Nervous System

Nervous SystemChapter 7

The Nervous system is the master controlling and communicating system of the body.

3 step process◦Sensory-uses sensory receptors to monitor changes inside and

outside of the body.

◦ Integration-processes and interprets the information and makes decision about what to do with the information (integration)

◦Motor-activation of muscles or glands in response to the stimuli

NERVOUS SYSTEM

Organization of the Nervous System

All nervous system organs are separated into two classifications by structure.

◦Central Nervous System

◦Peripheral Nervous System

Central Nervous System(CNS)

Made up of the brain and spinal cord

Main purpose is to interpret the incoming sensory information and relay tissue instructions based on past experiences or conditions.

Peripheral Nervous System(PNS)

Anything in the nervous system outside of the brain and the spinal cord.

Consists of mainly nerves◦2 types

Spinal Nerves- Carry impulses to and from the spinal cord.

Cranial Nerves- Carry impulses to and from the brain.

Functional ClassificationsThe functional classification is only

concerned with the peripheral nervous system

◦Sensory/afferent- send information from the sensory receptors to the CNS.

◦Motor/efferent-carry impulses from the CNS to the muscles and glands and initiate a response.

Motor DivisionsSomatic nervous system

◦allows conscious, voluntary movement of skeletal muscles.

◦Not all muscular activity is voluntary Skeletal muscle reflexes-stretch reflex

◦When a muscle spindle is stretched, it sends a message to the brain telling the brain to contract the muscle to prevent tearing. Patellar-tendon reflex

Stretch Reflex

Motor DivisionsAutonomic Nervous System

◦Regulates events that are automatic or involuntary Activity of cardiac muscles and smooth muscle Separated into two parts

◦Sympathetic-fight or flight, produce reactions under stress

◦Parasympathetic-rest and digest, all other autonomic functions, blood vessel dilation, pupil dilation.

Nervous Tissues◦ Astrocytes-

· Abundant, star-shaped cells· Brace neurons to their nutrient

supply· Form barrier

between capillaries and neurons

· Control the chemical environment of the brain by picking up excess ions and recapturing released

neurotransmitters.· Microglia-

· Spiderlike phagocytes that dispose of debris (dead cells)· Ependymal

· Line the cavities of the brain and spinal cord.· Their cilia help circulate CSF that fills those cavities

· CSF forms a protective cushion for the CNS

Oligodendrocytes-◦Produce a myelin sheath around nerve fibers in

the CNS◦Insulation/protection for nerves

Nervous Tissue: Support Cells

· Satellite cells· Protect neuron cell bodies by cushioning

cells

· Schwann cells· Form myelin sheath in the peripheral

nervous system

Structure of a Motor Neuron

Neuron Anatomy· Neurons = nerve cells

· Cells specialized to transmit messages· Major regions of neurons

· Cell body – nucleus and metabolic center of the cell

· Processes – fibers that extend from the cell body

Neuron Anatomy

· Body of the Cell· Metabolic center of the neuron· Nissl substance – specialized rough

endoplasmic reticulum· Neurofibrils – intermediate cytoskeleton

that maintains cell shape

Neuron Anatomy

· Extensions outside the cell body· Dendrites -carry messages toward the

cell body

· Axons –carry messages away from the cell body to another neuron

Axons and Nerve ImpulsesAxons transmit their information at their

terminal ends. All axons branch out at their end forming

thousands of axonal terminals. Once the impulse reaches the axonal

terminal it stimulates the release of neurotransmitters into the extracellular space.

Synapse

In Between each axonal terminal is a small gap called a synapse.

Neurons never touch other neurons.

Neurons

Most long nerve fibers are covered with a fatty material called myelin. ◦It protects and insulates the fibers and

increases the transmission rate.Axons outside of the CNS are insulated

(myelinated) by Schwann cells.◦Wrap themselves around axons for insulation.◦When it is wrapped around the axon, the

myelin sheath encloses the axon.

Neurons

The neurilemma is in between the myelin sheath and the Schwann cells.

The Myelin sheath is formed by many different Schwann cells, this leaves gaps of uncovered surface area that are called Nodes of Ranvier.

Multiple sclerosis

Myelin sheaths around the fibers are gradually destroyed.

Once destroyed they harden and become “scleroses”

This decreases the persons ability to control their muscles and their mobility decreases.

Central Nervous System

Clusters of neuron cell bodies and collections of nerve fibers are named nuclei when in the CNS.

They are well protected in the body within the skull or the spinal column.

These cells do not go through cell division after birth. If a cell dies, it is not replaced. Thus the need for the bony protective coverings.

CNS anatomy

Ganglia- small collection of cell bodies in the CNS.

Tracts- bundles of nerves running through

White matter-dense collections of myelinated tracts (fibers)

Gray matter-unmyelinated fibers and cell bodies

CNS

Sensory Neurons◦Afferent-go toward the brain/spinal cord for

processing◦Transmit information about outside stimuli to

the CNS◦Cutaneous receptors-skin◦Proprioceptors- muscle/tendon◦Nociceptors- pain impulses

Proprioceptors

Detect the amount of stretch or tension in skeletal muscles, tendons or joints.

Information is sent to the brain so that it can make adjustments for any changes in posture/balance.

CNS

Motor Neurons-◦Efferent, carry impulses from the CNS to the

muscles/glands for action.◦Relay the action message to the muscles

Association Neurons-◦Also known as interneurons.◦They connect the motor and sensory neurons in

neural pathways.

Sensory Receptors

Naked Nerve Endings- pain and temperature

Messner’s Corpuscles- touch receptorsPacinian Corpuscles-Deep PressureGolgi Tendon Organs (GTOs)-

proprioception (contraction)Muscle Spindle-proprioception (stretch)

Nerve Impulses-Phyisology

2 major functions◦Irritability- ability to respond to a stimuli and

convert it into a nerve impulse

◦Conductivity-ability to transmit the impulse to other neurons, muscles or glands.

Physiology

When at rest, the plasma membrane is polarized, meaning there are fewer positive ions sitting on the inner face of the membrane than on the outside.

The major positive ions on the inside of the cell are potassium (K+), and the positive ions on the outside, are sodium (Na+)

If the inside is more negative than the outside, the neuron remains inactive.

Physiology

Many types of stimuli are used to excite the neurons, to activate and create an impulse. ◦Light excites eye receptors◦Sound excited some ear receptors◦Pressure for cutaneous receptors, etc.

Physiology-depolarization

Regardless of the stimuli, the result is all the same, permeability of the cell membrane changes for a brief period.◦Once the neuron is activated, the sodium gates

of the plasma membrane open and allow the sodium (Na+) into the cell.

◦Law of diffusion- higher concentration of Na+ outside the membrane

◦Once inside the polarity of the inside of the cell changes, this process is called depolarization.

Depolarization cont.

If the stimulus is strong enough, and the rush of sodium is great enough, the neuron is activated through depolarization.◦Once depolarized the neuron will transmit the

nerve impulse (action potential).**This process is all or nothing**. The

impulse will either be sent all the way through the neuron, or not sent at all.

Repolarization

Almost immediately after the Na+ ions rush in, the membrane permeability changes◦It returns to being impermeable to Na+, but

permeable to K+, just as before.Now the K+ ions are free to float back out

into the tissue fluid. This happens rapidly, the quick outflow of these ions restores the electrical conditions of the cell. Returning to its resting (polarized) state. This process is called repolarization

Action potential

Conductivity of Neurons

Once an impulse is sent through the neuron and it reaches the axonal terminal, tiny vesicles containing the neurotransmitter chemical fuse with the axonal membrane, releasing the chemical transmitters.

These chemicals travel across the synapse and bind to the next neuron.

This will more often than not restart the action potential in that next neuron.

Reflex Arcs

Reflexes- rapid, predictable and involuntary responses to stimuli.

Autonomic reflexes-activity of smooth muscles, heart, glands◦Salivary secretion, pupil dilation◦Digestion, blood pressure, sweating

Somatic reflexes-◦Any reflex that stimulates skeletal muscles◦i.e. hand on a hot stove

Central Nervous System

The brain is the largest and most complex mass of nervous tissue in the body.◦Made up of four major regions

Cerebral hemispheres Diencephalon Brain stem Cerebellum

Cerebral Hemispheres

The largest of the four sections of the brain.

Covered in gyri, folds/ridges in the brain tissue.

The deeper grooves in the tissue are referred to as fissures.◦These fissures separate the hemispheres into

lobes, these lobes are named for the cranial bones that protect them.

Brain Anatomy

Controls somatic sensory activityRecognizes pain, cold or light touch.Center for cognition, speech and visual

perception.

**the body’s sensory pathways are crossed, meaning the left side of the sensory

cortex receives information from the right side of the body and vice-versa**

Brain Anatomy

Occipital Lobe-visual processing center◦Smallest of the four lobes of the brain◦Located in the rear of the skull

Temporal Lobe-center for auditory processing◦Important in the processing of both speech and

vision stimuli◦Contains the hippocampus which plays a key

role in long-term memory.

Brain Anatomy

Frontal Lobe-Primary motor area of the brain.◦Allows us to consciously move our skeletal

muscles. The axons of these motor neurons join together to

form the major voluntary tract, the cortico-spinal tract, which connects to the spinal cord.

◦Pathways are also crossed left brain controls right side of your body…

Frontal Lobe cont.

Broca’s area- located in the base of the precentral gyrus in the frontal lobe◦Specialized area which allows us to speak.

Usually found in the left hemisphere.◦If damaged, your ability to speak is severely

handicapped. You know what words you want to say, but are unable to vocalize them.

The frontal lobe houses the areas involed with language (word) comprehension.

Brain Anatomy

The higher level thinking/reasoning centers are believed to be in the anterior part of the frontal lobes.◦Complex memories are stored in the temporal

and frontal lobes.◦Speech areas are located in between the areas

between the temporal, parietal and occipital lobes. the frontal lobe is involved with the areas of

language comprehension, meaning of words.

Brain Anatomy

The Corpus Callosum is a large fiber tract that connects both cerebral hemispheres.◦It allows each hemisphere to connect with the

other. it increases function of the brain, because some

functions are controlled only by one side.

Brain Anatomy

White matter-myelinated tissue that makes up most of the cerebral hemisphere

Gray matter-unmyelinated cells or cell bodies of neurons found in the outermost area of the cerebral cortex, the cerebrum.

Brain Anatomy

Most of the gray matter is found in the cerebral cortex, but there are small areas of gray matter found in the cerebral cortex called the basal nuclei◦Helps control/regulate voluntary motor

activities by modifying the information sent from the CNS to skeletal muscle.

Diencephalon

Also called the interbrainFound above the brain stem and is

covered by the cerebral hemispheres.Made up of the thalamus, hypothalamus

and the epithalmus.

Thalamus

Encloses the third ventricleIt is a relay station for the sensory

information from the periphery to the sensory cortex.

Helps interpret whether the stimuli we are experiencing will be pleasant or unpleasant.

Hypothalamus

Translated “under the thalamus”Key role in regulation of body

temperature, water balance and the body’s metabolism

Center for emotional drives◦Hunger, thirst, sex, pain and pleasure

Controls the pituitary gland and mammillary bodies. Reflex centers involved in olfaction.

Epithalmus

Epithalmus forms the roof of the third ventricle

Key parts-◦Forms the roof of the third ventricle.◦Pineal body-small gland that produces

melatonin, regulates your sleep/wake patterns◦Choroid plexus-knots of capillaries within each

ventricle, together it forms the cerebrospinal fluid

Brain Stem

Approx. the size of a thumb in diameter, and about 3 inches in length.

Made up of the midbrain, pons and medulla oblongata

Provides a pathway for ascending and descending tracts of the nervous system

Also consists of many small gray matter areas, which make up the cranial nerves.

Midbrain

A relatively small aspect of the brain stemThe cerebral aqueduct travels through the

midbrain connecting the third and fourth ventricles.

The anterior aspect of the midbrain is composed of the cerebral peduncles (means little feet of the cerebrum)◦The peduncles relay impulses to the PNS

Pons

The pons is a rounded structure that is located just below the brain stem.

Translated pons means bridge◦Bridge between brain and spinal cord.

Made up mostly of fiber tracts.◦Center for breathing control.

Medulla Oblongata

Most inferior part of the brain stem. Merges with the spinal cord fluidly. Large fiber tract, regulate visceral activities.◦Center for heart rate, blood pressure,

breathing, swallowing and vomiting.

Reticular Formation

Mass of gray matter which extends the length of the brain stem.

Involved in motor control of the visceral organs.◦Reticular Activating System (RAS)- plays a role

in sleep/wake cycle. Damage to this area leads to a comatose state.

Cerebellum

Large, cauliflower like area, consists of two hemispheres like the cerebrum

Provides precise timiing for skeletal muscle and activity◦Center for balance and equilibrium◦Coordinated body movements

Center of autonomic activity

Cerebellum

Compare the brain’s intentions with the actual output of muscular activity◦Balance is judged by body position and amount of

tension in muscles, etc.If the cerebellum is damaged, direct blow to

the head, tumor, stroke, etc.◦Movements will become clumsy, disorganized

Called ataxia◦People with ataxia are not able to perform simple

tasks such as placing your finger to your nose with your eyes closed.

Protection of the CNS

Nervous system tissues are soft and delicate. Thus the need for the mass amounts of protection.◦CSF, skull, spine, etc.◦Meninges-outer layer of the dura matter

Outermost covering of the brain.◦Arachnoid mater- middle layer of the meninges

Span the subarachnoid space attach it to the innermost membrane

◦Pia matter-closest membrane to the actual brain and spinal cord. Where the CSF is housed.

◦**Meningitis leads to encephalitis**

CSF

Continually formed from blood by the choroid plexus. ◦Clusters of capillaries hanging in each of the brains

ventricles.Protects the brain and spinal cord as a watery

cushionConstantly flowing/moving through the spinal column

and skullChanges in the CSF can mean certain

diagnoses/problems◦Meningitis, tumors, MS◦Spinal Tap

Ventricles of the Brain

Blood Brain Barrier

Brain thrives on a constant, homeostatic environment

Any slight change in stasis could result in uncontrolled neural activity.

Composed of the least permeable capillaries in the whole body.

Brain dysfunction

Alzheimers- progressive degenerative disease of the brain. Brain tissue deteriorates and ultimately leads to dementia.◦Shortage of acetylcholine (Ach) and structural

changes.◦Mostly attacks centers for cognition and

memory.

Brain dysfunction

Parkinsons- problem at the basal nucleiResults of the degeneration of the

dopamine releasing neurons.This causes constant tremors, shuffling

gait and they have trouble initiating muscular movement

Brain Dysfunction

Huntington’s- degeneration of the basal nuclei, which leads to cerebral cortex.

The early stages are marked by wild, uncontrollable flapping movements of the limbs.

Later in the disease mental deterioration occurs.

Usually fatal within 15 years.

Traumatic Brain Injuries

Concussion- direct blow to the head, minor symptoms, dizziness, impaired vision, no permanent brain damage occurs.

Contusion- Bruise, marked tissue destruction/deterioration of the cerebral cortex. Usually results in loss of consciousness.

Concussions

Concussion grades-◦Grade 1-no loss of consciousness, appears dazed

or confused (bell rung)◦Grade 2-headache and confusion/vision

impairment lasting longer than 1 minute. Possible loss of consciousness Changes in cognition and emotional state, irrational.

◦Grade 3- Loss of consciousness for more than one minute, exhibit more extreme symptoms than grade 2.

Spinal Cord

Approximately 17 inches long, pathway for both afferent and efferent impulses.

Very well protected by the vertebrae and CSF.◦7 cervical vertebrae

Atlas-C1-supports the skull Axis-C2-allows for rotation

◦12 thoracic vertebrae◦5 lumbar vertebrae