The nervous system and the eye (chapter 10)

Components of Human nervous system

• The central nervous system (CNS) consisting of the brain and the spinal cord

• The peripheral nervous system (PNS) consisting of cranial nerves , the spinal nerves)

Neurones

1. Sensory neurone (receptor neurone)→ the sensory neurone transmits nerve impulses from the sense organs or receptors or the CNS.

2. Relay neurones (intermediate neurone)- The relay neurone transmits nerve impulses from the sensory neurone to the motor neurone. They are found within the CNS

3. Motor neurone(effector neurone)→ the motor neurone transmits nerve impulses from the CNS to the effectors

Synapse

There is a tiny space at the junction between two neurones. The junction between the two neurones is called a synapse. Nerve impulses cannot directly cross from one neurone to the next due to tiny space between them. Nerve impulses are transmitted across a synapse by chemicals reuses by neurones.

How do the brain, spinal cords and spinal nerves work together?

• In the spinal cords, relay neurones may synapse with the sensory neurones and motor neurones. They transmit nerve impulses from the sensory neurones in a reflex action.

• relay neurones may synapse with the sensory neurones and then transmit nerve impulses up the spinal cord to the brain. This enables us to feel sensations.

• Relay neurones may transmit nerve impulses about a voluntary action from the brain, down the spinal cord.

Reflex action definition

A reflex action os an immediate response to a specific stimulus without conscious control.

Reflex centres

• Cranial reflexes→ controlled by the brain and usually occur in the head region. eg: pupil reflex, blinking and salivation.

• Spinal reflexes. these are controlled by the spinal cord eg: sudden withdrawal of the hand on touching a hot object.

what happens when your hand touches a hot object

1. Receptors in your skin → the heat from the object stimulates the nerve endings in your skin. Nerve impulses are produces.

2. Sensory neurone → the sensory neurone transmits the nerve impulses to your spinal cord.

3. Spinal cord→ In the spinal cord, nerve impulses are transmitted first across a synapse to the relay neurones, then across another synapse to the motor neurone.

4. Motor Neurone→ the motor neurone transmits nerve impulses form the spinal cord to the effector

5. effector muscles→ your biceps muscle them contracts and causes your hand to withdraw a suddnely.

Reflex arc definition

A reflex arc is the shortest pathway by which a nerve impulses travel the receptor to the effector in a reflex action

Sclera

The sclera is the white of the eye. It is tough, outer white covering of the eyeball. It is continuous with the cornea, which is a dome-shaped transparent layer at the front of the eye. it protects the ye from external damage.

Conjunctiva

The conjunctiva is a thin transparent membrane covering the sclera in front. It is a mucous membrane. It secretes mucus, thus helping to keep the front of the eyeball moist.

Eyelashes

The eyelashes help to shield the eye from dust particles

Tear gland

The tear gland lies at the corner of the upper eyelid. It secrets tears so that:

• wash away dust particles

• keep the cornea moist for atmospheric oxygen to dissolve so that the dissolved oxygen can diffuse into the cornea

Eyelids

The eyelids protect the cornmeal from mechanical damage. They can be partly closed which is known as squinting. Squinting prevents too much light from entering the eyelid and damaging the retina. Blinking spreads tears over the cornea and conjunctiva, and wipes dust particles off the cornea

Iris

the iris is a circular sheet of muscles. It controls the size of the pupil and therefore the amount of light entertains the eye. It contains a pigment that gives eye its colour.

Pupil

Pupil is a hole in the centre of the iris. It allow light to enter the eye.

Retina

Retina is the innermost later of the eyeball. It is th light-sensitive layer on which o=images are formed. It contains light-sensitive cells or photoreceptors. The photoreceptors are connected to the nerve endings from the optic nerve.

Lens

It is a transparent circular and biconvex structure. It is elastic and changes its thickness to focus light on the retina.

Blind spot

It is the region where the optic nerve leaves the eye. It does not contain any rods or cones. therefore it is not sensitive to light. Yup will not be able to see an object off its image falls on the blind spot

Optic nerve

It is the nerve that transmits nerve impulses to the brain when the photoreceptors in the retina are stimulated.

Fovea

It is a small yellow depression in the retina. It is situated directly behind the lens. This is where images are normally focused. The fovea contains the greatest concentration of cones, but no rods. The fovea enables a person to have detailed colour vision in bright light.

Vitreous chamber

It is the space behind the lens. It is filled with vitreous humour. a transparent, jelly-like substance. Vitreous humour keeps the eyeball firm and helps to refract light onto the retina.

Choroid

It is the middle ayer of the eyeball

• It is pigmented black to prevent internal reflection of light.

• It contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products

Ciliary body

It is a thickened region at the front of the choroid. It contains ciliary muscles, which control the curvature or thickened of the lens.

Suspensory ligament

It is a connective tissue that attaches the edge of the lens to the ciliary body.

Cornea

It is a dome-shaped transparent layer continuous with the sclera or white of the eye. It refracts or bends light rays into the eye. The cornea causes the greatest refraction into the ey

Aqueous chamber

It is then space between the lens and the cornea. It is filled with aqueous humour, a transparent, watery fluid. Aqueous humour keeps the front of the eyeball firm and beckons to refract light into the pupil.

Pupil reflex

1. It is automatic, so no learning is required.

2. It prevents excessive light from entering the eye and damaging the retina

3. It is immediate esponse.

4. It allows enough light to enter the eye to allow us to see

How does the Iris control the amount of light entering the eye (reducing)

1. The circular muscles of the iris contarct

2. The radial muscles of the iris relax.

3. The pupil becomes smaller or constricts. This reduces the amount of light entering the eye.

How does the Iris control the amount of light entering the eye (increasing)

1. The radial muscles of the iris contract.

2. the circular muscles of the iris relax

3. The pupil enlarges or dilates. This increases the amount of light entering the eye.

Focusing definition

Focusing or accommodation is the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina.

Focusing on a distant object

1. Ciliary muscles relax, pulling on the suspensory ligament

2. Suspensory ligament become taut, pulling on the edge of the lens.

3. Lens become thinner and less convex.

4. Light rays from the distant object are sharply focused on the retina.

5. Photoreceptors are stimulated,=.

6. Nerve impulses produces are transmitted by the optic nerve to the brain. The brain interprets the nerve impulses, and the person sees the distant object,

Focusing on a near object

1. Ciliary muscles contract, relaxing their pull on the suspensory ligaments.

2. Suspensory ligament slacken, relaxing their pull on the lens.

3. The lens, being elastic, becomes thicker and more convex.

4. Light rays from the near object are sharply focused on the retina.

5. Photoreceptors are stimulated

6. Nerve impulses produced are transmitted by the optic nerve to the brain. the brain interprets the nerve impulses, and the person sees the near object