1. SENSORY
-senses changes in and outside the body

2. INTEGRATIVE
-interprets these changes

3. MOTOR
-responds to the changes by initiating action in the form of muscular contractions or glandular secretions

* most rapid means of maintaining homeostasis in your body

* study of the nervous system is called neurology

2 Main divisions

1. CENTRAL NERVOUS SYSTEM (CNS)
-brain and spinal cord
-all sensations have to be relayed here to be acted on
-muscle and gland stimulation
-control center for the entire system

2. PERIPHERAL NERVOUS SYSTEM (PNS)
-connection between the CNS and the receptors, muscles and glands
-split into 2 parts

A) Afferent system (ad-- toward fero-- carry)
-carry information from the periphery of the body to the CNS

B) Efferent system (effero-- to bring out)
-carry info from CNS to the muscles and glands
-split into 2 parts

a/ Somatic Nervous System (SNS)
-efferent neurons that carry impulses to skeletal muscles tissue ONLY
-considered voluntary

b/ Autonomic Nervous System (auto-- self nomas-- law)
-ANS
-efferent neurons that carry impulses to muscle, cardiac muscle and glands
-considered involuntary

   

2 principle types of cells

Neuroglia (glia-- glue)
-support and protect neurons
-wrap around neurons bind neurons to blood vessels
-can produce a myelin sheath--covers neurons and increases impulse speed

Neurons
-conduct nerve impulses from one part of the body to another
-info-processing units

3 Parts to the Structure

1. Cell body -contain a large nucleus surrounded by granular cytoplasm

2. Dendrites
-thick branched divisions of the cell body
-bring nerve impulses toward the cell body

3. Axon
-usually a single, longer process that conducts nerve impulses from the cell body
-terminated at another neuron, muscle, or gland
-may be up to a meter long

-axon terminal
-end of axon with many branching fibers
-expands at the end to form a bulb like structure called the synaptic end bulb

Nerve Fiber-common name for an axon and its myelin sheath

Myelin sheath
-formed by a type of neuroglia cell
-phospholipid segment that wraps around an axon
-protects the axon
-increases the speed of a nerve impulse along the axon

Schwann Cells
-actual cells that from the myelin sheath
-found only in the PNS

Function:
assist in repair of injured axons by providing a tube for the axon or dendrite to grow

** production of the myelin sheath starts during the 1st year of life

** amount increases from birth to maturity
-this is the reason that adults react quicker to certain stimuli

Nodes of Ranvier
-segments on the axon that are not myelinated
-gaps in the sheath

Membrane potentials
-ion concentrations outside of the neuron are very different than inside the plasma membrane

-neurons have an unequal distribution of potassium and sodium ions

-K+ concentration is 28 times greater on the inside of the plasma membrane

-Na+ concentration is 14 times greater on the outside of the neuron

-inside the membrane are large, nondiffusible, negatively charged ions

Sodium-Potassium Pump
-fights osmosis, transports Na+ out and K+ions in in a resting neuron

-3 Na+ go out for every 2 K+ that are pumped in

-active process that uses ATP energy

Resting Membrane Potential
-neuron with a NET positive charge outside and a NET negative charge inside the membrane

EXCITABILITY vs. STIMULUS
Excitability
-ability of neurons to respond to stimuli and convert them into impulses

Stimulus
-any condition in the environment that can alter the resting membrane
potential

1. If a stimulus is applied, the membranes permeability to Na+ increases at the point of stimulation

2. Na+ rush into the membrane through certain channels. (Na+ are attracted to the large negative ions in the membrane)

3. More Na+ coming in than being pumped out. Inside shifts from a negative to positive. (Depolarization)
4. Voltage-gated channels help restore the proper Na+ and K+ concentrations (Repolarization)

(Steps 1-4 occur in a wave motion traveling down the neuron membrane)

Refractory Period
-time in which the neuron cannot generate another impulse

**under normal conditions each fiber may conduct 10 to 500 impulses per second

**larger neurons conduct more, up to 2500 per second

Threshold stimulus
-any stimulus strong enough to initiate a nerve impulse

All or None
-once a neuron is stimulated the impulse travels the entire length of the neuron

-impulse along a myelinated fiber

-myelin sheath inhibits movement of ions

-Nodes of Ranvier allow for action potentials to be generated and conducted

-ionic current flows through the extra cellular fluid and triggers an impulse at the next node

-mechanism is the same as continuous conduction BUT the impulse skips from one node to the next

Valuable to Homeostasis
-speed on impulse greatly increased
- low energy expenditure by Na-K pump because there is not as much exposed membrane

Synapse
-junction between 2 neurons
-also called synaptic clefts
-essential in homeostasis because of the ability to transmit some impulses while inhibiting others
-brain diseases and many psychiatric disorders result from bad synaptic communication
-site that certain drugs effect

2 types of synapses
electrical and chemical

**most synapses in the CNS are chemical

Function:
-neuron secretes neurotransmitters across the synaptic cleft
-post synaptic neuron has receptors to match the transmitter
-when there is a match the impulse will continue

**synaptic vessicles store the neurotransmitters
**drugs trick the receptors, you feel or don’t feel things that aren’t really happening

ACETYLCHOLINE (ACh)
-most common neurotransmitter
-released by many presynaptic axons in the PNS
-neurotransmitter used to stimulate muscles

1. ACh released from the end bulb--calcium ions trigger the release of the synaptic vessicles

2. ACh crosses the cleft

3. Fits into the post synaptic receptors

4. Stimulates the neuron--increases the permeability toward Na+

5. Depolarization begins

6. Nerve impulse continues or the muscle contracts

-after 6 months of age, neurons lose their ability to reproduce

-myelinated peripheral neurons MAY regenerate damaged axons if the cell body remains intact

-central nervous system neurons are myelinated by oligodendrocytes-- don’t help in regeneration

therefore a damaged neuron in the CNS is functionally dead

-1300g
-one of the largest organs
-4 main parts

Brain Stem
-looks like a mushroom stalk
-consist of the medulla oblongata, pons, and mesencephalon

Diencephalon
-consist of the thalamus and hypothalamus

Cerebrum
-looks like the cap of a mushroom
-spread over the diencephalon
-7/8 of the total mass of the brain
-fills most of the cranium

Cerebellum
-inferior to the cerebrum and posterior to the brain stem

Cranial Meninges
-one layer adheres to the cranial bones
-one layer adheres to the brain directly

pia mater--transparent, fibrous with many blood vessels

Cerebrospinal Fluid (CSF)
-flows between the 2 meninges, around the brain and spinal cord
-80 ml to 150 ml in the CNS
-clear and watery, contains proteins, glucose and salts

homeostatic functions
1. protection
-shock absorber
-allows the brain to “float”

2. circulation
-delivers nutrients and removes waste

**CSF circulates between the meninges and through 4 ventricles (cavities)

2 lateral ventricles (one in each hemisphere)
1 between and inferior to the lateral ventricles
1 between the inferior brain stem and the cerebellum

-2% of your body weight (brain) uses 20% of the available O₂
-constant supply of glucose energy is needed
-1 to 2 minute interruptions of O₂ may lead to death of neurons

Medulla Oblongata
-continuation of the spinal cord just superior to the foramen magnum
-contains all tracts of ascending and descending neurons that communicate information between the brain and spinal cord

-decussation occurs at the inferior portion
-crossing over of neural tracts

-allows left side of the cerebral cortex to control motor movements
on the right side of your body

Reticular formation is a region that passes through the pons,
mesencephelon, diencephalon and into the spinal cord.
-controls consciousness and arousal from sleep


3 reflex centers (vital)

cardiac center-heart rate and force of contraction
medullary rhythmicity area-controls your breathing patterns
vasoconstrictor center-controls diameter of blood vessels (non-vital) such as swallowing, vomiting, coughing, sneezing, hiccuping

Pons “bridge”-superior to the medulla oblongata, anterior to the cerebellum

function:
connect the medulla oblongata to the brain and other parts of the brain to each other

Mesencephalon (mid-brain) between the pons and diencephalon

function:
-reflex centers for movements of the eyeball and head in response to visual stimulus
-reflex centers for movements of the head and trunk in response to auditory stimuli
-oculomoter nerves
-fine touch nerves

Diencephalon (2 parts)

Thalamus
-oval structure above the midbrain
-relay sensory impulses (except smell) to the cerebral cortex
-interpretation of pain, temperature and pressure

Hypothalamus
-small portion under the thalamus
-protected by the sella turcica

*regulates a ton of stuff

-water concentrations
-hormone concentrations
-blood temperature

homeostatic functions
1. regulates autonomic nervous system
2. reception and integration of sensory impulses from viscera
3. coordinates nervous and endocrine system
4. mind over body (stress--heart rate increases)
5. rage and aggression
6. regulates body temperature
7. regulates food intake (hunger and full feelings)
8. thirst
9. sleep patterns

-sits on brain stem and forms the bulk of the brain

Cerebral cortex
-is the surface of the cerebrum
-folds in the cortex are called gyri or convolutions
-shallow grooves between the convolutions are called sulci
-deep grooves in the cortex are called fissures

Longitudinal fissure separates the cerebrum into 2 hemispheres

Corpus callosum--internal bundle of fibers that connect the 2 hemispheres

result from displacement and distortion of neurons at the moment of impact

1. CONCUSSION
-abrupt but temporary loss of consciousness following a blow to the head or the sudden stopping of a moving head
-no visible bruising but post traumatic amnesia may occur

2. CONTUSION
-visible bruising of the brain due to trauma and blood leaking from microscopic vessels
-pia mater is torn
-results in unconsciousness for several minutes to many hours

3. LACERATION
-tearing of the brain , usually from skull fractures of gunshot wound
-large blood vessels bleed into the brain and can cause cerebral hematoma, and increased cranial pressure

Lobes and sections of the brain are named after the skull bones the areas are under

Sensory Areas
interpret senses

Primary Somesthetic or general sensory area
-located directly posterior to the central sulcus in the post central gyrus of the parietal lobe

function:
-localize exact points on the body the sensation is coming from

Somesthetic association area
-posterior to the primary somesthetic area

function:
-integrates input from thalamus and lower portions of your brain and combines it with info from the primary somesthetic area
-determine shape and texture of and object without seeing object
-sense relationship of body parts
-stores memories of past sense experiences

Primary Visual
-medial occipital lobe

function:
-receives input from eyes for shape, color and movement

Visual association area
-remainder of the occipital lobe

function:
-relates past visual experiences with present information form the primary visual area and the thalamus


Primary Auditory Area
-superior part of temporal lobe near the lateral cerebral sulcus

function:
interprets basic sound (pitch and rhythm)

Auditory association area
-inferior to the primary auditory

function:
-determines if sound is speech, music or noise
-interprets speech by translating words into thoughts

Primary Gustatory--taste

Primary olfactory--smell

Gnostic area
-located among the somesthetic, visual and auditory areas
-common integrative area

function:
-forms a common thought based on all of the senses
-transmits thought out other areas so appropriate actions will be taken

Clinical application: Positron Emission Tomography


Motor Areas

Primary Motor Area
-located in the pre-central gyrus of the frontal lobe

function:
-stimulation results in muscular contraction usually on the opposite side of the body
-controls specific muscle groups

Premotor Area
-anterior to the primary motor area

function:
-learned motor activities of a complex and sequential nature
-controls skilled movements

Frontal Eye Field Area--controls scanning movements of the eyes

Motor Speech Area (Broca’s Area)
-located in the frontal lobe just superior to the lateral cerebral sulcus on the left hemisphere
-impulses sent from here to the primary motor area to the muscles

function:
-translates thoughts into speech

Disorders in this area:
Aphasia--inability to speak
Agraphia--inability to write
Word deafness--inability to understand spoken words
Word blindness--inability to understand written words

Association Areas
-tracts that connect motor and sensory areas
-occupies greater portion of the lateral surface of the occipital, parietal, temporal and frontal lobes (anterior to the motor areas)

function:
memory, emotions, reasoning, will, judgment, personality traits and intelligence

Split Brain Concept

anatomically different:
-frontal lobe of the left hemisphere is smaller
-in left handed people the right parietal and occipital lobes are narrower

functionally different:
-right hemisphere:
left handed, music and artistic awareness, space and pattern perception, imagination

-left hemisphere:
right handed, language, numerical and scientific skills, sign language and reasoning

-2nd largest part
-separated from the cerebrum by the transverse fissure

function:
-coordinates subconscious movements of skeletal muscles
-coordinates information from receptors in muscles and tendons with impulses your motor areas are trying to have you do (ensures smooth movements of your body)
-maintains posture and equilibrium
-predicts future position of body part during movement

Damaged cerebellum:
-lack of muscle control
-change of speech pattern
-severe dizziness
-disturbances of gait (walking)

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