The Nervous System

The nervous system is the master controlling and communicating system of the body. Every thought, action, and emotion reflects its activity. Its cells communicate by electrical and chemical signals, which are rapid and specific, and usually cause almost immediate responses.

Functions of the Nervous SystemFunctions of the Nervous System Sensory input – gathering information

To monitor changes occurring inside and outside the body

Changes = stimuli

Integration To process and interpret sensory input and

decide if action is needed

Motor output

A response to integrated stimuli

The response activates muscles or glands

For example, when you are driving and see a red light ahead (sensory input), your nervous system integrates this information (red light means “stop”), and your foot goes for the brake (motor output).

Structural Classification of the Structural Classification of the Nervous SystemNervous System Two divisions:

Central nervous system (CNS)

Brain

Spinal cord

Peripheral nervous system (PNS)

Cranial nerves

Spinal nerves

Functional Classification of the PNSFunctional Classification of the PNS It divides them into TWO subdivisions:

1-Sensory (afferent) division

Nerve fibers that carry information to the central nervous system from:

- sensory receptors in the skin, skeletal muscles and joints (somatic sensory fibers).

- Sensory receptors in the visceral organs (visceral sensory fibers)

2-Motor (efferent) division

Nerve fibers that carry impulses away from the central nervous system ( to Muscles &Glands).

It has two subdivisions

1-Somatic nervous system = voluntary, it controls skeletal muscles

N. B. skeletal muscle reflexes are involuntary

2-Autonomic nervous system = involuntary, it controls smooth &cardiac muscles &glands

This also is divided into:

- sympathetic &

- parasympathetic

Organization of the Nervous SystemOrganization of the Nervous System

Histology of Nervous TissueHistology of Nervous Tissue

• Despite the complexity of the nervous system, there are only two functional cell types:

• Neurons - excitable nerve cells that transmit electrical signals

• Neuroglia (glial) cells (Astrocytes, Microglia, Ependymal cells, Oligodendrocytes ) are the supporting cells.

Neuroglia cells - 2 types in the Peripheral NS

Satellite cells

- surround neuron cell bodies in the periphery

- Protective , cushioning cells

Schwann cells (neurolemmocytes)

• surround axons/dendrites and form the myelin sheath around larger nerve fibers in the periphery

• similar to oligodendrocytes in function – insulators

Neuron (Nerve cell)Neuron (Nerve cell)Cells specialized to transmit messages

Differ structurally but have common features, but Differ structurally but have common features, but

all have :all have : A Cell body with nucleus and the usual organelles

Except centrioles

Extensions (proceses) outside the cell body Dendrites – conduct impulses toward the cell

body

Axons – conduct impulses away from the cell body

Neuron AnatomyNeuron Anatomy

Axons and Nerve ImpulsesAxons and Nerve Impulses

Axons end in axonal terminals

Axonal terminals contain vesicles which is full of chemical transmitter called neurotransmitters

Axonal terminals are separated from the next neuron (neuroneural junction) by a gap called Synaptic cleft (Synapse)

Nerve Fiber CoveringsNerve Fiber Coverings

**Most long nerve fibers areMost long nerve fibers are

covered with a covered with a whitish, fatty whitish, fatty

materialmaterial called called Myelin Myelin with with waxy appearance. It : waxy appearance. It :

-insulates the fiber &-insulates the fiber &

-Increases transmission rate-Increases transmission rate

**Axons outside CNS are Axons outside CNS are wrapped by Schwann Cellswrapped by Schwann Cells

Figure 7.5

Neuron Cell Body LocationNeuron Cell Body Location

Most are found in the central nervous system so CNS appears in two colors:

Gray matter – cell bodies and unmyelinated fibers

Nuclei – clusters of cell bodies within the white matter of the central nervous system

Ganglia – collections of cell bodies outside the central nervous system

White matter- collection of myelinated fibers.

Functional Classification of NeuronsFunctional Classification of Neurons1-Sensory (afferent) neurons carry impulses from :

Cutaneous sense organs (skin receptors)

Proprioceptors – receptors that detect stretch or tension in muscles and tendons and joints

to the CNS. Their cell bodies are in ganglia outside the CNS.

2-Motor (efferent) neurons carry impulses from the central nervous system to muscles and glands ,their cell bodies are always in CNS.

3-Interneurons (association neurons)

Their cell bodies are always found in CNS.

Connect sensory and motor neurons in neural pathways.

Neuron ClassificationNeuron Classification

Figure 7.6

Functional Properties of NeuronsFunctional Properties of Neurons

Irritability – ability to respond to stimuli

Conductivity – ability to transmit an impulse

The plasma membrane at rest is polarized that is:

Fewer positive ions are inside the cell than outside the cell.

Starting a Nerve ImpulseStarting a Nerve Impulse

Depolarization – a stimulus depolarizes the neuron’s membrane

A deploarized membrane allows sodium (Na+) to flow inside the membrane

The exchange of ions initiates an action potential in the neuron

The Action PotentialThe Action Potential

If the action potential (nerve impulse) starts, it is propagated over the entire axon

Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane

The sodium-potassium pump restores the original configuration This action requires ATP

Continuation of the Nerve Impulse Continuation of the Nerve Impulse between Neuronsbetween Neurons

Impulses are able to cross the synapse to another nerve

Neurotransmitter is released from a nerve’s axon terminal

The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter

So an action potential is started in the dendrite of the next neuron.

How Neurons Communicate at How Neurons Communicate at SynapsesSynapses

Figure 7.10

The Reflex ArcThe Reflex Arc

Reflex – rapid, predictable, and involuntary responses to stimuli.

Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector

Ref. arc have a minimum 5 elements:

Simple Reflex ArcSimple Reflex Arc

Figure 7.11b, c

Types of ReflexesTypes of Reflexes

Autonomic reflexes as:

Salivary gland secretion

Heart and blood pressure regulation

Changes in size of the pupil

Digestive system regulation

Somatic reflexes as:

Activation of skeletal muscles

REFEXES MAY USE :

• SPINAL CORD, as flexor reflex and knee jerk .

• BRAIN ( for proper evaluation) as response of pupils to light

• N.B.1, Exaggerated, Distorted or Absent reflexes indicate nervous system disorder.

• N.B.2, Reflex changes often occur before the pathological condition become obvious

Central Nervous System (CNS)Central Nervous System (CNS)

CNS develops from the embryonic neural tube

By the fourth week the anterior end begins to expand and brain formation begins, The rest of the tube becomes the spinal cord

The central canal becomes enlarged in 4 regions of the brain to form the ventricles:

-Four chambers within the brain

-Filled with cerebrospinal fluid

Regions of the BrainRegions of the Brain

Cerebral hemispheres

Diencephalon

Brain stem

CerebellumFigure 7.12

Cerebral Hemispheres (Cerebrum)Cerebral Hemispheres (Cerebrum)

Paired (left and right) superior parts of the brain

Include more than half of the brain mass

Figure 7.13a

Layers of the CerebrumLayers of the Cerebrum Gray matter(outside)

Outer layer

Composed mostly of neuron cell bodies

White matter(inside)

Axons(tracts)

Example: corpus callosum connects the two hemispheres

Basal nuclei – internal islands in the gray matter

Figure 7.13a

Cerebral Hemispheres (Cerebrum)Cerebral Hemispheres (Cerebrum)

The surface is made of ridges (gyri) and grooves (sulci)

Figure 7.13a

Specialized Area of the CerebrumSpecialized Area of the Cerebrum Cerebral areas involved in special senses

Gustatory area (taste)

Visual area

Auditory area

Olfactory area

Interpretation areas of the cerebrum

Speech/language region

Language comprehension region

General interpretation area

Specialized Area of the CerebrumSpecialized Area of the Cerebrum

Figure 7.13c

DiencephalonDiencephalon

Sits on top of the brain stem

Enclosed by the cerebral heispheres

Made of three parts Thalamus

Hypothalamus

Epithalamus

DiencephalonDiencephalon

Figure 7.15

ThalamusThalamus Surrounds the third ventricle

The relay station for sensory impulses (except olfaction)

Transfers impulses to the correct part of the cortex for localization and interpretation

HypothalamusHypothalamus (Under the thalamus)

Important autonomic nervous system center

Helps regulate body temperature

Controls water balance

Regulates metabolism

An important part of the limbic system (emotions)

The pituitary gland is attached to the hypothalamus

EpithalamusEpithalamus Forms the roof of the third ventricle

Houses the pineal body (an endocrine gland)

Includes the choroid plexus which forms the cerebrospinal fluid

Brain StemBrain Stem

Attaches to the spinal cord and is formed of:

Midbrain Pons Medulla oblongata

Brain StemBrain Stem

Figure 7.15a

MidbrainMidbrain Reflex centers for vision and hearing

PonsPons The bulging middle part of the brain stem

Mostly composed of fiber tracts

Includes nuclei involved in the control of breathing

Medulla OblongataMedulla Oblongata The lowest part of the brain stem Merges into the spinal cord Includes important fiber tracts Contains important control centers

Heart rate control Blood pressure regulation Breathing Swallowing Vomiting

CerebellumCerebellum

Two hemispheres with convoluted surfaces

Provides involuntary coordination of body movements

Protection of the Central Nervous Protection of the Central Nervous SystemSystem

Scalp and skin

Skull and vertebral column

Meninges

Cerebrospinal

fluid

Blood brain

barrier Figure 7.16a

MeningesMeninges• Three connective tissue membranes lie external to the

CNS

– dura mater,

-arachnoid mater, and

- pia mater

• Functions of the meninges

• Cover and protect the CNS

• Protect blood vessels and enclose venous sinuses

• Contain cerebrospinal fluid (CSF)

• Form partitions within the skull

MeningesMeninges

Figure 12.24a

Dura MaterDura Mater• Leathery, strong meninx composed of two

fibrous connective tissue layers

• The two layers separate in certain areas and form dural sinuses

• Double-layered external covering

• Periosteum – attached to surface of the skull

• Meningeal layer – outer covering of the brain

• Folds inward in several areas

Arachnoid MaterArachnoid Mater• The middle meninx, which forms a loose brain covering

• It is separated from the dura mater by the subdural space

• Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels

• Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood

Pia MaterPia Mater

Deep meninx composed of delicate connective tissue that clings tightly to the brain

Cerebrospinal FluidCerebrospinal Fluid

Similar to blood plasma composition

Formed by the choroid plexus

Forms a watery cushion to protect the brain

Circulated in subarachnoid space, ventricles, and central canal of the spinal cord

Ventricles and Location of the Ventricles and Location of the Cerebrospinal FluidCerebrospinal Fluid

Figure 7.17a

Blood Brain BarrierBlood Brain Barrier Includes the least permeable capillaries

of the body to exclude many potentially harmful substances

It can not prevent some substances Fats and fat soluble molecules Respiratory gases Alcohol Nicotine Anesthesia

Traumatic Brain InjuriesTraumatic Brain Injuries Concussion

Slight brain injury No permanent brain damage

Contusion Nervous tissue destruction occurs Nervous tissue does not regenerate

Cerebral edema Swelling due to inflammation of the brain May compress brain tissue causing death

Cerebrovascular Accident (CVA)Cerebrovascular Accident (CVA) Commonly called a stroke

Caused by ruptured or obstructed blood vessel supplying a region of the brain

Brain tissue supplied with oxygen from that blood source will die.

Loss of some functions or death may result

Alzheimer’s DiseaseAlzheimer’s Disease

• Progressive degenerative brain disease

Mostly seen in the elderly, but may begin in middle age leading to memory loss, irritability, confusion and ultimately, hallucinations and death

Spinal CordSpinal Cord

Extends from the medulla oblongata to below T12

Below T12 is the cauda equina (a collection of spinal nerves)

Enlargements present in the cervical and lumbar regions

Figure 7.18

Spinal Cord AnatomySpinal Cord Anatomy Exterior white mater – conduction tracts

Internal gray matter - mostly cell bodies

Dorsal (posterior) horns

Anterior (ventral) horns

Central canal filled with cerebrospinal fluid

Figure 7.19

Peripheral Nervous SystemPeripheral Nervous System Nerves and ganglia outside the CNS

Nerve = bundle of neuron fibers outside CNS bundled by connective tissue, may be:

-Afferent (sensory) nerves – carry impulses toward the CNS

-Efferent (motor) nerves – carry impulses away from the CNS

-Mixed nerves – both sensory and motor fibers

Cranial NervesCranial Nerves

12 pairs of nerves that mostly serve the head and neck

Numbered in order, front to back

Most are mixed nerves, but three are sensory only.

Distribution of Cranial NervesDistribution of Cranial Nerves

Figure 7.21

Cranial NervesCranial Nerves

I Olfactory nerve – sensory for smell

II Optic nerve – sensory for vision

III Oculomotor nerve – motor fibers to eye muscles

IV Trochlear – motor fiber to eye muscles

Cranial NervesCranial Nerves

V Trigeminal nerve – sensory for the face; motor fibers to chewing muscles

VI Abducens nerve – motor fibers to eye muscles

VII Facial nerve – sensory for taste; motor fibers to the face

VIII Vestibulocochlear nerve – sensory for balance and hearing

Cranial NervesCranial Nerves

IX Glossopharyngeal nerve – sensory for taste; motor fibers to the pharynx

X Vagus nerves – sensory and motor fibers for pharynx, larynx, and abdominal viscera

XI Accessory nerve – motor fibers to neck and upper back

XII Hypoglossal nerve – motor fibers to tongue

Spinal NervesSpinal Nerves

There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs

Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord

Spinal nerves are named for the region from which they arise.

Spinal NervesSpinal Nerves

Figure 7.22a

Examples of Nerve DistributionExamples of Nerve Distribution

Figure 7.23

Autonomic Nervous SystemAutonomic Nervous System

The involuntary part of the peripheral nervous system

Consists of motor nerves only

Divided into two divisions

Sympathetic division

Parasympathetic division

Differences Between Somatic and Differences Between Somatic and Autonomic Nervous SystemsAutonomic Nervous Systems Nerves

Somatic – one motor neuron

Autonomic – two neurons ,preganglionic and postganglionic nerves

Effector organs

Somatic – skeletal muscle

Autonomic – smooth muscle, cardiac muscle,and glands

Nerurotransmitters

Somatic – always use acetylcholine

Autominic – use acetylcholine, epinephrine, or norepinephrine

Figure 7.24

Sympathetic Division (thoracolumberSympathetic Division (thoracolumber))

Slide 7.70

Originates from T1 through L2

Norepinephrine and epinephrine are neurotransmitters to the effector organs

Parasympathetic Division(craniosacralParasympathetic Division(craniosacral))

Originates from the brain stem and S1 through S4

Always uses acetylcholine as a neurotransmitter

Sympathetic PathwaysSympathetic Pathways

Slide 7.71

Figure 7.26

Anatomy of the Autonomic Nervous Anatomy of the Autonomic Nervous SystemSystem

Figure 7.25

Autonomic FunctioningAutonomic Functioning Sympathetic – “fight-or-flight”

Increase activities

Remember the “E” division = exercise, excitement, emergency, and embarrassment

Parasympathetic – housekeeping activities

Conserves energy

Maintains daily necessary body functions

Remember as the “D” division - digestion, defecation, and diuresis

Development Aspects of the Nervous SystemDevelopment Aspects of the Nervous System The nervous system is formed during the first

month of embryonic development so any early maternal infection can have extremely harmful effects on the baby.

The hypothalamus is one of the last areas of the brain to develop.

No more neurons are formed after birth, but growth and maturation continues for several years.

The brain reaches maximum weight in young adults (1300-1400gr.)