Download To allow an immediate response to stimuli in the

The Function of the Nervous System:
To allow an immediate response to stimuli in the environment
The Cells of the Nervous System:
A. Neurons (aka “Nerve cells”)
-specially designed to transmit an electrical signal
-Consist of 3 parts:
1. The Cell Body (which contains the nucleus)
2. The Dendrites (branches which carry the signal
TOWARDS the cell body)
3. The Axons (branches which carry the signal AWAY
from the cell body)
-A “nerve” consists of one or more neurons
-two neurons in a row, are separated by a “synapse”
(a gap)
-when the nerve signal gets to the end of the axon, it releases
chemicals called “neurotransmitters” which cross the
synapse, and initiate the signal in the dendrites of the next
neuron
-Often, axons will be wrapped in a “Myelin sheath” (fat)
-Gaps in this sheath are known as “nodes of Ranvier”
B. Neuroglia
-“supporting” cells; support, insulate and protect neurons
-“Schwann cells” = neuroglia which produce the myelin sheath
The Impulse of a Neuron
Neurons are said to carry an electrical impulse, which is unlike a wire
carrying an electrical current
To understand this impulse, we must focus on a small section of the
neuron’s dendrite or axon:
When this small section is at rest (not carrying an impulse), we find
there is a charge difference inside vs. outside the membrane
-this charge is slightly negative on the inside of the membrane,
slightly positive outside of the membrane
-this charge is called the “resting membrane potential”
-it is caused by an uneven distribution of ions:
-Outside the membrane there is a high concentration of
Sodium (Na+) ions, and a low conc. of Potassium (K+) ions
-Inside the membrane, there is a low conc. of Sodium (Na+)
ions, and a high conc. of Potassium (K+) ions
-Other ions are involved as well
-Diffusion of these ions does not occur in or out of the
membrane, as the membrane is impermeable to these ions
The Neuron’s Action Potential
As a result of a stimulus, the membrane of the section of the neuron
becomes alternatively permeable to these ions
-First, Sodium “gates” in the membrane open, allowing Na+ ions
to enter the cell
-As the Na+ ions enter, the section “depolarizes”
-The Sodium gates then close
-Next, Potassium gates open, allowing K+ ions to leave the cell
-As the K+ ions exit, the section “repolarizes”
-The Potassium gates then close
-Finally, sodium-potassium pumps in the membrane pump
sodium ions out, and potassium ions in, restoring concentrations
This change in the membrane potential of this section of the neuron is
known as an “Action Potential”
-An action potential then occurs in the next section of the neuron
-The sequence of action potentials appearing to “travel” down a
neuron is known as the neuron’s “Impulse”
The Effect of Myelination
As a result of the myelin sheath around the neuron, the movement of
ions (and therefore, action potentials) can NOT occur in these areas
-The only places they can occur is in the Nodes of Ranvier
-therefore, the impulse seems to “jump” from node to node
-As a result, the impulse travels approx 10 times faster
-This is known as “Saltatory” conduction of an impulse
The Organization of the Nervous System
1. The Central Nervous System (CNS)
A. The Brain
B. The Spinal Cord
2. The Peripheral Nervous Sytem (PNS)
A. The Sensory or Afferent Division
-impulses lead from sense organs towards the CNS
B. The Motor or Efferent Division
-impulses lead away from the CNS towards “effectors”
(muscles and/or glands)
1. Somatic Nervous System (voluntary)
-controls the skeletal muscles
2. Autonomic Nervous System (involuntary)
-controls the cardiac and smooth muscles
a. Sympathetic system
-typically, “speeds up” effectors
-the “fight or flight” response
b. Parasympathetic system
-typically, “slows down” effectors
The Reflexes
Reflexes are rapid, predictable, involuntary responses to stimuli
Reflexes may be somatic (deal with skeletal muscles) or autonomic
(deal with smooth muscles, glands, or the heart)
Reflexes are a result of the “reflex arc”
1. The sensory receptor
2. The sensory neurons
3. The integration center (either the brain OR the spinal cord)
-determines what the stimulus “means” and what the response
should be
4. The motor neurons
5. The effector organs (muscle or gland)
The Central Nervous System
Protection:
1. Bone
2. The Meninges (3 layers of connective tissue)
-Cerebrospinal fluid acts as an intermediate between
blood and the CNS (the blood-brain barrier)
Structures of the Central Nervous System:
The Brain
The origin of 12 pairs of nerves, which are called the “cranial nerves”,
as well as the spinal cord
Consists of 4 major sections: The cerebrum, the diencephalon, the
brain stem, and the cerebellum
The Cerebrum:
-Contains many gyri (outfoldings) and sulci (grooves)
-Is the largest portion of the human brain
-Is responsible for most memory, logic, emotional response,
interpretation of sensation, and voluntary movement
-Is divided into left and right cerebral hemispheres by the longitudinal
fissure
-Each cerebral hemisphere is divided into lobes: The Frontal lobe and
the Parietal lobe (both deal with speech, memory, sensory, and motor
functions), the Temporal lobe (hearing and olfaction functions) and
the Occipital lobe (visual functions)
-The frontal lobe contains a major portion of the “limbic system”
which
-The two cerebral hemispheres are connected (and communicate) as a
result of the corpus callosum
The Diencephalon:
-Sits atop of the brain stem
-allows the brain stem to communicate with the cerebrum
-Consists of the thalamus, the hypothalamus, and the pituitary gland,
and the pineal gland
-The the hypothalamus helps to regulate autonomic functions
-The thalamus screens out “unimportant information” to the
cerebrum
The Cerebellum:
-Coordinates the contractions of skeletal muscles’ movement which
was initiated by the cerebrum
The Brain Stem:
-Consists of the midbrain, the pons, and the medulla oblongata
-The pons controls breathing
-The medulla oblongata controls heart rate, blood pressure, breathing
rate, swallowing, and other involuntary functions
The Spinal Cord
-Acts as the connection between the brain and most of the PNS
-Stretches from the brain down to the L1 vertebra
-Inferior to the L1 vertebra, it degrades into a group of individual
nerves called the “cauda equina”
-The origin of 31 pairs of nerves, called “spinal nerves”, which exit
between the vertebrae
-Consists of sensory (afferent) nerve tracts, motor (efferent) nerve
tracts, as well as integration sites for spinal reflexes
The Special Senses
Vision:
The result of light causing photoreceptors to produce action potentials
These action potentials are then sent to the brain via the Optic nerve
Structures associated with Vision:
The photoreceptors: two types – Rods which detect low intensity light
in degrees of “black and white”, and Cones which detect color, but
only in high intensity light
The Retina: The back wall of the eyeball, which contains all the
photoreceptors
The fovea centralis: The portion of the retina that contains primarily
cones; outside the fovea, there are mainly rods
The optic disc: the portion of the retina where the optic nerve
attaches; there are no rods or cones located here
To get a focused image of the proper light intensity to fall upon the
retina, the following structures of the eyeball are required:
The cornea: Bends (refracts) light
The pupil: Allows light into the eyeball
The Iris: The colored portion that regulates the size of the pupil
The lens: refracts light, but can muscles can change its shape to focus
the image (accommodation)
Hearing
The result of liquid pushing upon mechanoreceptors, producing action
potentials that are sent to the brain via the Vestibulocochlear nerve
Structures associated with Hearing:
The Pinna: collects the sound
The Tympanic membrane: vibrates in response to sound
The Pharyngotympanic (auditory) tube: opens up into the throat,
allowing equal air pressure on both sides of the tympanic membrane
The Ossicles (the malleus, the incus, and the stapes): vibrate in
response to the tympanic membrane vibrating
The stapes pushes on the oval window of the cochlea, which causes
waves of the liquid “endolymph” within the cochlea
These waves of endolymph push upon a membrane (the tectorial
membrane) that pushes down on the mechanoreceptors
Chemical Senses: Taste and Smell
The result of chemicals binding to chemoreceptors, producing action
potentials that are sent to the brain
Olfaction (smell): Caused by olfactory receptors in the nasal cavity
The impulses travel to the brain via the olfactory nerve
In the brain, the olfactory pathways are integrated with the limbic
system, thus explaining the strong emotional response to smell
Gustation (taste): Caused by groups of chemoreceptors called taste
buds
These taste buds are actually located within little hills (papillae)
located upon the tongue
When stimulated, the impulses are sent to the brain via the 3 cranial
nerves
Taste, is a function of 5 different chemoreceptors, which detect the
following taste sensations: sweet, sour, bitter, salty, and “Umami”
(delicious – as in steak)