Download The Nervous System

The Nervous System
(Section 9.1)
Although closely linked, the endocrine system and nervous
system play different roles. In comparison,
Complexity
Structure
Communication
Response Time
Nervous System
Endocrine System
More structurally
complex; can integrate
vast amounts of
information and
stimulate a wide range
of responses
Less structurally
complex
System of neurons that
branch throughout the
body
Endocrine glands
secrete hormones into
the bloodstream where
they are carried to the
target organ
Neurons conduct
electrical signals directly
to and from specific
targets; allows find pinpoint control
Hormones circulate as
chemical messengers
throughout the body
cells via the blood
stream; most body cells
are exposed to the
hormone and only target
cells with receptors
respond
Fast transmission of
nerve impulses up to
100/second
Make take minutes,
hours, or days for
hormones to be
produced, carried by
blood to target organ,
and for response to
occur
The Nervous System
The nervous system contains more than 1 billion nerve cells in
the brain alone!
Overview: of the Nervous System:
Nervous System
Central Nervous System
brain
Spinal cord
Peripheral Nervous System
Somatic (voluntary) Nerves
acts as a coordinating
system for incoming and
outgoing messages
The Nervous System has three
overlapping functions:
sensory
motor
Autonomic (involuntary) Nerves
sympathetic
parasympathetic
relay information
between the CNS and
organs of the body
1. Sensory input: is the conduction of signals from sensory
receptors to integration centers of the nervous system.
2. Integration: is a process by which information from
sensory receptors is interpreted and associated with
appropriate responses of the body.
3. Motor input: is the conduction of signals from the
processing center to effector cells (muscle cells, gland
cells) that actually carry out the body’s response to
stimuli
These functions involve both parts of the nervous system:
The Central Nervous System
(Section 9.3)
Consists of the BRAIN and SPINAL CORD
 Spinal Cord = carries sensory nerve messages from sensory
receptors to the brain, and relays motor nerve messages
from the brain towards target cells in muscles and organs
o Emerges from skull from foramen magnum, extends
down through a channel in your backbone
o Consists of white matter (consists mostly of myelinated
axons) and grey matter (consists of mostly of cell
bodies, mostly unmyelinated axons, glial cells, and
blood capillaries)
 Cerebrospinal fluid = surrounds CNS (brain and spinal
cord); acts as a sock absorber and transport medium
(nutrients, chemicals, removal of wastes, etc)
o is a connection between CNS and endocrine system
 Brain = formed from
a concentration of
nerve tissue and acts
as a coordinating
center of the nervous
system
o is protected by
the skull and
meninges (a
protective three
layer thick
membrane that surrounds the brain and spinal cord)
o meninges controls which chemicals can ultimately
reach the brain
o consists of three distinctive regions: forebrain,
midbrain, and hindbrain.
Sections of the Brain
Forebrain:
 reason, intellect, memory, language, and personality
 information on right side does not = info on left
 generally on right (visual patterns or spatial awareness)
 generally on left (verbal skills)
 hemispheres are joined by a bundle of nerves called corpus
collosum = allows communication between hemispheres
Sections
Olfactory lobes
Cerebrum
Cerebral Cortex
Cerebrum
Lobes
Frontal Lobes
Temporal Lobe
Parietal Lobe
Occipital Lobe








Function
(x2) process information about smell
2 hemisphere; largest and most developed
coordinates sensory info & motor actions
largest and most developed
divided into four (4) lobes (see below)
surface of cerebrum
made of grey matter
highly folded (deep folds = fissures)
Function
 motor area control movement of voluntary
muscles (e.g walking and speech)
 linked to intellectual activities and
personality
 sensory areas are associated with vision
and hearing
 linked to memory and interpretation of
sensory information
 sensory areas are associated with tough
and temperature awareness
 linked to emotions and interpreting speech
 sensory areas are associated with vision
 interprets visual information
Midbrain:
 located directly below cerebral cortex
 relay center for eye and ear reflexes
Sections
Thalamus

Hypothalamus

Hippocampus
Basal Ganglia






Function
integrative center connecting many
different parts of the brain together
master control center of automatic
nervous system (ANS)
integrates the ANS and endocrine system
short term memory
many parts; responsible for crude motor
movements
injury leads to rigidity, Parkinson’s and
Huntington’s disease
mediates emotional feelings
mediates between forebrain & hindbrain
The Hindbrain:
Sections
Medulla
Oblongata




Pons “bridges”

Cerebellum


Function
joins spinal cord to cerebellum
controls involuntary muscle action
coordinating centre for the ANS
e.g breathing, heart rate, blood vessel
activity, swallowing, vomiting, digestion…)
acts as relay station by sending nerve
messages between the cerebellum and
medulla
controls limb movements, balance and
muscle tone
e.g. walking, hand-eye coordination, etc)
Peripheral Nervous System
(Section 9.1)
 The peripheral nervous system (PNS) resides or extends
outside the central nervous system (CNS) - brain and spinal
cord.
 main function of the PNS is to connect the CNS to the limbs
and organs.
 PNS is not protected by bone or by the blood-brain barrier,
leaving it exposed to toxins and mechanical injuries.
 divided into:
o the somatic nervous system (voluntary) – sensory
system- brings info from sensory receptors to CNS,
and motor system- carries signals from CNS to target
cells
o the automatic (involuntary) nervous system –
sympathetic – increase energy expenditure, and
parasympathetic – increase activity of energy
concerving activities (saves energy)
 both consits of a collection of nerve cells
Nerve Cells:
 The nervous system includes two main types of cells:
o Neurons = conduct messages; identified as excitable.
 Specialized for transmitting chemical and
electrical signals from one location in the body to
another.
o
Supporting cells = (glial and Schwann cells) provide
structural reinforcement as well as protect, insulate,
and assist neurons; identified as non-excitable (nonconducting)
Structure of the
Neuron:
 All neurons contain
three parts:
o Dendrites:
receive info
form sensory
receptors or
other nerves and conduct impulse to cell body; short
and extensively branched
o Cell body: (aka Soma) contains nucleus, organelles
and cytoplasm. The cell body of most neurons are
located in the CNS (others in ganglia)
o Axon: extension of cytoplasm: carries impulse away
from cell body to other neuron or effector; branched
into synaptic terminals
 Many axons are covered by a fatty protein called the myelin
sheath = insulation of nerve impulse (these axons are said
to be myelinated)
 Myelin sheath is formed from specialized glial cells called
Schwann cells
*purpose is to prevent loss of charge – insulate
 Areas between the myelin sheath = nodes of Ranvier. The
nerve impulse actually jumps from node to node (speeding
impulse)  see diagram on next page
 All PNS nerves have a thin membrane called the
neurillemma = promotes regeneration of damaged axons.
 Some nerve cells within the brain and spinal cord do NOT
have myelin or neurillemma (celled grey matter), therefore
these cells are incapable of regeneration. Damage to these
cells is permanent.
 There has been some success with reattaching and
regenerating nerves. Stem cell research has also led to new
possibilities for neuron regeneration.
*See pg. 415 for more info.
There are three major classes of neurons:
1. sensory neurons (aka afferent): convey information awy
from sensory receptors towards the CNS; most synapse
with interneurons
2. motor neurons (aka efferent): convey impulses away from
CNS towards efferent cells.
3. interneurons: act as a link between sensory neurons and
motor neurons (located predominantly in CNS - the spinal
cord and brain)
Organization of Neurons
Neurons are arranged in groups, or circuits, of two or more
kinds of neurons:
 Complex circuits = like those associated with most
behaviours) involve integration by interneurons in the CNS
 Convergent circuits = messages from several neurons
come together at a single neuron; permits integration
of information from several sources
 Divergent circuits = messages from a single neuron
spreads out to several neurons; permits transmission
of information from several sources
 Reverberating circuits = circular circuits in which
the signal returns to its original source; believed to
play a role in memory storage
 Reflex Arc = simplest circuit (group arrangement)/pathway
 does not involve the brain = very quick
 involves synapses between sensory neurons and motor
neurons (e.g. knee jerk reflex)
 Nerve body cells are also often arranged into clusters, which
allow for the coordination of activities by only a part of the
nervous system
o a nucleus is a cluster of nerve cell bodies within the
brain
o a ganglion is a cluster of nerve cell bodies in the
peripheral nervous system
The Electrochemcial Impulse
(Section 9.2)
 Neurons send messages using electrical charges (ions)
 Electrochemical gradient across the membrane creates a
voltage potential across the membrane
 The electrical nature of the nervous system was first
identified by Luigi Galvani (1910)
o During experiments he discovered that the leg muscle
of a dead frog could be made to twitch by electrical
stimulus
HOW?
 Nerve cells have a rich supply of negative and positive ions
in and outside of the cell, but the ion concentration are
dissimilar in the inner and outer environments
 The membrane of the nerve cell however, is permeable to
only SOME ions in the solution – opportunity for an
electrochemical gradient
Formation of the electrical impulse
 Signal transmission along neurons depends on voltage
potentials created by ionic imbalance across the membrane
 Ranges from – 50 to -100 mV (millivolts) in animals
1. Resting Neuron: the membrane of a neuron at rest is
said to be charged (polarized) because of a potential
difference across the membrane (called the resting
potential)
 neuron has a rich supply of positive and negative ions
within and outside of the cell, but membrane is
impermeable to negative ions (stay on inside of cell)
 potential difference is created by an unequal
concentration of positive ions on both sides of the
membrane
o high concentration of K+ inside
o high concentration of Na+ outside
 resting membrane is more permeable to K+, thus more
K+ ions leave than Na+ ions enter – inside becomes
more negative
 active transport by sodium-potassium pump help
maintain this gradient (i.e actively pumps K+ into cell)
 resting potential = -70mV (millivolts)
2. Stimulation of the nerve leads to depolarization
 the action potential (nerve impulse) is the rapid change
in the membrane potential of an excitable cell, caused by
stimulus triggered opening/closing of voltage-gated ion
channels
o stimulus causes Na+ ion channels to open, allowing
Na+ to rush back into cell (concentration gradient)
– increases membrane permeability to Na+
o Once the voltage inside the cell becomes positive
the ion channels slam shut!
o Depolarization = diffusion of sodium ions into cell
resulting in a charge reversal
o Sodium-potassium pump restores resting potential
o Repolarization = process of restoring the original
polarity pf the nerve membrane
 The action potential moves in a wave of polarization down
the axon (nerve fibre)
o See figure 7, p. 421 for direction of movement of
action potential
 Threshold potential = a stimulus must be a above a
critical intensity and duration to stimulate a nerve
o Vary with the type of neuron
o Increasing the intensity of the stimulus will NOT
increase the speed of the transmission nor the
intensity of the response (all or nothing response)
o Different intensities of stimulus are detected
because:
 Different neurons have different thresholds
 Frequency of the stimulus changes
 Refractory period = brief interval of time following the
firing of a neuron which it is incapable of a second
response (waiting period)
Synaptic Transmission
 Active potential down the axon, but then what?
 There are small spaces between neurons, or between neurons and
their effectors (i.e muscle cells), called synapses (synaptic cleft)
 A single neuron may branch many times a tits end plate (axon
terminals) and join (make connections with) many different neurons
 Small vesicles containing chemicals called neurotransmitters are
located in the end plates of axons.
 When the nerve impulse reaches the end of the axon it causes the
chemicals to be released into the synaptic cleft.
 Chemicals diffuse across synapse to the dendrites of the next neuron
causing a depolarization in that neuron..signal continues
Note: the neuron that releases the chemicals is called a presynaptic
nerve, while the neuron after the synaptic cleft is called the
postsynaptic nerve.
Neurotransmitters
Allow neurons to communicate across synapses:
Acetylcholine = acts as an excitatory neurotransmitter
 causes Na+ channels to open in postsynaptic neuron
 resulting in depolarization
How does this stop?
Cholinesterase = is an enzyme released from the postsynaptic
membrane to destroy acetylcholine – Na+ channels closed