3.5.3 Responses in the Human Nervous System

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Humans use two systems to respond to stimuli: the nervous system for fast action and the hormonal system for slower

responses.

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• A stimulus is any change in your environment

• e.g. a flash of light, a noise, a fly landing on your nose.

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Nervous Endocrine

Electro/chemical messages

Chemical messages

Carried in nerves Carried in blood

Fast acting Slow acting

Short-term effect Long-term effect

Single target Many targets

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Nervous system

• Central NS (Brain & Spinal Cord)

• Peripheral NS (Cranial & spinal nerves)

– Somatic NS (voluntary control of external environment)

– Autonomic NS (involuntary control of external environment)

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Central Nervous System

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The Nervous System

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Functions:

• Detect changes in external and internal environment.

• Interpret these changes and respond in a coordinated manner.

• Store information gained by experience.

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• The CNS processes messages and controls our responses.

• The PNS carries messages to and from the CNS.

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• Receptors are cells sensitive to a stimulus e.g. pain receptors, eye.

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• An effector responds to a stimulus e.g. a muscle contracts or a gland

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Nerve

• A nerve (nerve fibre) is a bundle of neurons (nerve cells) - both sensory and motor neurons.

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There are three types of Neuron :

Motor neuron –carries impulses from the CNS to muscles and glands.

Interneuron –connects sensory and motor neurons and so carries messages within the CNS.

Sensory neuron –carries messages from the sense organ to the central nervous system (CNS).

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Dendron is a short fibre that receives information and carries it towards the cell body.

Dendrites small branches of a dendron.

Cell body

Axon very long fibre that conducts impulses away from the cell body.

Myelin sheath insulates the neuron from electrical impulses flowing in other neurons.

Schwann cell wrap their fatty cell membranes around an axon

Nodes of Ranvier gaps in myelin sheath which speed up the message – electrical impulse ‘jumps’ from node to node.

Neurotransmitter swellings (synaptic knobs)

release chemicals that carry the impulse from one nerve to another.

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Sensory Neuron

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Motor neuron

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Inter Neuron

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NS ----- Co-ordination & Response

An impulse is an electrical message that is carried along aneuron.

A neuron is a specialised cell that carries electrical messages(impulses) around the body.

A stimulus is any change in your environment

A receptor is a nerve cell that detects the stimulus

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Movement of impulse

• Threshold: This is the minimum stimulus required to start a nervous impulse.

• All or Nothing Law: The size of the stimulus (provided it is above the threshold level) has no effect on the size of the impulse.

• Either a full message is carried or no message.

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Refractory period

• Is a short timespan after a neuron has carried an impulse during which a stimulus fails to cause a response.

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Synapse

• A region where two neurons come into close contact

• Usually found between the axon terminals of one neuron and the dendrites of another

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Synaptic cleft is the tiny gap between the two neurons at a

synapse

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• Electrical impulses cannot simply cross a synapse

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1. Activation of Neurotransmitter• Ions stimulate the neurotransmitter swellings to release

a chemical substance called neurotransmitters, that diffuse rapidly across the synaptic cleft.

• Neurotransmitters are contained in vesicles in the neurotransmitter swellings.

• Once released across the synapse, it then combines with receptors on the other neuron and causes the electrical impulse to be regenerated.

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2. Inactivation of neurotransmitter

• The neurotransmitter is then broken down by enzymes.

• They are reabsorbed in the neurotransmitter swellings and allows them to be recycled.

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Diagram

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Synaptic cleft—the gap between two neurons, bridged by chemicals (neurotransmitters).

Neurotransmitter—chemical released across a synaptic cleft tocarry a signal from one neuron to another.

The chemical is then destroyed or removed

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Benefits?• Allows transit of Impulses.• Permit impulses in one direction only – neurotransmitters only

present on one side of the synapse.• Allow localisation of a response rather than a total body response

(chaos!).• Protect against over-stimulation, as they will slow down if

overloaded.• Their complicated interconnections allow for learing and memory.• They ignore low-level stimulations – effectively removing

‘background noise’ from nervous system.

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Disadvantages

• Synapses are relatively slow and their number is often minimised by developing long axons and dendrons.

• Allows chemicals to affect N.S. e.g. hallucinatory drugs, painkillers, anaesthetics and certain poisons.

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Effect of Neurotransmitters• Acetycholine released from motor neurons, triggers muscle

contraction. It has an inhibitory effect on cardiac muscle, resulting in a decreased heart rate.

• Noradrenaline, serotonin and dopamine affect mood. Their imbalance has been linked to depression, attention deficit disorder (ADD) and psychosis (where behaviour and personality are altered e.g. schizophrenia). Antidepressants and other mood-affecting drugs work by altering the levels of these neurotransmitters in the brain.

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Drugs - many affect transmission of impulses across synapse by

increasing/decreasing the production of the

neurotransmitter or by affecting the rate of breakdown of the

neurotransmitter

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• Ectasy affects nerve cells that produce serotonin. It causes the nerve cells to release all the stored serotonin at once – this can cause damage to the axons. Serotonin regulates temp. as well as mood. If body temp. reaches 430C (dancing) the blood starts to coagulate, and death can follow. Ectasy affects memory too.

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• Cannabis. Marijuana – a hallucinogen – (from the dried leaves) and hashish (resin from the flowers). In low doses it is a depressant – impairs co-ordination, perception, timing and short-term memory. It slows down motor activity and causes mild euphoria. It also causes disorientation, increased anxiety (panic), delusions (paranoia) and hallucinations. Over time, marijuana can suppress the immune system, impair mental functions and lower sperm and testosterone levels.

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• Cocaine, interferes with the normal breakdown of dopamine. Dopamine is involved with pleasurable feelings. If it is not broken down the synapse keeps on transmitting messages and euphoria follows. The body reduces its production of dopamine which results in addiction as the user has to take more cocaine to produce enough dopamine to feel ‘normal’. Body becomes tolerant to cocaine.

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The brain is composed of over 100 billion neurons, eachreceiving messages simultaneously from thousands of otherneurons.

The brain is protected by the skull bones, meninges (three membranes) and cerebrospinal fluid.

The Brain

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The cerebrum is our conscious brain, with different parts havingdifferent jobs to do.

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Conscious actions

Stimulus Receptor Sensory Neuron Coordinator

Motor Neuron Effector Response

1) Receptors in your skin detect a stimulus

3) Here another sensory neuron carries the signal to the brain

4) The brain decides to move away the hand

5) This impulse is sent by MOTOR NEURONS to the hand muscles (the effectors) via the spinal chord…

2) The impulse is carried by SENSORY NEURONS to the spinal chord

6) Which then moves the hand away

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The cerebellum co-ordinates processes that we have learned to do automatically, such as speaking.

The medulla oblongata co-ordinates involuntary, automatic processes — such as breathing, heartbeat.

cerebellumMedulla oblongata

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The pituitary ‘[master] gland secretes hormones that stimulateother glands to release their hormones.

The hypothalmus is the centre for the regulation of the internal organs.

The spinal cord is well protected by the vertebrae, meninges (three membranes) and cerebrospinal fluid.

It transmits impulses to and from the brain and controls many reflex actions.

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A cross-section through the spinal cord shows a small central canal, filled with cerebrospinal fluid, surrounded by an area of grey matter, shaped somewhat like the letter H.

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Outside the grey matter, the spinal cord consists of white matter (containing axons only).

Grey matter contains cell bodies and dendrites (regions of a neuron that have no white myelin covering).

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In humans, 31 pairs of spinal nerves branch off from the spinal cord.

The dorsal root ganglion is a swelling that consists of the cell bodies of the sensory neurons.

Each spinal nerve has a dorsal root and a ventral root.

The dorsal root consists of nerve fibres carrying information into the spinal cord from the senses.

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The ventral root consists of nerve fibres carrying information out from the spinal cord, to the muscles and glands.

The cell bodies of the motor neurons are positioned within the grey matter of the cord.

The spinal cord transmits impulses to and from the brain and controls many reflex actions. 53

A reflex action is a quick, automatic response to a particular stimulus.

Interneuron

REFLEX ACTION --- The Reflex Arc

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Suppose you touch a hot flame.

In this brief instant, a message has been carried by a sensory neuron from pain receptors in the skin to the spinal cord.

Almost instantly you pull your hand away.

Interneuron

REFLEX ACTION --- The Reflex Arc

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In the spinal cord, the message is passed on to an interneuron and then to a motor neuron, and so into muscles that respond by contracting and pulling your hand from the flame.

Interneuron

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The response is called a reflex action, as it does not involve conscious control, and is predictable and automatic.

Many of the activities of the body, such as breathing and keeping our balance, are regulated by reflex actions.

This response saves the body from injury.

Interneuron

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Nervous System Disorder

It is caused by the brain reducing the normal amount of dopamine that it makes.

Parkinson’s disease is a nervous system disorder, normally seen in older people, in which muscles become rigid and movement is slow and difficult, with persistent tremors [shaking].

There is at present no means of preventing it, but giving L-dopa (which the body changes into dopamine) can relieve the symptoms in many patients.

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SENSE ORGANS

Animals have specialised senses to provide them with information about their environment.

The five senses are sight, hearing, touch, taste and smell.

A receptor is a cell that can detect a stimulus

A stimulus is any change in your environment, e.g. light, sound.

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Smell

Taste

Touch

Hearing

Sight

Sense

Smell

Taste

Touch

Hearing

Sight

Sense

Nose

Tongue

Skin

Ear

Eye

Organ

Smell

Taste

Touch

Hearing

Sight

Sense

Nose

Tongue

Skin

Ear

Eye

Organ

chemicals [receptors in the nasal cavity detect vapours]

chemicals [taste buds detect sweet, sour, salt and bitter].

touch, pressure, temperature and pain [receptors spread throughout body]

sound [receptors in cochlea]

light [by rods and cones in the retina]

Stimulus detected

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Eyelids can cover and protect the eyes.

The EYE

Conjunctiva — thin transparent lining protecting the cornea.

Conjunctiva

Eyelid

Cornea—front transparent part of the sclera. It focuses light rays on the retina.

Cornea

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Sclera—tough fibrous outer layer – the ‘white’ of the eye; it maintains the shape of the eyeball.

Choroid—contains blood vessels supplying food and oxygen to the cells of the eye.

Retina—the innermost layer that contains the receptor cells [rods and cones].

Sclera

Choroid

Retina

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The front region of the choroid is specialised into the iris

Iris—contains blood vessels and melanin [giving us our eye colour], and controls the amount of light entering the eye [through the pupil].

Iris

The fovea is where our best vision is [mainly cones] Fovea

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In bright light, pupil constricts.

In dim light, the pupil dilates.

Pupil

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Ciliary body [muscle] — thickened edge of the choroid that controls the shape of the lens

Lens—like a magnifying glass, it focuses the light rays on the retina.

Suspensory ligaments —hold the lens in place.

Ciliary muscle

Suspensory ligaments

Lens

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Lens—focuses the light rays on the retina.

Accommodation is the ability of the lens to change its shape (focal length) to form a clear image. 67

For close vision, the ciliary muscle contracts, the suspensory ligaments relax, the lens becomes thicker.

Close Vision

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When the eye is at rest, the lens is thin, has a long focal length and is adapted for seeing distant objects.

Distant Vision

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Accommodation is the ability of the lens to change its shape (focal length) to form a clear image.

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Seeing things at different distances

For distant objects, the ciliary muscle relaxes and so the

suspensory ligaments pull tight, pulling the

lens thinner – the light doesn’t bend as

much.

For close objects the ciliary muscle

contracts, allowing the lens to go fat, thus bending the light

more.

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Aqueous humour—watery liquid that supplies the lens and cornea with nutrients and helps keep the shape of the cornea and lens.

Vitreous humour—gel that helps maintain the shape of the eye.

Aqueous humour Vitreous humour

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When light rays focus on the retina, receptor cells are stimulated and impulses are carried along the optic nerve to the brain.

Blind spot—where the optic nerve fibres pass through the retina and there is no room for receptors.

Optic nerve

Blind Spot

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Long-sighted :You are long-sighted if you can clearly see objects a long way off, but you cannot see things close by.

Reading glasses [convex lenses] can correct the problem.

Eye Defects

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Short-sightedYou are short-sighted if you can clearly see objects close to you, but you cannot see things in the distance.

Glasses with concave lenses can correct the problem.

Eye Defects

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Pinna—outer visible ear, funnels sound into the ear canal.

The EAR

Ear canal —tube leading to the ear drum. It has hairs and wax glands to trap dirt and germs.

Eardrum—membrane of skin that vibrates when sound waves hit it.

EardrumPinna

Ear Canal

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Middle ear—air-filled cavity containing three small bones [ossicles] and the Eustachian tube

Eustachian tube—keeps air pressure equal on each side of the eardrum.

It opens when we swallow, cough, etc.

Ossicles— 3 small bones [hammer, anvil and stirrup], that amplify the sound.

Middle Ear

Ossicles

Eustachian tube

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Inner ear—contains a coiled, fluid-filled tube called the cochlea and the semi-circular canals.

Semi-circular canals—help us keep our balance and posture.

Cochlea—contains nerves that convert sound vibrations into electrical impulses.

Inner Ear

Cochlea

Semi-circular canals

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In turn, this vibrates the hammer, anvil and stirrup bones, which amplify the sound.

The pinna (ear lobe) channels the sound (vibrations in the air) towards the eardrum, which then vibrates.

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The brain interprets these as sounds, and we ‘hear’.

The stirrup pushes on the oval window of the cochlea, moving the liquid inside.

Special hairs on 30,000 receptor cells detect the movement and send signals to the brain along the auditory nerve.

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Semi-circular canals—help us keep our balance and posture.

The three semicircular canals are curved tubes, each about 15mm long and filled with fluid. 82

Head movements are detected by nerves inside the canals.

This helps us keep our whole body balanced as we move.

The brain responds by sending messages through the cerebellum, which trigger reflex actions in our muscles.

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Deafness

Deafness can be caused by long exposure to a high level of noise, drugs, or ear infections.

Damage to the eardrum, ossicles [bones], and cochlea, which can be caused by loud sounds, produces incurable deafness.

Ear Defects

Workers exposed to prolonged sounds of over 90 decibels [dB] are obliged by law to wear ear protection.

Any exposure to 140 dB causes immediate damage to hearing. 84

The SKIN as a Sense Organ

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