The Nervous System

And the Senses

Human Organ System Overview

Nervous System Integumentary System Skeletal System Muscular System Circulatory System

Human Organ System Overview (continued)

Nervous System Integumentary System Skeletal System Muscular System Circulatory System

Human Organ System Overview (continued)

Nervous System Integumentary System Skeletal System Muscular System Circulatory System

Human Organ System Overview (continued)

Nervous System Integumentary System Skeletal System Muscular System Circulatory System

Human Organ System Overview (continued)

Nervous System Integumentary System Skeletal System Muscular System Circulatory System

Human Organ System Overview (continued)

Respiratory System Digestive System Excretory System

Endocrine System Reproductive System Lymphatic System

The Nervous System

• Basic and smallest unit of nervous system is the neuron

– The nervous system forms a communication and coordination network throughout an animal’s body.

– Neurons are nerve cells that carry electrical signals from one part of the body to another.

Three Types of Neurons

• Sensory neurons – carry impulses from body to spinal cord and brain

• Motor neurons – carry response impulses away from brain and spinal cord to a muscle or gland

• Interneurons – found within the brain and spinal cord – connect sensory and motor neurons

The Neuron

• Neurons conduct impulses throughout nervous system and consists of three regions – cell body, dendrites, and axons– Dendrites – receive impulses and carry them

to the cell body– Cell body – where nucleus of cell is– Axon – carries impulses away from cell body

to other neurons, muscles, or glands

The Neuron

Figure 27.2

Signal directionDendrites

Nucleus

Cellbody

Signalpathway Synaptic

terminalsSupportingcell

Myelin sheath

Axon

Action Potential

– A resting neuron has potential energy that can be put to work to send nerve signals from one part of the body to another.

– This difference in charge (voltage) across the plasma membrane of a resting neuron is the resting potential.

– A stimulus is any factor that causes a nerve signal to be generated.

– A stimulus of sufficient strength can trigger an action potential, a nerve signal that carries information along a neuron.

– If the stimulus is strong enough, a sufficient number of channels open to reach the threshold, the minimum change in a membrane’s voltage that must occur to trigger the action potential.

– Action potential is like a “domino effect” along a neuron.

Action Potential

Figure 27.3a

Neuron interior

Resting potential A stimulus opens thefirst set of ion channels;if threshold is reached,an action potential istriggered.

1 2

Action Potential

Figure 27.3b

Additional channelsopen; in that region ofthe neuron, the interiorof the cell becomesmore positive than theoutside.

3 4 The first set of channelscloses and inactivates;a second set ofchannels opens andpositive ions rush out;the membrane returnsto resting potential.

Figure 27.4Axon

Action potential

Action potential Action potential

1

2 3

Connections Between Neurons

• Neurons don’t touch one another – there is a tiny space between axons and dendrites called a synapse – need neurotransmitters (chemicals) to cross this gap and carry the “message” on

Connections Between Neurons

Figure 27.5

SYNAPSE

Synapticterminal ofsendingneuron

Dendrite ofreceiving neuron

Sending neuron

VesiclesActionpotentialarrives.

Neuro-transmitter is released intosynaptic cleft.

Neuro-transmitterbinds to receptor.

NeurotransmittermoleculesIon channels

Receivingneuron

Synapticcleft

Synapticterminal

Vesicle fuseswith plasmamembrane.

Neurotransmitter

Receptor

Ion channel opens and triggers or inhibits a new action potential.

Ion channel closes.Neurotransmitter isbroken down andreleased.

Ions

1

23

4

5 6

Connections Between Neurons

Neurotransmitters

A Nerve Impulse

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS) • CNS – brain and spinal cord

• PNS – all nerves carrying “messages” to and from CNS

– The vertebrate peripheral nervous system is divided into two functional components, the

1. motor system and

2. autonomic nervous system.– The motor system

• carries signals to and from skeletal muscles and • mainly responds to external stimuli.

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS) – The autonomic nervous system

• regulates the internal environment and• controls

– smooth and cardiac muscles and– organs and glands of the digestive, cardiovascular,

excretory, and endocrine systems.

Parts of the Brain

• Cerebrum – thinking and learning center – conscious activities of the body

• Cerebellum – balance and coordination

• Medulla – involuntary activities

The Brain

Pons

Pituitary gland

Hypothalamus

Cerebrum

Medulla oblongata Spinal cord

Cerebellum

Pineal gland

Thalamus

The Senses

• Eyes – – Complex organ sensing light– Retina – thin layer of tissue made up of light receptors

and sensory neurons– Two types of light receptor cells:

• Rods – detect shape and movement• Cones – detect color

The Eye

Choroid

Retina

Blood vessels

Optic nerve

Fovea

Vitreous humor

Sclera

Ligaments

Iris

Pupil

Cornea

Aqueous humor

Lens

Muscle

Visual Problems

– The most common visual problems are• nearsightedness, the inability to focus well on distant

objects,• farsightedness, the inability to focus well on near

objects, and• astigmatism, blurred vision caused by a misshapen

lens or cornea.

The Senses

• Ears – – Complex system detecting sound and helping

to maintain a sense of balance/equilibrium

The Ear

Auditory canal

Tympanum Round window Eustachian tube

Bone

Cochlea

Cochlear nerve

Semicircular canals

Oval window

StirrupAnvilHammer

Hearing Problems

– Deafness, the loss of hearing, can be caused by

• middle ear infections,• injury, such as a ruptured eardrum,• stiffening of the middle-ear bones (a common age-

related problem), or• overexposure to loud noises.