The Nervous System

It is comprised of 3 basic components

Brain Spinal cord Nerves

General Functions of the Nervous System

Sensation Monitors changes/events occurring in and outside the

body. Such changes are known as stimuli and the cells that monitor them are sensory receptors.

Integration The parallel processing and interpretation of sensory

information to determine the appropriate response (Conscious or subconscious)

Reaction Motor output or the activation of effectors such as muscles

or glands (typically via the release of neurotransmitters).

Organization of the Nervous System

2 big initial divisions:1. Central Nervous

System The brain + the spinal

cord The center of integration

and control

2. Peripheral Nervous System

The nervous system outside of the brain and spinal cord

Consists of: 31 Spinal nerves

Carry info to and from the spinal cord

12 Cranial nerves Carry info to and

from the brain

What is the basic functional unit of the nervous system?

Neuron! What does the neuron do? Carries messages throughout

the body How does it carry the messages? By conducting electrical signals What are these signals called? Nerve impulses!

Neuron Anatomy Three parts to a

NEURON:

1. Cell body: Large, central portion of the neuron where all organelles are located.

What is its job? Interpret incoming

signals

Cell Body

Neuron Anatomy

2. Dendrites:Short, highly branched fibers What is its job? Carries impulses

toward the cell body Referred to as the

afferent process

Dendrites

Neuron Anatomy

3. Axon:Long, slightly branched fiber What is its job? Carry impulses away from the cell body Referred to as the efferent process

Neuron Anatomy

Three parts of the Axon:A. Axon hillock: site of initiation of an

action potential (point where axon and cell body meet)

B. B. Axon fiber: the main portion of the axon

C. C. Axon terminal:branched end of the axon (point of communication with other cells)

The Axon

Axon Terminals

Axon Hillock

fiber

Neuron AnatomyAdditional Parts of a Neuron

Dendrite

Cell body

Axon terminal

Myelin sheath

Nodes ofRanvier

AxonNucleus

Structure of a Typical Neuron

Neuron anatomyWhat do you remember?

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1

2

4

5

6

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dendrites

nucleus

Cell body

axon

Nodes ofRanvier

Myelinsheath

Axonterminals

Direction of impulse

3 Types of Neurons

A. Sensory Neurons: Receive incoming stimuli

Five types of sensory neurons:o Thermo-receptorso Mechano-receptorso Chemo-receptorso Photo-receptorso Pain-receptors

Thermo-receptors

Location: Skin Hypothalamus Body Core

Function: Sensation of hot and

cold Detects change in

body core temp.

Mechano-receptors Location:

Skin Skeletal muscle Inner ear

Function: Touch Pressure Muscle movement Motion Sound

Chemo-receptors

Location: Nose Tongue (taste buds) Blood vessels

Function: Smell Taste Detects levels of CO2

in blood

Photo-receptors

Location: Eyes

Function: Allow vision thru detection of light

Pain-receptors

Location: Everywhere, except

the brain Function:

Sensation of pain Detects chemicals

released by damaged cells

Three Types of Neurons

B. Motor Neurons:Carry impulses to muscles and glandsCause a response to some stimuli

C. Interneurons:Connect sensory and motor neuronsAllow for quick response (reflex action)

Three types of neurons

Structural diversity in neurons Multipolar- many dendrites, one axon

Most neurons in CNS Bipolar- one dendrite, one axon

Sensory organs Unipolar- sensory

Axon termini extend into CNS

Neurons

What is grey matter? Collective cell bodies and

dendrites of all neurons

What is white matter? Myelinated nerve fibers Axons of all neurons Can be approximately

one meter in length

White matter

Gray matter

What is a Nerve? Bundle of axons

held together by connective tissue.

What color isa nerve?

White! Why? Because axons

are white matter and they composenerves

How are nerves held together? Connective tissue What is this connective tissue called? Neuroglial cells (nerve glue) Approximately half of the

volume of the brain is composed of neuroglial cells

Most brain tumors develop in mesoglial cells – NOT neurons

What do neuroglial cells do? Support the axons Insulate the electrical impulses Like electrical tape insulates

electric wires this prevents “leaking”of electric signals

Four types of neuroglia in CNS Oligodendrocytes

Myelinating cells Astrocytes

Connects neurons and blood vessels together

Microglia Phagocytes (from bone marrow)

Ependymal cells Line ventricles of brain; produce

cerebrospinal fluid (CSF)

Neuroglia of the PNS

Schwann cells Myelinating cells Help direct axon

regeneration Satellite cells

Support, protection, regulation of molecular exchange

“Filter out” other stimuli

The Nerve Impulse

Resting potential – the charge that exists across a neuron’s membrane while at rest. -70 mV. This is the starting point for an action

potential.

The Nerve Impulse

A nerve signal or action potential is an electrochemical message of neurons. An all-or-none phenomenon – either the

fiber is conducting an action potential or it is not.

Across its plasma membrane, every cell has a voltage called a membrane potential.

The inside of a cell is negative relative to the outside.

The Nerve Impulse

Neuron at rest – active transport channels in the neuron’s plasma membrane pump: Sodium ions (Na+) out of the cell. Potassium ions (K+) into the cell.

More sodium is moved out; less potassium is moved in. Result is a negative charge inside the cell. Cell membrane is now polarized.

Sodium-Potassium Exchange Pump

Na+ flows into the cell during an action potential, it must be pumped out using sodium pumps so that the action potential will continue.

The Nerve Impulse A nerve impulse starts when pressure or

other sensory inputs disturb a neuron’s plasma membrane, causing sodium channels on a dendrite to open. Sodium ions flood into the neuron and the

membrane is depolarized – more positive inside than outside.

The Nerve Impulse This moving local reversal of voltage is called an

action potential. A very rapid and brief depolarization of the cell

membrane. Membrane potential changes from -70 mV to +35 mV.

After the action potential has passed, the voltage gated channels snap closed and the resting potential is restored. The membrane potential quickly returns to -70 mV

during the repolarization phase. An action potential is a brief all-or-none

depolarization of a neuron’s plasma membrane. Carries information along axons. An action potential is self-propagating – once started it

continues to the end.

Synapses: Junctions Between Nerves

Eventually, the impulse reaches the end of the axon.

Neurons do not make direct contact with each other.

The junction between the axon of one neuron and the dendrite of the next is called a synapse .

Synaptic Pathways

Presynaptic neurons bring action potentials toward the synapse.

Postsynaptic neurons carry action potentials away from the synapse.

A synaptic cleft is the small gap between the two neurons.

Neurotransmitters

Chemical messengers called neurotransmitters carry the message of the nerve impulse across the synapse.

Neurotransmitters

Neurotransmitters are released into the synapse and bind with receptors on the postsynaptic cell membrane, which cause ion channels to open in the new cell.

Acetylcholine – Example Neurotransmitter

Reflex Arc A simple reflex produces a very fast motor

response to a stimulus because the sensory neuron bringing information about the stimulus passes the information directly to the motor neuron.

Reflex Arc Usually, there are interneurons between

sensory and motor neurons. An interneuron may connect two neurons on the

same side of the spinal cord, or on opposite sides.

The Central Nervous System

Meninges – are membranes that protect the brain and spinal cord Dura mater (outermost layer) Arachnoid membrane ( middle layer) Pia mater (innermost layer)

The Central Nervous System Cerebrospinal Fluid (CSF) Located between the arachnoid mater and

pia mater is an area called the subarachnoid space

Continuously secreted from specialized cells (ependymal cells) in the choroid plexus in ventricles

Functions: Physical and chemical protection of the CNS

45Spinal Cord

Slender structure that is continuous with the brain

Descends into the vertebral canal and ends around the level of the first or second lumbar vertebra.

Function of the spinal cord is to carry sensory information to and from the brain

31 spinal segments: 8 cervical segments 12 thoracic segments 5 lumbar segments 5 sacral segments 1 coccygeal segment  

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Spinal Cord - Ascending and Descending Tracts

Ascending tracts - carry sensory information up to the brain

Descending tracts - carry motor information down from the brain to muscles and glands

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The Brain

Four Parts:Four Parts: Cerebrum Diencephalons Brain stem Cerebellum

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The Brain - Cerebrum

Largest part of the brain

Two halves cerebral hemispheres

Thick bundle of nerve fibers called the corpus callosum connect the two hemispheres

Lobes Frontal Parietal Temporal Occipital

Cortex Ventricles

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The Brain - Diencephalons

Thalamus - relay station for sensory information that heads to the cerebral cortex for interpretation

Hypothalamus - maintains balance by regulating many vital activities such as heart rate, blood pressure, and breathing rate.

Located between the cerebral hemispheres and is superior to the brain stem Located between the cerebral hemispheres and is superior to the brain stem

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The Brain - Brain stem

Midbrain Midbrain - controls both visual and auditory reflexes

PonsPons - regulates breathing

Midbrain Midbrain - controls both visual and auditory reflexes

PonsPons - regulates breathing

Medulla oblongataMedulla oblongata - controls many vital activities such as heart rate, blood pressure, and breathing

Connects the cerebrum to the spinal cordConnects the cerebrum to the spinal cord

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The Brain - Cerebellum

Inferior to the occipital lobes of the cerebrum

Posterior to the pons and medulla oblongata

Inferior to the occipital lobes of the cerebrum

Posterior to the pons and medulla oblongata

Coordinates complex skeletal muscle contractions that are needed for body movements

Coordinates complex skeletal muscle contractions that are needed for body movements

Diseases and Disorders of the Nervous System Cerebrovascular Accident:

CVA Commonly called a stroke The result of a ruptured

blood vessel supplying (cerebral hemorrhage) a region of the brain or a vessel is obstructed by a clot.

Brain tissue supplied with oxygen from that blood source dies, swelling occurs in the brain due to leaking of blood from vessels.

Loss of some functions or death may result

This is due often to elevated blood pressure or hypertension.

Diseases and Disorders of the Nervous System Epilepsy: This disease is

due to random, mis-firing of neurons within the brain affecting sensory and motor regions of the brain. Ranging in effects from sleep-like state of consciousness (narcolepsy), muscle paralysis and spasms (Petit mal and Grand mal seizures). Still not understood why this disease occurs. However in some cases it can result from brain trauma or injury.