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Nervous System, Sensory System, terms: synapse = transmission of…
Nervous System
Central Nervous System (CNS)
The CNS is made up of the
brain and the spinal cord
. The brain is the largest and most complex portion of the nervous system. It
controls perception, movement, sensation, thinking, and many other physiological aspects
.
BRAIN PROTECTION
Cerebrospinal Fluid
CSF surrounds the brain and spinal cord, maintaining a stable ionic concentration and protecting CNS structures. The brain floats in CSF, which cushions it and prevents the bottom of the brain from being crushed by its own weight. The CSF also helps to nourish the brain and may assist in carrying chemical signals concerning sleep and appetite.
Blood Brain Barrier
It acts to selectively allow certain molecules to pass and to keep others from reaching the brain. The maintenance of a constant environment keeps the brain’s neurons from firing uncontrollably.
Between the bony coverings and the soft brain tissues are layered membranes known as meninges that protect the brain and spinal cord.
Basal Nuclei (basal ganglia)
Relays motor impulses from cerebral cortex, passing into brain stem & spinal cord
Assists in coordination of voluntary movement
Diencephalon
Thalamus: Relays sensory impulses ascending from other areas of nervous system into cerebral cortex
Hypothalamus: Helps maintain homeostasis by regulating visceral activities + Links nervous and endocrine system
Cerebrum
Controls higher brain functions
Interprets sensory impulses
Initiates muscular movements
Stores memories
Handles reasoning
Develops intelligence
Brain Stem
Midbrain
= Reflex centers, move eyes, head and maintain posture
Pons
= Relays impulses between medulla oblongata & cerebrum, and helps regulate breathing rate and depth
Medulla Oblongata
= Conducts ascending, descending impulses between brain & spinal cord + contains cardiac, vasomotor, respiratory control centers including various non-vital reflex control centers
Brain Structure
Brain and spinal cord are connected via the brain stem, which allows communication to
flow in both directions
. The spinal cord also provides a
two-way communication, between the CNS and PNS
.
The brain is surrounded first by
grey matter
and then by
white matter
. The grey matter (cortex) consists mostly of
neuron cell bodies
, whereas the white matter consists of
myelinated fiber tracts
.
gray matter is located in the center with the white matter outside
Spinal Cord
The spinal cord is a thin column of nerves leading from the brain to the vertebral canal.
It provides two ways of communication, to and from the brain, and contains the spinal reflex centers.
Spinal Cord
Cervical Cord, Cervical Nerves (8 pairs), Thoracic Cord, Thoracic Nerves (12 pairs), Conus Medullaris, Cauda Equina, Lumbar Nerves (5 pairs), Sacral Nerves (5 pairs) & Coccygeal Nerves
Structure & Function of Nervous System
Integration
The nervous system processes and interprets this information to determine how it should react (integration). It will tell you to run (flight response).
Neurons
Each neuron consists of a cell body, soma, with extensions called an axon and one or more (sometimes many) extensions called dendrites.
Axons conduct nerve impulses away from the cell body. Most neurons have a single axon arising from the axonal hillock on the cell body.
Neurons are closely associated with neuroglia.
Dendrites receive electrical nerve impulses and conduct them toward the cell body and the axon.
Neuroglia comprise almost half the brain and spinal cord tissue and are much more numerous than neurons. They support, nourish, and protect neurons.
CNS neurons are not able to replace themselves if injured or destroyed, although there may be a few exceptions.
Functional Classification
Sensory (
a
fferent)
Conduct action potentials toward the CNS. Sensory neurons detect the internal and external environments (such as from the skin and viscera) and facilitate motor coordination (such as in joints and muscles).
Motor (
e
fferent)
conduct action potentials away from the CNS toward muscles or glands
Interneuron
Conduct action potentials within the CNS from one neuron to another, primarily between sensory and motor neurons.
Glial Cells
Types of Neuroglia
CNS
ependymal cells = assist in production & movement of cerebral spinal fluid
astrocytes = provide structural & metabolic support, participate in repair
oligodendrocytes = produce myelin & provide electrical insulation
microglia = participate in defense & immune responses
Myelin Sheath
Certain neuroglia wrap themselves around the
axons of neurons
to create a structure known as the myelin sheath.
Myelin is a whitish protein-lipoid. An axon with a
myelin sheath
is called
myelinated
, whereas those without myelin sheaths are called
unmyelinated
.
Cell Membrane Potential
A cell membrane’s surface is usually electrically charged (polarized) compared with its inner contents. This is due to unequal amounts of positive and negative ions.
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Divisions of Nervous System
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Neurons Communication
Synapses
Axodendritic synapses are those between the axon endings of a neuron and the dendrites of other neurons.
Axosomatic synapses are those between axon endings of one neuron and soma of others.
Neuromuscular Junction
Synapse between a neuron and a muscle cell is known as a neuromuscular junction
Post-synaptic Neuron
The neuron receiving this impulse is called a postsynaptic neuron. The process of the impulse crossing the synaptic cleft is called synaptic transmission.
Pre-synaptic Neuron
A neuron carrying an impulse into a synapse is called a presynaptic neuron.
Synaptic Cleft
A synapse is a junction between any two communicating neurons. The actual gap between neurons is known as the synaptic cleft.
Chemical Synapses
Chemical synapses
release and receive chemical neurotransmitters.
Information is transferred across chemical synapses beginning when an action potential arrives at an axon terminal.
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Peripheral Nervous System
Satellite cells = provide structural & metabolic support for cell bodies of neurons
Schwann cells = produce myelin & provide electrical insulation
Motor Output
The information from the brain activates the effector organs, which in this case would be your muscles. Running away from the bulldog is the motor output.
Sensory Input
nervous system receives sensory input from the environment and sends it to the brain
Peripheral Nervous System
It allows us to process information between our bodies and our environments.
Sensory (afferent)
Motor (efferent)
Sympathetic & Parasympathetic
Parasympathetic Nerves
constrict pupils
stimulate saliva, activity of stomach, gallbladder, activity of intestines
slow heartbeat
contrict airways
contract bladder
Sympathetic Nerves
Dilate pupils
Inhibit salivation, activity of stomach, gallbladder, activity of intestines
increase heartbeat
relax airways
secrete epinephrine & norepinephrine
relax bladder
Made up of:
made up of all neural structures that are outside the brain and spinal cord. The somatic nervous system oversees conscious activities, whereas the autonomic nervous system oversees unconscious activities.
Sensory Receptors
The sensory receptors of the PNS are specialized to respond to stimuli. They are sensitive to certain types of environmental changes.
C
hemoreceptors-respond to chemicals in solution, including smelled or tasted molecules, changes in blood chemistry, and changes in interstitial fluid chemistry.
M
echanoreceptors- respond to mechanical forces such as pressure, touch, stretching, and vibrations.
N
ociceptors- respond to stimuli that may be damaging, such as extreme heat or cold, excessive pressure, and inflammatory chemicals, resulting in pain.
T
hermoreceptors- respond to temperature changes
Cows Moo Not True
General Senses
The general senses of touch, pressure, temperature, and pain are spread throughout the body via muscle, joint, skin, and visceral receptors.
General sensory receptors are nerve endings of two types: 1. nonencapsulated (free) - mostly respond to temperature and painful stimuli in the skin and internal tissues, except for the brain
2.encapsulated
Reflex Arc
a fast, automatic response to a specific stimulus. Reflex activity in the human body can be either inborn (innate) or learned (acquired).
Inborn reflexes are rapid and predictable motor responses to stimuli that are formed between neurons during human development.
They are involuntary and subconsciously maintain body posture, help to avoid pain, and control visceral activities.
Sensory impulses from receptors can reach their effectors without being processed by the brain. Some reflex arcs use interneurons. The five basic components of a reflex arc are a receptor, a sensory neuron, an integration center, a motor neuron, and an effector.
Somatic & Autonomic Nervous System
Sensory (Afferent)
Impulses are carried toward the CNS from the body’s sensory receptors.
Somatic sensory fibers transmit impulses from the joints, skeletal muscles, and skin.
Visceral sensory fibers transmit impulses from the visceral organs of the ventral body cavity.
This sensory division informs the CNS of all events happening inside and outside the body.
Motor (Efferent)
Impulses are carried from the CNS to the effector organs, activating muscles to contract and glands to secrete.
They affect (cause) motor responses.
The two main parts of this motor division are the somatic nervous system and the autonomic nervous system (ANS).
SNS
Somatic motor fibers transmit impulses from the CNS to the skeletal muscles.
Includes 12 pairs of cranial nerves, which connect sensory organs.
The cranial nerves that only have sensory functions include the olfactory and optic nerves. Each of these has no parasympathetic fibers.
31 pairs of spinal nerves, which bring information into the spinal cord and carry messages from the cord to the effectors
Voluntary nervous system
ANS
The ANS contains visceral motor nerve fibers regulating glandular, cardiac muscle, and smooth muscle activity.
Subdivides into the sympathetic and parasympathetic divisions.
Involuntary nervous system
Effects of Ageing of Nervous System
Risk of Stroke
As fatty deposits accumulate in the blood vessels, there is a decrease in blood flow to the brain.
This can increase the chances that an affected vessel will rupture, leading to symptoms of a stroke (cerebrovascular accident)
Slower reaction of Time
Because the brain processes nerve impulses more slowly, performance of certain tasks and reaction times often become slower
An elderly person who exercises (both mentally and physically) often loses fewer nerve cells in the brain.
Consumption of two or more drinks of alcohol every day reduces brain function.
Loss of Neurons
Neuronal loss occurs. The amount and location of the loss varies substantially among different regions of the brain.
Some neurons may shrink.
Neuronal loss and shrinkage causes a decrease in brain weight and size.
The nervous system has large numbers of neurons, more than we probably ever use, so age-related neuron loss may not affect behavior appreciably until very old age.
Increase in Neuroglia & Deposits
Neuroglia increases
Brain neurons accumulate abnormal intracellular deposits. Extracellular plaques may affect memory processing. When deposits and plaques exceed normal amounts caused by aging, clinical abnormalities may occur.
These particular changes are also characteristic of the brains of those with Alzheimer’s disease
Changes in Transmission Efficiency
Decreased numbers of functional nerve cells may reduce the strength of the message being transmitted.
Fewer nerve cells result in more space to cross, and the coherence of the message may be disrupted, or random background noise (neural noise) could interfere with the clarity of the message.
The motor part of the older cerebral cortex may continue to respond for a time after stimulation ceases, and such aftereffects could blur or interfere with subsequent incoming messages. These changes could then account for the increased time older people usually need to perform simple tasks as well as for their poorer retention and increased susceptibility to distraction in learning and memory tasks.
Sensory System
what is it?
The central nervous system (CNS)
processes
and
interprets nerve impulses from sensory receptors
that detect environmental changes.
Feelings and sensations are the body’s responses to these nerve impulses. Receptors that function in the special senses are much more complex than those functioning in the general senses.
The special senses are
smell, taste, hearing, equilibrium, and sight.
Olfaction - Sense of Smell
Olfactory receptor cells have
hair-like cilia
, which help to
differentiate among odors
.
Olfactory Cortex & Neuronal Pathways
Action potentials generated olfactory neurons conducted
along the olfactory pathway
to the central
olfactory cortex areas in the temporal and frontal lobes
of the cerebrum.
Odorant molecules must partially condensate from
gases to fluids before receptors can detect
them. Impulses are analyzed by
olfactory bulbs
and interpreted in the
olfactory complex
of the brain.
The mechanism for the sense of smell concerns
olfactory receptors
, which work with the sense of taste as well. The olfactory organs are
masses of epithelium covering the upperpart of the nasal cavity
,
superior nasal conchae
, and part of the
nasal septum
.
Auditory - Sense of Hearing
External Ear
Auricle (pinna)
:
A funnel-shaped structure composed of elastic cartilage, thin skin, and small amounts of hair; most people refer to this structure as “the ear.” The auricle functions to funnel sound waves to the external acoustic meatus.
External acoustic meatus
:
Glands secrete cerumen, a yellow-brown waxy substance commonly referred to as earwax. Cerumen helps to trap foreign particles and repel insects from entering the ear.
Eardrum (tympanic membrane)
:
A semitransparent membrane covered by thin skin on the outside and mucous membrane on the inside that actually moves back and forth in response to sound waves; it is the boundary between the outer and middle ear.
Middle Ear
The middle ear
(tympanic cavity
) inside the petrous portion of the temporal bone is filled with air and contains the
auditory ossicles
(the malleus, incus, and stapes). These bones are attached to the
tympanic cavity wall
by
ligaments and bridge the eardrum and inner ear to transmit vibrations
Vibrations
are passed to the auditory ossicles, which is held to an opening (the oval window) by
ligaments
. Vibration of the stapes moves fluid within the inner ear to stimulate
hearing receptors
Internal Ear
The internal ear is complex, with chambers and tubes forming the
bony labyrinth
Inside the labyrinth structures are
three semicircular canals
, which aid in equilibrium, and a
cochlea
, which functions in hearing.
The v
estibule and its two expanded chambers (the utricle and saccule)
,with a tiny macula containing many sensory hair cells function in equilibrium.
When the head is upright, the hairs project upward into a
gelatinous material
. When the head bends forward, backward, or to one side, the
hairs bend to signal nerve fibers
.
The organs of dynamic equilibrium are the
three semicircular canals in the labyrinth
. A swelling called an
ampulla houses sensory organs
, each known as a crista ampullaris. Hair cells within a cupula are bent to signal the brain.
Visual - Sense of Sight
Eyebrow
The eyebrows consist of coarse, short hairs overlying the supraorbital skull margins and help to shade the eyes from light and to trap perspiration from the forehead.
Eyelid
Each eyelid has skin, muscle, connective tissue, and conjunctiva layers that collectively protect the anterior portion of the eye. The clinical term for eyelid is palpebrae.
The eyelid is the thinnest portion of skin on the body.
Conjunctiva
The conjunctiva is a thin, transparent mucous membrane associated with the eyelids and the exposed areas of the eye. The secretions of the conjuctiva help lubricate the surface of the eye.
Extrinsic Eye Muscles
move the eye within the orbit depends on the extrinsic muscles
Refraction
When light waves bend to focus, the phenomenon is called refraction
It occurs when light waves pass at oblique angles from one optical density medium to a different one
Lens has a convex surface, it causes light waves to converge
Most refraction occurs when light reaches the corneal tissues (with same density as water)
Additional refraction occurs when the light
passes from the aqueous humor into the lens
This causes focus of light rays from an object toward a
focal point
, which is a certain point of intersection on the retina
Distance between center + focal point = Focal distance of lens
Focal distance is determined by the
distance of the object from the lens
and the
shape of the lens
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Neuronal Pathways for Vision
Ganglion cell axons
extend to the lateral geniculate ganglion of the thalamus, where they synapse. From there, neurons form the optic radiations that project to the visual cortex.
Neurons from the nasal visual field (temporal retina) of one eye and the temporal visual field (nasal retina) of the opposite eye project to the same cerebral hemisphere.
Axons from the nasal retina cross in the optic chiasm, and axons from the temporal retina remain uncrossed.
Iris
Middle layer
- The iris is the visible colored membrane of the eye, lying between the cornea and lens.
Pupil
The pupil is the round central opening of the iris and allows light to enter the eye. When the pupil contracts, less light enters, controlling the amount of light the eye needs to see in specific conditions.
Cornea
Outer layer- cornea , which helps to focus entering light rays and is continuous along its circumference with the white sclera.
The cornea lacks blood vessels and cannot be affected by the immune system activities
Retina
The neural layer of the retina contains the
rods and cones
.
Rods
have long, thin projections and provide
black and white vision
. Rods are hundreds of times more
sensitive to light
than cones, providing vision in dim light without color.
Cones
have short, blunt projections and provide
color vision
. Cones provide
sharper images
, whereas rods provide more general outlines of objects.
Gustatory - Sense of Taste
Papillae is the rough bumps on the tongue
The sense of taste is actually mostly based on the
sense of smell to about 80%
. Food lacks taste when the nasal passages are congested. (e.g. cannot taste food when having flu)
Without smell, the sense of taste would be very inefficient.
The five primary taste sensations are
sweetness, sourness, saltiness, bitterness, and umami (deliciousness)
.
Taste pores have tiny projections called
taste hairs
, which are the sensitive parts of the taste receptor cells. Stimulation triggers an impulse on a nearby
nerve fiber
traveling to the brain.
terms:
synapse = transmission of electric nerve impulses