Download UNIT 6: RECEPTORS AND EFFECTORS.

UNIT 6: RECEPTORS AND EFFECTORS.Receptors are responsible for:
-
Detecting changes (stimuli) in the world around us
Converting the changes into nerve impulses
Sending the nerve impulses to the higher nerve centre
Types of receptors.We can classify receptors in different ways:
-
According to its structure:
 Simply groups of nerves
 Special cells, which join together to make the sensory organs.
- According to the type of stimuli they receive:
 Photoreceptor: detect light stimuli and are found in the eyes.
 Mechanoreceptor: detect mechanic stimuli like pressure, contact or sound
waves. These include receptors in skin and the ears.
 Chemoreceptor: pick up information from chemical changes. These include
taste and smell receptors.
 Thermoreceptor: detect changes in temperature. These include some skin
receptors.
 Receptor of pain: detect pain.
Characteristic of the receptors.- Receptors detect stimuli only if they have a minimum intensity called threshold
intensity.
- Some receptors have adaptation. This means that if a stimulus persists in time
the receptor don’t catch it and it doesn’t send information about it to the higher
nerve centers
The eye.The eye detects stimuli like changes in light intensity and color.
Each eye is made up of:
- The eyeball, a hollow spherical structure, located in a cavity in the skull called
orbit.
- Accessory organs like eyebrows, the eyelids (folds of skin that protect the
eyeball), eyelashes, the ocular muscles (which allow the eye to move) and the
lachrymal glands (which keep the eye moist and also secrete bactericide
substances).
Receptors in the eye are the photoreceptor cells found in the retina:
- Rods: they are stimulated by any kind of visible light, but do not distinguish
colors. They are responsible for seeing in black and white.
- Cones. They distinguish colors, but they need a greater intensity of light.
Structure of the eyeball.It has spherical shape and from the outside to inside we can distinguish the following
layers:
1) The sclera: The outer part of the eye. It is white and at the very front it becomes
transparent and forms the cornea. This layer has a protector function.
2) The choroid: It is the second layer. It is black and it has a lot of blood vessels
that send nutrients and oxygen to the eye.
In front is the iris (behind the cornea), that is a different color for each person. In
the centre of the iris there is an opening the pupil. The diameter of the pupil is
variable. It regulates the amount of light that goes inside the eye.
3) The retina: It is the internal layer, where are the photoreceptor cells. These cells
are rods and cones. Inside the retina we can distinguish:
- The fovea: the area of the retina where sensitivity is at its maximum and there
are a large number of cones.
- The blind spot: the part of the eye where there is no vision because there are no
photoreceptor cells.
- The optic nerve: a group of nerve cells which communicate the cones and rods
with the brain.
4) Inside the eye there are two chambers that are separated by the crystalline lens.
The crystalline lens is located behind the iris. It is a biconvex lens which is held
onto the inside wall of the eyeball by tiny muscles.
The anterior chamber is filled with a watery liquid (aqueous humour) and the
posterior chamber has a more viscous substance, which is also transparent
(vitreous humour).
How the eye words.Before light reaches the retina, it crosses the eyeball and two things happen:
1) Regulation of the intensity of the light. The pupil opens more or less depending
upon the intensity of the light.
2) Focusing images. This happens thanks to the modification of the thickness of the
crystalline lens.
Some problems with sight are: myopia, hypermetropia, squinting, astigmatism,
cataracts, etc.
The ear.The ear is the sensory organ located on both side of the head and found in the bone
cavities in the temporal bones of the skull.
It picks up two types of stimuli: sounds and changes in the body’s position (balance).
Parts of the ear.1) Outer ear: this is made up of the pinna (auricle) and the ear canal, which
penetrates the bone. In the canal there are glands that produce protective wax.
The ear canal ends in the tympanic membrane, also called the eardrum.
2) Middle ear: this is a cavity in the temporal bone which stars at the eardrum and
reaches two small membranes called the oval window and the round window.
There are three canal connected bones in this part of the ear: the hammer, the
anvil and the stapes. The hammer is joined to the eardrum and the stirrup to the
oval window.
The Eustachian tube connects the ear with the pharynx and it equalizes the
pressure of the ear in both sides of the eardrum.
3) Inner ear: This is the deepest part of the ear and is made up of labyrinthine
membranes. These form two complicated cavities in the temporal bone. There is
a fluid called perilymph between the two labyrinth cavities, and within the
membranes there is another liquid called endolymph.
The first part of the labyrinth is the cochlea, which is responsible for picking up
sound. It has the sensitive cells that make the organ of Corti. It sends the
information to the brain by the auditory nerve.
The other part is the vestibule, which is responsible for spatial control and
balance. The vestibule is made up of three channels or semicircular canals and
two vesicles called the utricle and the saccule.
How is hearing produced?
Sound waves (vibrations) reach the eardrum, make it vibrate, transmitting the vibration
through the three bones of the middle ear, which transfer it to the oval window. This,
makes the perilymph fluid in the inner ear vibrate, stimulating the interior cells in the
cochlea, which make up the organ of Corti. A nerve impulse stars there and travels to
the brain, where the information is interpreted.
How is balance controlled?
The sense of balance tells us the position of our body (the control of the position of the
body is produced in the utricle and in the saccule) and of the movement we make (the
detection of movement takes place in the semicircular canals).
The skin, touch organs.The touch picks up the following stimuli:
- The form, shape and texture of objects.
- The pressure
- Heat or cold (temperatures that are higher or lower than normal).
- Pain.
All the skin receptors are found in dermal corpuscle, except pain receptors which are
nerve endings.
The skin receptors haven’t an uniform distribution. There are places with a high
concentration of receptors like lips or fingertips, that are high sensitivity, and there are
other places with low concentration of receptors such us the back with less sensitivity.
All the skin receptors have adaptation except the pain receptors.
The sense of taste.This sense allows us to detect the chemical substances dissolved in saliva.
The sensation of flavour is a mix of taste, smells and touch.
There are a lot of flavours, and all of them are the result of combinations of four basic
tastes: sweet, salty, sour and bitter.
Taste receptors are cells grouped together on taste buds, which form the taste papillae
that are found all over the tongue.
The smell.The sense of smell allows us to detect gaseous molecules that reach the nose. These
substances need to be in high concentration and to be soluble in the mucus.
There are 3000 different smells and they are a mix of seven basic smells.
The receptors of smell are inside of the nasal cavities, and they are made up by the
olfactory cells, which are grouped together in the olfactory bulb.
The information of the smell is sending to the brain by the olfactory nerve.
This sense has adaptation.
Effectors organs: locomotion.Effectors organs do the responses. They are muscles and bones that do the movements
of our body. Muscles have an active part and bones have a passive part in the process.
Bones.They are made up by bone tissue, which cells contain calcium salts, which make them
hard.
There are two types of bone tissues:
- Spongy bone tissue has cavities and gives bone their particular characteristic.
- Compact bone tissue: is solid and has no holes in it.
Bones and their shape.There are three types of bones according to their shape:
- Long bones: they have spongy bone tissue at the ends (epiphysis) and compact
bone in the central part (diaphysis). Examples of these are the bones of the
limbs.
- Short bones: they are rounded and they are made of compact bone tissue on the
outside and spongy bone tissue on the inside. Examples of short bones are the
vertebrae and the wrist bones.
- Flat bones: they are flat like a sheet, and they are made of an internal layer of
spongy bone tissue which is restricted by two layers of compact bone tissue.
Their function is to protect, like the skull and the shoulder blade.
Muscles.They are made of muscular tissue.
Muscles depending on their shape can be fusiform or long (biceps and triceps), flat
(pectorals and rectus abdominals) and circular or sphincters (some muscles in the eyes
and in the lips)