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.)