The Nervous System

Chapter 7

The nervous system allows the body to detect, interpret and respond to stimuli.

◦ A stimulus is any change in the external or internal environment.

Overview

Functions of the Nervous System

1. Sensory input · To monitor changes occurring inside and outside the

body using sensory receptors

2. Integration· To process and interpret sensory input

3. Motor output

· To produce a response, if needed, that activates muscles or glands (effectors)

Functions of the Nervous System

Organization of the Nervous System

• Two main divisions:• Central nervous system

(CNS)

• Brain

• Spinal cord

• Peripheral nervous system (PNS)

• Nerves outside the brain and spinal cord

Organization of the Nervous System

Nervous Tissue

• Nervous tissue is made up of neuroglial (supporting ) cells and neurons.

• Neuroglial cells outnumber neurons.

• Neuroglial cells do not conduct impulses; neurons do.

• Neuroglial cells generally support, insulate and protect delicate neurons.

• Neuroglial cells are capable of dividing; neurons are not.

Supporting Cells

Neurons

• Neurons = nerve cells

• Specialized to transmit messages (impulses)

• Major regions of neurons:

• Cell body – nucleus and metabolic center of the cell

• Processes – fibers that extend from the cell body

• Dendrites – carry impulses towards the cell body

• Axon – carries impulse away from the cell body

Neurons

Neurons

· Myelin sheath – fatty material surrounding axons produced by Schwann cells

· Insulates axons

· Speeds up transmission

Classification of Neurons

· Sensory (afferent) neurons

· Carry impulses from sensory receptors to the CNS

· Motor (efferent) neurons

· Carry impulses from the CNS to muscles or glands (effectors)

· Interneurons (associative neurons)

· Connect sensory and motor neurons

Classification of Neurons

The Nerve Impulse

· The plasma membrane at rest is polarized (resting potential)

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

· Active transport by the sodium-potassium pump maintains this polarity

Starting the Nerve Impulse

• A stimulus depolarizes the neuron’s membrane

• Sodium ions (Na+) rush inside the membrane

• This initiates an impulse (action potential) in the neuron

Continuing the Nerve Impulse

· The impulse continues to move toward the cell body

· Potassium ions rush out of the neuron which repolarizes the membrane

· The sodium-potassium pump then establishes the original polarity

The Threshold

· A minimum stimulus (threshold) is needed to start an impulse

· If the threshold is met, a nerve impulse starts, and continues over the entire axon (all or none response)

· Impulses travel faster when fibers have a myelin sheath

Neuron to Neuron Communication

• Neurons do not touch each other because of a gap called the synapse.

• Impulses cross the synapse with the help of chemicals called neurotransmitters.

• Neurotransmitters released from the axon terminal diffuse across the synapse

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

• An action potential (impulse) is started

Neuron to Neuron Communication

The Central Nervous System

· Brain and spinal cord

Regions of the Brain

· Cerebral hemispheres

· Diencephalon

· Brain stem

· Cerebellum

Cerebrum◦ Voluntary activities (motor initiation), intelligence,

learning, judgment, sensory interpretation◦ Two hemispheres connected by the corpus

callosum Right hemisphere controls the left side of the body

and visa-versa◦ Deep folds and grooves increase surface area

Gray matter – outer layer of dense nerve cell bodies White matter – inner layer of myelinated nerve fibers

Cerebrum

Lobes of the Cerebrum

· Fissures (deep grooves) divide the cerebrum into lobes

· Surface lobes of the cerebrum

· Frontal lobe

· Parietal lobe

· Occipital lobe

· Temporal lobe

Specialized Areas of the Cerebrum

· Primary sensory area (postcentral gyrus)– receives impulses from the body’s sensory receptors

· Primary motor area (precentral gyrus)– sends impulses to skeletal muscles

Sensory and Motor Areas of the Cerebral Cortex

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

· Broca’s area – speech

· Gustatory area - taste

· Visual area - sight

· Auditory area - hearing

· Olfactory area – smell

· Interpretation areas of the cerebrum

· Speech/language region

· Language comprehension region

· General interpretation area

Specialized Area of the Cerebrum

· Two hemispheres with convoluted surfaces

· Provides involuntary coordination of body movements

· http://www.youtube.com/watch?feature=player_detailpage&v=F7Yw26zHoEw

Cerebellum

· Sits on top of the brain stem· Enclosed by the cerebral

hemispheres· Made of three parts

· Thalamus – sensory relay station

· Hypothalamus – controls hunger, thirst, fatigue, anger, temperature, coordinates with endocrine system

· Epithalamus – pineal gland and choroid plexus (forms cerebrospinal fluid)

Diencephalon

• Attaches to the spinal cord

• Controls vital functions – blood pressure, heart rate, breathing, swallowing, vomiting

• Parts of the brain stem

• Midbrain

• Pons

• Medulla oblongata

Brain Stem

Scalp and skin Skull and vertebral column Meninges Cerebrospinal fluid (CSF) Blood-brain barrier

Protection of the Central Nervous System

Figure 7.17a

Superiorsagittal sinus

Subduralspace

Subarachnoidspace

Skin of scalp

Periosteal

Bone of skull

MeningealDuramater

Arachnoid materPia mater

Arachnoid villus

Bloodvessel

Falx cerebri(in longitudinal

fissure only)

Periosteum

(a)

Figure 7.17b

Skull

Scalp

Superiorsagittal sinusDura mater

Transversesinus

Temporalbone

Occipital lobeTentoriumcerebelli

Arachnoid materover medulla

oblongata(b)

Cerebellum

Cerebrospinal Fluid (CSF)

Similar to blood plasma composition Formed by the choroid plexus

◦ Choroid plexuses–capillaries in the ventricles of the brain

Forms a watery cushion to protect the brain Circulated in arachnoid space, ventricles,

and central canal of the spinal cord

Figure 7.18a

Septumpellucidum

Inferiorhorn

Lateralaperture

Lateral ventricle

Anterior horn

Interventricularforamen

Third ventricle

Cerebral aqueduct

Fourth ventricle

Central canal

(a) Anterior view

Figure 7.18b

Lateral ventricle

Anterior horn

Interventricularforamen

Third ventricle

Cerebral aqueduct

Fourth ventricle

Central canal

Posteriorhorn

Inferior horn

Medianaperture

Lateralaperture

(b) Left lateral view

Blood-Brain Barrier

Includes the least permeable capillaries of the body

Excludes many potentially harmful substances

Useless as a barrier against some substances◦ Fats and fat soluble molecules◦ Respiratory gases◦ Alcohol◦ Nicotine◦ Anesthesia

Traumatic Brain Injuries (TBI)· Concussion

· Slight or mild brain injury· Bleeding & tearing of nerve fibers · Recovery likely with some memory loss

· Contusion· A more severe TBI· Nervous tissue destruction occurs· Nervous tissue does not regenerate

· Cerebral edema· Swelling from the inflammatory response· May compress and kill brain tissue

Cerebrovascular Accident (CVA)

· Commonly called a stroke

· The result of a ruptured blood vessel supplying a region of the brain

· Brain tissue supplied with oxygen from that blood source dies

· Loss of some functions or death may result

Alzheimer’s Disease

· Progressive degenerative brain disease· Mostly seen in the elderly, but may begin in middle age· Structural changes in the brain include abnormal protein

deposits and twisted fibers within neurons· Victims experience memory loss, irritability, confusion

and ultimately, hallucinations and death

PNS: Cranial Nerves

Twelve pairs of nerves that mostly serve the head and neck

Only the pair of vagus nerves extend to thoracic and abdominal cavities

Most are mixed nerves, but three are sensory only

PNS: Cranial 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 one eye muscle 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 IX Glossopharyngeal nerve—sensory for taste; motor fibers to

the pharynx X Vagus nerves—sensory and motor fibers for pharynx, larynx,

and viscera XI Accessory nerve—motor fibers to neck and upper back XII Hypoglossal nerve—motor fibers to tongue

Figure 7.24

I Olfactory II Optic

III Oculomotor

IV TrochlearVI Abducens

V Trigeminal V Trigeminal

VII Facial

Vestibularbranch

Cochlearbranch

VIII Vestibulocochlear

X Vagus

IX GlossopharyngealXI AccessoryXII Hypoglossal

PNS: Cranial Nerves Device

Oh – Olfactory Oh – Optic Oh – Oculomotor To – Trochlear Touch – Trigeminal And – Abducens Feel – Facial Very – Vestibulocochlear Green – Glossopharyngeal Vegetables – Vagus Ah – Accessory Ha – Hypoglossal

Spinal Cord

• Extension of brain stem• Controls reflexes• Surrounded by meninges (membranes)• Central canal is filled with spinal fluid• Protected by vertebrae.• Gray matter – mostly cell bodies• White matter – myelinated cell fibers

Spinal Cord

· Extends from the medulla oblongata to the region of T12

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

· Enlargements occur in the cervical and lumbar regions

Spinal Cord

· Central canal filled with cerebrospinal fluid

The Reflex Arc

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

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

· No brain involvement

Peripheral Nervous System

· Nerves and ganglia outside the central nervous system

· A nerve consists of neuron fibers bundled by connective tissue

· There are twelve pairs of cranial nerves and 31 pairs of spinal nerves.

· Somatic nerves control voluntary functions and autonomic nerves control involuntary functions

A nerve has layers of connective tissue.

The endoneurium is a delicate connective tissue wrapping around each fiber.The perineurium is a coarser wrapping around a bundle of fibers (fascicle).The epineuriun is a tough connective tissue wrapping around all the fascicles.

Nerves may contain just sensory neurons, just motor neurons, or both (mixed).

PNS: Spinal Nerves

There is a pair of spinal nerves at the level of each vertebrae.

Formed by the combination of the ventral and dorsal roots of the spinal cord

Named for the region from which they arise

PNS: Spinal Nerve Plexuses

Plexus–networks of nerves serving motor and sensory needs of the limbs

Form from ventral rami of spinal nerves in the cervical, lumbar, and sacral regions

Four plexuses:◦ Cervical◦ Brachial◦ Lumbar◦ Sacral

Cervicalnerves

Thoracicnerves

Lumbarnerves

Sacralnerves

Ventral rami formcervical plexus

(C1 – C5)

Ventral rami formbrachial plexus

(C5 – C8; T1)

No plexusformed

(intercostalnerves)(T1 – T12)

Ventral rami formlumbar plexus

(L1 – L4)

Ventral rami formsacral plexus

(L4 – L5; S1 – S4)

(a)

C1

T1

L1

2345678

23456

789

1011

12

2

3

4

5

S1

234

Figure 7.25a

PNS: Autonomic Nervous System

Motor subdivision of the PNS◦ Consists only of motor nerves

Also known as the involuntary nervous system◦ Regulates activities of cardiac and smooth

muscles and glands Two subdivisions

◦ Sympathetic division◦ Parasympathetic division

PNS: Autonomic Functioning

Sympathetic—“fight or flight”◦ Response to unusual stimulus◦ Takes over to increase activities◦ Remember as the “E” division

Exercise, excitement, emergency, and embarrassment

Parasympathetic—“housekeeping” activites◦ Conserves energy◦ Maintains daily necessary body functions◦ Remember as the “D” division

digestion, defecation, and diuresis

Anatomy of the Autonomic Nervous System

Comparison of Somatic and Autonomic Nervous Systems