Transcript Major Divisions in the Central Nervous System

Major Divisions in the Central Nervous System
SPINAL- large mixed nerve
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White Matter- outer portion of the
mylenated fibers running up and
down.
Grey Matter- cell bodies of
associative and motor neurons.
– 31 pairs of spinal nervessensory fibers enter dorsal root
(surface)- than into gray matter.
– Destruction of dorsal root= loss
of sensation in parts which
supply sensory neurons.
– Destruction of ventral rootsmuscular paralysis of body part
supplied by motor neuron.
– Two main functions of spinal
cord:
a) Coordinating center- reflexes
b) Connects peripheral nervous
system to brain
BRAIN- meninges,
cerebralspinal fluid (lymph)
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4 Ventricles- cavities; fluid circulates
12 pairs of cranial nerves
2 large hemispheres (longitudinal
halves)
Divided into three region: Forebrain,
midbrain, hindbrain.
Meninges- protective membrane
a) 3 membranes- Durameter,
Arachnoid layer, Piameter
b) Protect brain and spinal cord
STRUCURAL DIVISIONS OF THE HUMAN BRAIN
HINDBRAIN
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MEDULLA- essential to life (vital center)
– Controls breathing rate (vagus, phrenic
nerve)
– Controls heart rate (vagus-acetylcholinslower rate; accelerator- adrenalin)
– Blood Pressure (diameter of arterioles)
– ½ of the cranial nerves originate here
– Swallowing, vomiting, coughing, salivation
center
– Reticular activating system- nerves to
activate or awaken forebrain. Forebrain
cannot react unless awakened by reticular
formation. Destruction leads to death or
coma. More extensive than inhibitory center
– Reticular inhibitory system- inhibition of
voluntary movements (movements induced
by cerebral cortex, reflexes, depressions of
respiration and blood pressure.)
CEREBELUM- The Little Brain
• Largest Segment of the hindbrain
• Receives impulses from proprioreceptors in muscles,
tendons, joints- coordinates muscle activity )impules
away from motor cortex of cerebrum)
• Coordinates impulses from the eyes, semicircular
canals to maintain balance
*In birds- large, intricate, used for balance in
flight
Pons- “bridge”
• *a) relay station to carry impulses from
one side of the cerebellum to another
• b) coordinates left side of brain with
right
• c) some cranial nerves originate here
Midbrain
• 1. In man - smaller and inconspicuous
• ( optic lobes of fish, frogs, reptiles, birds)
• 2. Contains all afferent and efferent nerve
fibers that pass between spinal cord and
higher brain centers
• 3. Contains all nuclei involved in many
autonomic reflexes
Forebrain
• Consists of the hypothalamus, pituitary,
thalamus, cerebrum.
Hypothalamus: Homeostatic Center
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homeostatic center - receives impulses from cerebral cortex
a) regulates body temperature ( dilate or constrict skin arterioles)
*b) regulates water balance - sensitive to osmotic pressure of blood stimulates ADH secretion
- regulates many autonomic functions
c) regulates food intake - feeding and satisfy center
d) regulates gastric secretion - if overactive leads to ulceration-hemorrhage
e) regulates adenohypophysis - releasing factor
ACTH - RF - hormonal (cortex) / neural adrenal medula
TSH - RF - fight-or-flight
Prolactin - RF
LH - RF
FSH - RF
STH - RF
*f) integration center for sex drive, anger, thirst, pleasure
*g) produces posterior pituitary hormone ( ADH and oxytocin)
Thalamus – Main Relay Center
• a) main relay center between brainstem (midbrain, pons,
medulla, and cerebrum)
• b) thalamus and cerebral cortex act as a unit ( many
functions cannot be separated)
• c) pain center (from free nerve endings)
• *d) relay station between sense organs and cerebrum restricts many impulses (during sleep) that normally
reach cortex
• Memory - may be stored by chemical and structural
changes in neurons of sensory cortex
• Emotions - may originate from interactions between
cerebral cortex and limbic system
CEREBRUM- sets man apart from other
vertebrates. Volume is approx 1400g
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Part of learning, reasoning, memory, judgment (10-14 billion
associative neurons)
Speech and writing area- hearing center, visual center,
associative area.
Central Sulcus (fissure of Rolando)- bewtween frontal and
parietal lobes
a) Broca’s Area
Frontal- voluntary motor responses- motor control of
speech
Parietal- Sensory cortex- recieves impulses from sensory
neurons for touch, taste.
Left Frontal- Broca’s Area (motor)
Left Parietal- Wernicke’s area (comprehension of spoken and
written language) 2 areas joined by nerve bundle.
Right Frontal and Parietal- associated with music and spatial
relations
– Temporal Lobe- delineated by lateral sulcus (Fissure of
Sylvius). Auditory (taste, smell, hearing, speech).
– Occipital Lobe- visual cortex. Neurons are specific to one
aspect of the shape of an object.
– Left Brain- Speech, logic, writing, math.
– Right Brain- discrimination of shape and form.
Autonomic Nervous System
• The autonomic nervous system is a
functional division of the peripheral
nervous system. It consists of motor
neurons which control involuntary organs
(heart, smooth muscles of viscera digestive, respiratory, excretory, circulatory
organs)
• Purpose: To maintain homeostasis constant internal environment
Divisions
SYMPATHETIC DIVISION: neurons which enable the body
to react to stress of emergencies (“fight-or-flight”)
PARASYMPATHETIC DIVISION: neurons which return the
body to basic “non-emergency” level
1. Heart: neurons secrete adrenalin at pacemaker.
Increase heart rate
1. Heart: vagus nerve releases more *acetylcholine (main
neurotransmitter of parasympathetic system).
Decreases heart rate.
2. Liver: glycogen is broken down to glucose and glucose is
released into blood.
Increase blood glucose.
2. Does not affect the liver.
3. Lungs: dilation of bronchioles.
Increases oxygen intake and carbon dioxide output.
3. Lungs: constriction of bronchioles.
Decreases oxygen intake and carbon dioxide output.
4. Digestive Organs: Decreased peristalsis.
Slows digestive process.
4. Digestive Organs: increased peristalsis.
Increases digestive process.
5. Skeletal Muscles: dilation of arterioles.
Increases blood supply.
5. Skeletal Muscles: constriction of arterioles.
Decreases blood supply.
6. Breathing Rate: increases number of impulses to rib muscles
and diaphragm.
6. Breathing Rate: decreases number of impulses to rib muscles
and diaphragm.
Decreases breathing rate.
7. Kidney: decreases urine volume
7. Does not affect the kidney.
Summary: Visual
Peripheral Nervous System: PNS
• The peripheral nervous system is composed of the
cranial and spinal nerves. (The autonomic fibers,
sometimes classified as a separate cranial nerves
connected with the brain. They are structurally different
form the spinal nerves, of which there are 31 pairs.
Some of the cranial nerves are entirely sensory
(afferent), some of which there are 31 pairs. Some of
the cranial nerves are entirely sensory (afferent), some
are wholly motor (efferent); the remainder contain both
sensory and motor fibers and are known as mixed
nerves. Although the number differs, the cranial nerves
are sufficiently similar in fish, amphibians, reptiles, birds,
and man to have the same names. The names are
based partly on the system innervated and partly on the
structure and path of the nerves.
Cranial Nerves
I. Olfactory nerve:*
Sensory; transmits impulses arising in receptor cells of the nasal
cavity.
II. Optic nerve:*
Sensory; transmits impulses arising in the retina.
III. Oculomotor:*
Motor; innervates 4 of the 6 muscles of the eye, and the ciliary
muscle of the eye. Eye movement.
IV. Trochlear:*
Motor; innervates 1 of 6 muscles in the eye. Eye movement.
V. Trigeminal:
Mixed; both sensory and motor. Sensory fibers innervate front
part of scalp, upper and lower jaw (including teeth), and
tongue. Motor fibers innervate muscles used in chewing.
VI. Abducens:*
Motor; innervates one of the 6 eye muscles. Eye movement.
VII. Facial nerve:*
Mixed; innervates *muscles of the face and scalp, outer ear, and
neck. Stimulates secretion of *parotid gland. Sense of
*taste in first 2/3 of tongue. Facial expressions.
VIII. Auditory: (acoustic)*
Sensory; transmits impulses from inner ear.
IX. Glossopharyngeal:
Mixed; sense of taste for back 1/3 of tongue, sense of touch and
temperature in palate, tonsils, and pharynx. Moves
muscles in the pharynx.
X. Vagus:*
-heart rate
-breathing rate
-receives impulses from alimentary canal
Mixed; movement of muscles in larynx, and pharynx.
Movement of muscles of heart, bronchi, esophagus,
stomach, pancreas, gall bladder, small intestine, fist 1/3 of
colon. Stimulate gastric and pancreatic secretions.
XI. Accessory:
Motor; accessory to vagus. Innervates breathing organs,
pharynx, digestive tract.
XII. Hypoglossal:*
Motor; innervates muscles of tongue.
Nerves Cells and Conduction
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Neuron – nerve cell; basic unit of structure and function
Neuron Parts:
a. Dendrites – branched extensions that carry signal towards
cell body
b. Cell body (soma or cyton) - contains the nucleus and other
organelles
c. Axon - convey outgoing messages from the neuron to other
cells (longer than dendrites)
The functional kinds of neurons within the reflex arc:
a. Sensory (afferent) – receptor neurons – carry impulses to the
brain and spinal cord (CNS)
b. Interneuron (associative) – connector neurons – found within
the CNS; transmit signals between sensory, motor, and other
interneurons
c. Motor (efferent) – effector neurons – carry impulses from CNS
to effector
More Nerve Cells
Nerve: Organ composed of•
Hundreds or thousands of neurons
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Other tissues – epithelium, connective, blood vessels
Types of nerves (3)
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Sensory Nerve – consists of sensory neurons (ex:
Auditory or optic nerve conducts to the CNS)
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Motor nerve – consists of all motor neurons (Signal
from CNS conducts to effector muscle or gland)
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Mixed nerve – consists of both sensory and motor
neuron bundles – neurons separated by connective
tissues (ex: All 31 pairs of spinal nerves)
Look at Your Diagram
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Motor Neuron – Longitudinal View (Numbers correspond with additional
diagram)
1. dendrites – transmit impulse towards cell body
2. nucleus – essential to cell functions
3. cyton – synthesizes the neurohumor or neurotransmitter (ex: adrenaline or
acetylcholine)
4. neurofibrils – protein tubules which carry impulses throughout cell
5. schwann cell – cell around axon – membrane (neurilemma) essential to
regeneration of neuron
6. myelin sheath – lipid layer around axon; an insulator and increases rate of
conduction
7. axis cylinder – composed of neurofibrils – carry impulse throughout neuron
8. nodes of ranvier – gaps between successive schwann cells
9. motor end plate – (axon terminals) site where neurotransmitters are stored
and released through the synapse to an effector
10. axon – carry impulses away from the cell body to the synapse or to the
effector
Stimulus - Impulses
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Stimulus – environmental change which causes a response;
usually a form of energy (ex: radiant, electrical, pressure, sound,
chemical)
Impulse – electrochemical change along a neuron
Threshold level stimulus – minimum strength needed to initiate a
nerve impulse (different for different neurons and individuals)
Nerve impulse – “all or none”
Once the impulse is initiated by the threshold stimulus, the neuron
responds 100%; impulse rate is steady and doesn’t vary in
strength
Signal conduction stages:
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Before impulse –
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nonconducting (polarized membrane) – “resting potential”
Sodium pump uses ATP to maintain high Na+ outside, and K+ inside, making
the inside more negative in charge
Continued
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Threshold Stimulus causes action potential –
disrupts the semipermeability of the membrane
Impulse – wave of reverse polarity along the
neuron
Na+ rushes in during conduction while Cl- and
K+ leave
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After conduction –
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the sodium-potassium pump shuttles sodium ions out and
potassium ions in, re-establishing the resting potential
refractory period – lapse of time required for neuron to
restore original charges (1/250sec to 1/2500sec in
humans, depending on cell)
cell is temporarily insensitive to stimulation
Conduction – Impulses
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Rate of conduction – 2 factors
diameter of neuron – larger cell diameter, the faster the rate of conduction
myelin sheath – enhances rate
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non-myelinated cells – slowest conduction rate (sensory neurons)
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myelinated cells (motor) – faster rate
At axon terminal of neuron – axon stores and releases neurohumors into adjacent tissues:
May be an effector (muscle or gland) or an adjacent nerve cell (dendrites or cell body)
To release of neurohumors across synapse:
1. Influx of Ca2+ on presynaptic neuron
2. Causes synaptic vesicles to fuse with the membrane of the presynaptic neuron.
3. The vesicles release neurotransmitter molecules into the synaptic cleft. These molecules diffuse across the cleft and
bind to the receptors of ion channels embedded in the postsynaptic membrane.
4. Neurotransmitter opens specific ion channels, causing a Na+ influx that depolarizes the postsynaptic membrane.
5. The neurotransmitter molecules are quickly degraded by enzymes or are taken up by another neuron
Synapse – neurotransmitter either activates or inhibits adjacent neuron here
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excitatory postsynaptic potential (EPSP) - electrical change that is caused by the binding of
neurotransmitter to the receptor on postsynaptic cell (helps conduct signal to next neuron)
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inhibitory postsynaptic potential (IPSP) – binding of neurotransmitter hyperpolarizes the
membrane --(more negative than resting potential) --making it more difficult for an action potential to
be generated in postsynaptic cell
Summation of postsynaptic potentials –
Summation – occurs when synaptic terminals acts simultaneously, or in rapid succession, on the same postsynaptic
cell
Types:
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temporal summation - when transmitters from the same or multiple cell stimulate the postsynaptic
cell in rapid succession, helping cause another action potential
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spatial summation – when transmitters from different cells act on a postsynaptic cell at the same
time, helping cause another action potential
Major types of neurotransmitters –
Acetylcholine – motor neurons
Adrenaline (epinephrine) – motor, sensory, associative
Noradrenalin (norepinephrine) – motor, sensory, associative
Serotonin – associative neurons
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