Sensitivity – the ability to respond to stimulus.
Response to stimulus in verterbrates
Involuntary actions: activities not under control of the will, e.g. peristalsis, heartbeat :
Voluntary actions: activities under control of the will NERVOUS SYSTEM IN MAMMALS
Nervous system coordinates and controls the work of all systems in the body Nervous system in mammals comprises of:
• Central Nervous System (CNS) comprising brain and spinal cord
• Peripheral Nervous System (PNS) comprising cranial nerves from brain to all head parts, and spinal nerves from spinal cord and receptor organs
The Nervous Tissue
It is made up of nerve cells called neurones. There are 3 types of neurons:
• Sensory/Receptor neurones – transmit impulses from sense organs to CNS
• Motor/Effector neurones – transmit impulses from CNS to effectors
• Intermediate/Relay neurones – found in CNS
a nerve fibre
A neurone consists of a cell body containing nucleus and nerve fibres.
Nerve fibres:
- There are 2 types of nerve fibres:
- Nerve fibre that transmits impulses away from cell body – axon - Nerve fibre that transmits impulses towards cell body – dendrons - The terminal branches at the ends of nerve fibres are called dendrites - Nerve fibres are enclosed by a layer of fatty substances – myelin sheath
- Sheath serves as insulating layer
- It is surrounded by thin membrane neurilemma – providing nourishment for fibre - It is not continuous like nerve fibres; it is seperated by nodes of Ranvier where
neurilemma sinks down and comes in contact with the fibre. The nodes speed up transmission of impulses along fibre
The Central Nervous System
Nervous tissue of CNS has 2 distinct regions: grey matter and white matter
- Grey matter consists of cell bodies of neurons and forms outer layers of brains and central portions of spinal cord.
- White matter consists of nerve fibres and forms the central part of brain and outer layers of the spinal cord.
The Synapse
Electric pulses are transmitted from axon of a neuron to dendron of another neurone across a small gap called synapse (a junction between 2 neurones)
Nerve impulses that reach axon end are converted to chemicals neurotransmitters into the synapse. These chemicals trigger impulse in dendrites of following neurone.
If neurone ends in muscle or gland (effectors), the chemicals stimulate the effectors.
- Receptors keep CNS informed of change in surroundings, produce message as electrical impulses: nerve impulses, which is sent to CNS by nerves. If impulse is sent to muscles, then muscle moves the body part away. Muscles are therefore effectors
The Brain of a Mammal
The mammalian brain is divided into 3 parts:
- Forebrain – includes cerebrum, thalamus, hypothalamus and pituitary gland - Midbrain
- Hindbrain – consists of cerebellum and medulla oblongata
Cerebrum of brain is made up of 2 cerebral hemispheres – the largest parts in brain.
Forebrain
- They concerned with intelligence, memory, learning, voluntary actions & emotions Thalamus sorts data, suppresses signals and enhnces others before sending the sorted information to appropriate brain centre for further interpretation and integration.
The floor of cerebral hemisphere is hypothalamus
- It controls and monitors body’s activities like thermoregulation & osmoregulation.
Other functions are:
• Controlling automatic nervous system (control involuntary actions in the body)
• Regulating food intake through hunger sensation
• Inducing sleep or wakefulness
• Controlling emotions
• Controlling endocrine system through its control of pituitary gland
Pituitary gland is attached to hypothalamus, secreting ADH and other hormones.
Consists of optic lobes. They deal with input from various organs:
Midbrain
1. Relay messages from ears
2. Controls visual reflexes such as movement of eyeballs
3. Filters sensory input before they reach concious regions of brain (e.g. a mother can sleep although there’s loud traffic noise but awake when her baby cries)
Cerebellum lies behind optic lobes with many folds and is large.
Hindbrain
- It controls muscular co-ordination especially in maintaining body balance
Medulla oblongata lies below cerebellum. Its posterior narrows down into spinal cord - It controls involuntary actions such as heartbeat, breathing, blood circulation and
contraction and relaxation of blood vessels. A direct blow here could cause death.
Spinal cord extends from medulla oblongata to end of vertebral columns.
Spinal Cord and Spinal Nerves
Spinal nerves emerge at intervals along length of spinal cord. (PNS&CNS diagram)
Structure of the spinal cord:
- Has grey and white matter, where grey matter is inside while white matter outside - In the middle of spinal cord runs a narrow central canal containing cerebrospinal
fluid which brings nutrients to the cord
- Relay/intermediate neurones lie in grey matter of spinal cord, where they form synapses with receptor and effector neurones which enable impulses to be transmitted from receptor neurones to effector neurones
Structure of the spinal nerves:
- Spinal nerve divides into 2 roots before it joins the spinal cord
• Dorsal roots join dorsal part of spinal cord and contain receptor neurones
Cell body of the neurones aggregate in small swelling – dorsal root ganglion
The axon end in grey matter of spinal cord; dendron become sensory fibres in dorsal root and spinal nerve
• Ventral roots attached to ventral part of spinal cord and contain effector neurones
Cell bodies of neurones lie in grey matter of spinal cord.
Axon leave spinal cord to enter ventral root and spinal nerve
- Nerve fibres from both roots converge into spinal nerve, where it contain both receptor and effector neurones. It is called mixed fibre
- Spinal nerve subdivide into branches supplying nerve fibres to various parts of body - Sensory and motor neurones soon separate going to their destinations
- Receptors in hand stimulated – impulses transmitted through receptor neurons to grey matter in spinal cord.
How these work hand-in-hand to make you feel someone touches your hand
- Neurones in spinal cord conduct impulses to brain and the touch is felt when impulses reach cerebrum
Voluntary Actions – the response to a specific stimulus with conscious control Reflex Actions – immediate response to a specific stimulus without conscious control The events of reflex action on the picture on the right is as follows:
1. Heat on object stimulates nerve endings (receptor) in the skin.
2. Impulses are produced. Impulses travel along receptor neurone to spinal cord.
3. In spinal cord, impulses are transmittes across synapse to relay neurone then across another synapse to effector neurone.
4. Impulses leave spinal cord along effector neurone to effector
5. Biceps muscle (effector) contracts to suddenly withdraw hand from the object.
Spinal and Cranial Reflexes
Spinial reflex actions are controlled by spinal cord.
Cranial reflex actions are controlled by brain and usually occur in head region.
Reflex Arc
It is the nervous pathway by which impulses can travel from receptor to effector.
Reflex arc consists of: - receptors
- receptor neurone
- reflex centre (spinal cord/brain) - effector neurone
- effectors
Below is a simple reflex arc of the example given above
Conditioned reflex
It is the reflex action to a fixed stimuli acquired from past experience
e.g.: Plums might be sour and not tasty for the first sight but once we tried it for a number of times, we find it mouth watering and salivary gland secretes saliva.
12.2 THE ENDOCRINE SYSTEM Hormones
Hormones are chemical messengers in animals which assist the nervous system in co-ordinating various parts of the body, ensuring they develop and work harmoniously ENDOCRINE GLANDS
Exocrine glands are glands having duct to carry the secretion to be poured out of the body (e.g. salivary glands, sweat glands)
Endocrine glands are glands which do not have duct to carry the secretion. The hormone secreted passes directly into bloodstream to be distrubuted around the body. They secrete gland internally (e.g. adrenal glands, thyroid gland)
- Pituitary gland is the master gland whih secretes hormones which controls activity of a particular gland
- Some glands don’t function until the organism reaches certain age e.g. gonads - Divided into 2: short-lived hormone(adrenaline) and long-term hormone(thyroxine) Hormonal and Nervous Control
Hormonal control serves as means of co-ordination within the body.
- Stimulus causes transmission of messages to target organ (effector) Differences between nervous and hormonal controls
Nervous control Hormonal control
- Involves nervous impulses (electrical
signals) - Involves hormones (chemical substances)
- Impulses transmitted by neurones - Hormones transported by blood
- Quick response - Slow response
- Response short-lived - Response may be short-lived or long-lived - May be voluntary or involuntary - Always involuntary
- Usually localized (only one target
organ affected) - May affect more than one target organ The table next shows endocrine glands and their secretion
12.3 THE EYE
Eyeball is located within a depression in the skull (the orbit) so only front part’s visible.
It’s attached to orbit by 6 rectus muscles for rotating eyeball without moving head.
Structure Function
Conjunctiva Thin transparent membrane covering the exposed part of eyeball Eyelids
Movable flap which can control the amount of light entering the eye, prevents mechanical damage to eyeball and spreads tears throughout the eye to lubricate conjunctiva, everytime we blink
Structure Function
Tear gland Lies on the outer corner of the upper eyelid to secrete tears which lubricates the eye and cleanse eye from dust particles
Tear duct Drains excess tears into nasal cavity
Eyebrow Shades eye & catches drops of sweat from entering to irritate eye Eyelashes Stiff hairs along eyelids to trap dust particles and irritants
Sclera It’s the outermost layer of the eye which is a tough, white, fibrous coat, forming protective layer for eye and gives eye its shape Cornea Transparent coat of eye on the visible outer part which allows light
to pass through and refracted
Choroid Layer under sclera containing network of capillaries to nourish the eye and pigmented black to prevent total internal reflection of light Ciliary muscles Anterior part of choroid which work with suspensory ligaments to
alter the length and thickness of lens Iris
Circular disc-shaped structure which gives eye colour and alters size of pupil to control the amount of light which can enter. This also prevents excessive exposure to light which may damage eye Pupil It’s a hole in the centre of iris which allows light to enter the eye Retina It’s the innermost layer of eye where image is formed as it
consists of photoreceptors connected to optic nerve Yellow spot
Also called fovea. It is a shallow yellow depression where cones are most concentrated so it gives sharpest vision when light is focused here.
Blind spot Found just over optic nerve and has no photoreceptors so it’s insensitive to light causing no image formed when light falls here Aqueous humour Watery fluid in the small chamber in front of iris and lens used to
refract light into the eye and keeps eyeball firm
Vitreous humour Transparent jelly-like fluid in the large chamber behind the lens used to refract light into the eye and keeps the eyeball firm
Optic nerve Transmit impulses to brain when retinal photoreceptors stimulated Controlling pupil size
Circular muscles arranged in circle around the pupil works antagonistically with radial muscles arranged radially around the pupil
In dim light
Circular muscles relax while radial muscles contract, pulling the pupil to dilate, maximising the amount of light entering the eye.
In bright light
Circular muscles contract while radial muscles relax, decreasing size of pupil to prevent excessive light from damaging the retina.
About our Retina
The retina consists of light-sensitive cells – photoreceptors – where image is formed.
Photoreceptors:
(a) Rods
Contains visual purple which is a pigment helping for vision in dim light. The pigment bleaches in bright light and hence when we look into the dark from bright room, we may not be able to distinguish objects as it takes time for visual purple to reform. The reform needs vitamin A, so without it, we may not be able to see in dim light, suffering night-blindness.
(b) Cones
Contains less light-sensitive red, green and blue cones having separate pigments which absorb different light wavelengths for colour vision in bright light. They work together to create different colours hence completing spectrum of light on retina.
Controlling lens size for Accomodation
Accomodation means the ability of the eye to adjust the lens so that images of any distances are formed accurately on the retina.
Light reflected from object falls onto retina after refracted by cornea and aqueous humour and further refracted to converge and focus on retina by lens. The image formed stimulates the rods or cones to create impulses to be transmitted via optic nerve to the brain which interprets the impulses so that we can see it upright and same size because real image formed on retina is inverted, reversed and diminished.
Lens is adjusted by ciliary muscles and suspensory ligaments (antagonistic muscles) so that light rays are accurately focused onto the retina
Focusing Distant Objects
Ciliary muscles relax to pull suspensory ligaments so that it contract and pull the lens to flatten and becomes less convex. This allows the almost “parallel” distant light rays to be less refracted so that it can be sharply formed on retina.
Focusing Near Objects
Ciliary muscles contract to slacken the pull on suspensory ligaments so that the lens attached to it thickens and becomes more convex. This diverges the light rays further so that images are formed sharply on retina.
The closer the object, the more ciliary muscles contract to make lens more convex so that it can make the object as focused as possible on retina, but when the contraction is at its maximum, if the object is placed any more nearer, the image will be blurred as the lens can’t be further adjusted. The point where the image formed starts to blur is near point of the eye.