Chapter 10: Nervous system and the eye Flashcards
What is a stimulus
A change in the environment that causes an organism to react. An organism’s reaction to a stimulus is called a response
What is sesitivity
The ability to respond to a stimulus
Role of nervous system
regulates bodily functions and activities; and how we react to our surroundings
Components of nervous system
- Central Nervous System (CNS): brain & spinal cord
- Peripheral Nervous System (PNS): Cranial nerves (from brain) , spinal nerves (from spinal cord), sense organs
Types of neurones
- Sensory neurone (receptor neurone)
- Relay neurone (intermediate neurone)
- Motor neurone (effector neurone)
Sensory neurone
Transmits nerve impulses from sense organs or receptors to the CNS
Relay neurone
Transmits nerve impulses from the sensory neurone to the motor neurone. They are found in the CNS
Motor neurone
Transmits nerve impulses from the CNS to the effectors
Structure of motor neurone
- Cell body: contains a nucleus, cytoplasm, cell membrane and organelles. It’s irregular in shape
- Nerve Fibre: A strand of cytoplasm extending from the cell body
Structure of sensory neurone
- Cell Body: Circular
- Nerve fibre: one long one extending towards receptor and one short one extending towards the relay neurone in CNS
What is a synapse
The junction between 2 neurones. Nerve impulses are transmitted across the synapse by chemicals released by the neurones
How do the brain, spinal cord and spinal nerves work tgt
- In spinal cord, relay neurones may synapse w/ sensory & motor neurone. They transmits nerve impulses from the sensory neurones to the motor neurones in reflex action
- Relay neurones synapse w/ sensory neurones and transmit nerve impulses up spinal cord to the brain. This causes sensations
- Relay neurones may transmit nerve impulses about a voluntary action from brain, down spinal cord
Nerve impulses pathway (sensation)
Receptor in skin ➡️ sensory neurone ➡️ Relay neurone in spinal cord ➡️ Brain
Nerve impulses pathway (Voluntary action)
Brain ➡️ Relay neurone in the spinal cord ➡️ Motor neurone ➡️ Effector
What is reflex action?
An immediate response to a specific stimulus without conscious control
Types of reflex actions
- Cranial reflexes: Controlled by brain and usually occur in head region (Ex: Pupil reflex)
- Spinal reflexes: Controlled by the spinal cord, occurs below the neck (Ex: Knee-jerk reaction)
What happens when your hand touches a hot object (for example)
- Touch and pain receptors in the hand will detect the hot object (stimulus)
- Receptors in the skin are stimulated and produces nerve impulses
- Sensory neurone transmits the nerve impulses produced to the spinal cord
- In the spinal cord, nerve impulses are transmitted first across a synapse to the relay neurone, and then across another synapse to the motor neurone
- Motor neurone transmits the nerve impulses from spinal cord to the effector, which is the hand muscles
- Muscles in hand contracts and the hand moves away from hot object
What is reflex arc
It is the shortest pathway by which nerve impulses travel from the receptor to the effector in a reflex action
Pathway of nerve impulses (reflex arc)
Receptor ➡️ Sensory neurone ➡️ Relay neurone (CNS) ➡️ motor neurone ➡️ effector
External structure of eye
- Sclera
- Conjunctiva
- Eyelashes
- Tear Gland
- Eyelids
- Iris
- Pupil
Sclera
- “white” of eye
- tough, white outer covering of the eyeball
- is continuous with the cornea
- protects the eyeball from mechanical damage
Conjunctiva
- thin transparent membrane covering sclera in front
- a mucous membrane
- secretes mucus, help keep front of eyeball moist
Eyelashes
- shields the eye from dust particles
Tear gland
- lies at the corner of the upper eyelid
- secretes tears that wash away dust particles; keep cornea moist; and lubricate the conjunctiva to reduce friction when eyelids move
Eyelids
- protects the cornea from mechanical damage
- squinting prevents too much light from entering the eye and damaging the retina
- blinking spreads tears over the cornea and conjunctiva, and wipes dust particles off the cornea
Iris
- a circular sheet of muscles
- controls the sizes of the pupil 👉 controls amount of light entering eye
- contains pigment that gives the eye its colour
Pupil
- a hole in the centre of the iris
- allows light to enter the eyes
Internal structure of eye
- Retina
- Lens
- Blind Spot
- Optic Nerve
- Fovea
- Vitreous Chamber
- Choroid
- Ciliary Body
- Suspensory ligament
- Cornea
- Aqueous Chamber
Retina
- innermost layer of the eyeball
- light sensitive layer on which images are formed
- contains photoreceptors
- photoreceptors are connected to nerve endings from optic nerve
Lens
- transparent, circular and biconvex structure
- it is elastic
- changes its thickness to focus light onto the retina
Blind Spot
- region where the optic nerve leaves the eye
- does not contain any rods or cones 👉 it’s not sensitive to light
- one will not be able to see an object if the image falls into the blind spot
Optic nerve
- nerve that transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated
Fovea
- small yellow depression in retina
- situated directed behind the lens
- where images are normally focused
- contains greatest concentration of cones, but not rods
- enables a person to have detailed colour vision in bright light
Vitreous chambre
- space between the lens
- filled with vitreous humour (a transparent, jelly-like substance)
- vitreous humour keeps the eyeball firm and helps to refract light onto the retina
Choroid
- middle layer of the eyeball (in between sclera and retina)
- pigmented black to prevent internal reflection of light
- contains blood vessels that bring oxygen and nutrients to eyeball and remove metabolic waste products
Ciliary body
- thickened region at the front end of the choroid
- contains ciliary muscles, which controls the curvature/thickness of the lens
Cornea
- dome-shaped transparent layer continuous with the sclera
- refracts light rays into the eye
- causes greatest refraction of light into the eye
Aqueous chamber
- space between lens and cornea
- filled with aqueous humour (a transparents, watery fluid)
- aqueous humour keeps front of the eyeball firm and helps to refract light into the pupil
What is pupil reflex
- a reflex action that causes pupils to change in size in response to changes in light intensity
- controlled by iris
Benefits of pupil reflex
- it is automatic, so no learning is required
- prevents excessive light from entering the eye and damaging the retina
- it is an immediate response
- allows enough light to enter the eye to let us see
How iris control amount of light entering our eyes?
- using circular and radial muscles
- they are antagonistic muscles
What happens in bright light?
- size of pupil decreases 👉 less light enters the eye
- circular muscles contract
- radial muscles relax
( when it’s bright, happy marriage, hence CC and RR)
What happens in dim light?
- size of pupil increases 👉 more light enter eye
- circular muscles relax
- radial muscles contract
(when it’s dim, life is sad, there is a divorce, hence CR and RC)
Nerve impulses pathway for pupil reflex
Stimulus (Change in light intensity) ➡️ Receptors (Retina) ➡️ Sensory neurone in optic nerve ➡️ Brain ➡️ Motor neurone ➡️ Effector (Iris Muscles)
How do we see
- light rays are refracted through the cornea and the aqueous humour onto the lens
- lens causes further refraction and the rays are brought to a focus on the retina
- the image on the retina stimulates either the rods or cones, depending on intensity of light
- image formed on retina is inverted, laterally inverted and diminished
What is focusing
- adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina
Focusing on distant objects
- ciliary muscles relax, pulling on suspensory ligament (CR)
- Suspensory ligament become taut, pulling on edge of lens (ST)
- Lens becomes thinner and less convex
- Light rays from the distant object are sharply focused on the retina
- Photoreceptors are stimulated
- Nerve impulses produced are transmitted by the optic nerve to the brain. The brain interprets the nerve impulses and the person sees the distant object.
Focusing on near objects
- Ciliary muscles contract, relaxing their pull on the suspensory ligaments (CC)
- Suspensory ligament slacken, relaxing their pull on the lens (SS)
- The lens, being elastic, becomes thicker and more convex
- Light rays from the near object are sharply focused on the retina
- Photoreceptors are stimulated
- Nerve impulses are transmitted by the optic nerve to the brain. The brain interprets the nerve impulses and the person sees the near object
