid = a fluid formed from blood plasma in special areas in the walls of the ventricles. It protects and nourishes the CNS and prevents cells from drying out.


The Nervous System

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