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The Nervous System

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Objectives:

You should be able to:

define the terms “receptors” and “effectors”;

explain the relationship between the receptor, the central

nervous system and the effector; and

describe the role of the sensory, relay and motor neurones.

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The nervous system has three main functions:

• Receive stimuli from external and internal environments

• Integrates input of stimuli from receptors

• Responds to stimuli

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The nervous system is made up of two main parts:

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

• The peripheral nervous system (PNS) consists of two sets of nerves - the

cranial nerves from the brain and the spinal nerves from the spinal cord.

The PNS links the CNS with the receptors and effectors.

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The nerve cell is known as a neurone.

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A neurone usually has:

• a cell body containing nucleus, cytoplasm, cell membrane and

organelles

• one long thin nerve fibre called an axon which transmits impulses

away from the cell body

• one or more short nerve fibres called dendrons that conduct impulses

to cell body

• nerve endings (dendrites at the end of dendrons and axon terminals at

the end of axons).

• The myelin sheath which serves as an insulating layer for nerve fibres

• Nodes of Ranvier separating the myelin sheath to speed up impulse

transmission

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There are three main functional types of neurones – sensory neurones, motor neurones and intermediate or relay neurones.

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Synapses – the connection between neurones

A synapse is found at the point where an axon of one neurone comes into

very close contact with the dendrite of another neurone.

Transmission across a synapse is by chemical means – known as

neurotransmitters.

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Synapses – the connection between neurones

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The Pathway of Nervous Transmission

Components of a general neural pathway:

Receptors detect stimuli

Information is sent via sensory neurones to spinal cord

Relay neurones in spinal cord conduct nerve impulses to the brain

Motor neurones signal effectors to respond

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The Central Nervous System

You should be able to:

describe the functions of the main regions of the brain; and

describe the role of the autonomic nervous system.

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The Human Brain

The brain is surrounded by 3 membranes:

• Meningas – protects brain along with bones of skull

• Grey matter – contains cell bodies of neurons

• White matter – contains nerve fibres of neurons

The mammalian brain can be divided into three main parts:

• Forebrain – consists of cerebrum, thalamus and hypothalamus

• Midbrain

• Hindbrain – consists of cerebellum and medulla oblongata

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Structure Function

Cerebrum controls voluntary movements, memory, speech, intelligence, reasoning ability, personality

Thalamus relays sensory information from all parts of the body to the cerebral cortex

Hypothalamus regulates body temperature, appetite, thirst, emotionscontrols release of hormones from pituitary gland

Midbrain controls reflex movements of eye muscle, head and neck

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Structure Function

Cerebellum controls and coordinates muscular activityimportant in balance, posture and movement

Medulla oblongata

controls involuntary actions such as heart rate, breathing rate, peristalsis

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Peripheral Nervous System

Somatic nervous system

◦ Consists of peripheral nerves linking the sense organs and skeletal

muscles to the CNS.

◦ Under conscious control

Autonomic nervous system

◦ Responsible for the regulation of involuntary actions, such as heart rate,

breathing rate, digestion and pupil dilation.

◦ Brings about homeostasis

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The Reflex Action

You should be able to:

describe a reflex arc; and

distinguish between a cranial reflex and a spinal reflex.

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• A reflex action is an immediate response to a specific stimulus

without conscious control.

• The shortest pathway of nerve impulses from the receptor to the

effector is known as the reflex arc.

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• A simple reflex arc consists of the following:

– Receptor (sense organ)

– Sensory or receptor neurone

– Reflex centre (CNS)

– Motor or effector neurone

– Effector (muscle or gland)

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There are two main types of reflex actions:

Spinal reflexes

◦ Controlled by spinal cord

◦ E.g. withdrawal reflex

◦ E.g. knee-jerk reflex

Cranial reflexes

◦ Controlled by the brain and occur in the head region (but not under

conscious control)

◦ E.g. blink reflex, pupil reflex and salivation

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Our Five Senses

You should be able to:

identify the main sense organs and the stimuli to which they

respond.

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Sense Organ Stimulus to which it responds

-Light-sensitive cells found on the retina are stimulated by light, and sends the information via the optic nerve to the brain.

-Sound waves are directed as pressure changes to the ear drum.-Vibrations generate nerve impulses via movement of sensory hair.- Nerve impulses reach the auditory nerve to the brain.

-Taste buds have receptors for the four tastes of sweet, sour, salty and bitter.

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Sense Organ Stimulus to which it responds

-Olfactory receptors lining the nasal cavity are stimulated by odours we breathe in.

-Nerve endings in the skin detect sensations of pain, pressure, heat and cold.

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The Eye

You should be able to:

• relate the structure of the human eye to its functions as a sense organ;

• explain how the eye produces a focused image of near and distant

objects on the retina;

• describe the pupil reflex in response to bright and dim light; and

• explain sight defects and their corrections.

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External structure of the human eye

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Side view of the human eye

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Structure Function

Cornea Refracts light towards the retina

Sclera Protects the eyeball

Choroid Prevents internal reflection of light within the eye

Ciliary muscles Changes the shape of the lens during accommodation

Iris Controls size of the pupil

Lens Refracts light onto the retina

Retina Light-sensitive layer where images are formed

Fovea Most of the light is focused here, where vision is sharpest

Blind spot Point where optic nerve leaves the eye

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The Cells Which Enable Us to See

Retina is the innermost, light-sensitive layer on which images are formed

Photoreceptors convert light waves into nerve impulses which relay the

information to the brain.

There are two kinds of photoreceptor cells – rods and cones.

Rods

◦ Important for vision in dim light

Cones

◦ Used for vision in bright light

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Fovea centralis - shallow depression of the retina with the highest

concentration of cones

Blind spot – where the optic nerve joins the eyeball. No

photoreceptors are located here, so light rays falling on this spot

would not be detected.

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Stereoscopic vision – why do we need two eyes?

• The brain integrates the information from the overlapping visual fields of both eyes and interprets them as one image.

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Accommodation

The adjustment of the lens of the eye so that clear images of

objects at different distances are formed on the retina.

The lens is attached to the ring of ciliary muscles by suspensory

ligaments.

Contraction and relaxation of these muscles change the tension in

the ligaments, which then affects the pull on the lens.

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Looking at a close object

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Looking at a distant object

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Pupil Reflex

A cranial reflex

The automatic change in the size of the pupil in response to changes in light intensity.

The pupil size is controlled by the action of the circular and radial muscles of the iris.

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Vision defects

People with short-sightedness are unable to see distant objects clearly.

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People with long-sightedness are unable to see near objects clearly.

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Vision defects

AstigmatismCaused by irregular curvature of the cornea or lens, so light cannot be focused at one point, causing blurred vision

GlaucomaBuildup of pressure of fluid in eyeball can damage optic nerve and cause vision loss

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CataractsClouding of the lens, causing blurred vision