Biology 12
Nervous System
Major Divisions of Nervous System
1. Central nervous system (CNS) - includes the brain and spinal cord
2. Peripheral nervous system (PNS) - includes the cranial and spinal nerves
Neurons: the nervous system consists of 3 main types of nerve cells called neurons
•
Have three parts; dendrites, cell body, and axon.
Parts of a Neuron:
•
Dendrite - conducts nerve impulses toward the cell body.
•
Cell body - helps to organize the impulses.
•
Axon - conducts nerve impulses away from the cell body.
Types of Neurons:
1. Sensory neurons - take messages from a sense organ to the central nervous system. They have
long dendrites and short axons.
2. Motor neurons - take messages away from the central nervous system to a muscle fibres or
glands. They have short dendrites and long axons.
3. Interneurons - are always found completely within the central nervous system and convey
messages between parts of the system.
Reflex Arc: DOESN'T INVOLVE BRAIN SO ENABLES QUICK ACTION IN EMERGENCY.
1. Sense receptor (like those found in the skin) - generates a nerve impulse.
2. Sensory neuron take the nerve impulse to the central nervous system. The impulse moves along
the dendrite and proceed to the cell body (in the dorsal root ganglion) and then go from the cell
body to the axon.
3. Interneuron (which is always found in the central nervous system), receives this nerve impulse
from the sensory neuron by its dendrites and passes it through its cell body to its axons .
4. Motor neuron receives the nerve impulse from the axons of the interneuron by its dendrites and cell
body and passes it along its long axon to the effector.
5. Effector causes an action ex. knee jerk reflex (the effectors are the muscles that moves the lower leg)
Nerve Impulse:
• a nerve impulse along a neuron consists of a wave of change of polarity or charge along it's
length.
• This change can be measured by using an oscilloscope which shows the voltage changes
along the neuron. It records changes in polarity.
Resting Potential:
• When a neuron is not conducting a nerve impulse the oscilloscope records a potential
difference across the membrane of -60 millivolts (mV).
• It is called Resting Potential because the neuron is not conducting an impulse.
• Resting Potential is maintained by the Na+/K+ pump which actively transports Na+ out of neuron and
K+ into neuron (requires ATP)
• In this situation there is also a higher concentration of Sodium ions on the outside of the membrane
than on the inside and also a higher concentration of Potassium ions on the inside than on the outside.
• In the centre of the neuron are large negatively charged units which are responsible for the net negative
potential in the resting state. These units do not move even when an impulse is traveling.
Action potential:
• At certain point on membrane of the neuron, stimulation changes its electro-chemical nature.
• This causes Na+ gates or channels (carrier proteins) to open and allow sodium ions to move in
toward the centre of the neuron. The effect of this is to change the net negative charge of the
neuron from -60 mV to +40 mV.
• When this is shown on an oscilloscope it appears as an up-swinging curve and is known as the
"UP-SWING" or depolarization part of the ACTION POTENTIAL.
• Immediately following the entry of sodium ions, potassium ions move to the outside of the
membrane through the K+ gate or channel.
• This causes the net charge of the neuron to return to the -60 mV level. This is shows as a
down swinging curve on the oscilloscope and is known as the "DOWN-SWING" or repolarization part
of the ACTION POTENTIAL.
Recovery (Refractory) phase
• The Na+/K+ pump transport Na+ out of the neuron and transports K+ into the neuron.
• This pump requires ATP
Summary:
1. resting potential - ion balance (Na+ on outside and K+ on inside of neuron) maintained by
active transport (requires ATP).
2. depolarization (upswing) - membrane of neuron is stimulated at certain point, special Na+
gates (channels) open in the membrane and sodium ions are allowed to move in. When sodium
moves in, the inside of the membrane becomes more positive (from -60 mV to +40 mV).
3. repolarization (downswing) - just after the sodium ions arrive on the inside, K+ gates
(channels) open to allow potassium ions to move out. When potassium moves out it returns the
membrane potential to -60 mV.
4.
These two changes (depolarization/repolarization) are called an action potential.
5.
recovery phase - active transport of Na+ to outside of neuron and K+ to inside of neuron re-establishes
resting potential (requires ATP)
All or None Response
• Depolarization to a threshold level causes an action potential to travel along a neuron.
• Action potentials are of uniform strength, there are neither strong action potentials nor weak ones.
•
If the intensity (amount) of stimulus was increased, this would increase the frequency of
impulses but would not affect the strength of individual action potentials.
Polarity Changes during a Nerve Impulse
Nerve Impulse:
• Action potential travels - these two changes (depolarization/repolarization) occur in a
progressive wave along the length of the neuron.
Myelinated Neurons:
• Most of the Neurons that are outside the central nervous system are wrapped with a special
fatty sheath called a myelin sheath.
• At the Nodes of Ranvier Na+ and K+ ion interchange can take place as happens with the
previously described impulse conduction. The Myelin sheath restricts this. A local current is
set up between each node and the impulse appears to jump quickly between nodes. The effect is a
faster rate of nerve impulse conduction.
Synapse:
• Each axon branches into many fine terminal branches, each of which is tipped by a small
swelling or terminal knob. Each region is called a synapse and the knob is called the synaptic ending.
• The membrane of the knob (axon) is called the presynaptic membrane.
• The small gap is called the synaptic cleft.
• The membrane on the dendrite or cell body is called the postsynaptic membrane.
• The post synaptic membrane has special receptor sites where neurotransmitters can bind.
• Transmission of nerve impulses across a synaptic cleft is carried out by chemicals called
neurotransmitter substances.
Transmission of a nerve impulse across a Synapse:
• When an impulse reaches a synaptic ending it modifies the membrane in such a way that that
calcium ions diffuse into the pre-synaptic ending.
• The calcium ions appear to interact with contractile protein to causes them to pull the synaptic vesicles to
the edge of the membrane where they then burst to release their contents.
• The contents are neurotransmitter substances which diffuse across the cleft.
• These neurotransmitters then bind with receptor sites in a lock and key manner. This causes either an
excitatory or inhibitory effect.
• Excitatory effect - action potential generated in post synaptic membrane. (Na+ gates (channels) open
on post synaptic membrane)
• Inhibitory effect - action potential does not continue in post synaptic membrane. (K+ gates (channels)
open causing hyper polarization of post synaptic membrane)
• Enzymes on the post synaptic membrane, breakdown the neurotransmitters from the synaptic
cleft and the receptor sites. This stops excitatory or inhibitory stimulation of postsynaptic membranes.
Division of autonomic nervous system
Sympathetic nervous system
Neurotransmitter
noradrenalin (NA)
Synaptic enzyme
Monoamine oxidase
Parasympathetic nervous system
acetylcholine (Ach)
Acetyl cholinesterase AChE)
Divisions of the Nervous System
1. Central nervous system (CNS)
Brain and spinal cord
2. Peripheral nervous system (PNS)
Cranial and spinal nerves
Somatic nervous system
to skeletal muscles (bone & skin)
consists of sensory & motor nerves
can be voluntary or involuntary
Autonomic Nervous System
to smooth muscles and internal organs
consists of motor nerves only
involuntary (no need for conscious intervention)
There are two divisions of the autonomic nervous system:
1.
Sympathetic system - fight or flight
2.
Parasympathetic system – rest and digest
Sympathetic nervous system
• promotes reaction to “fight or flight” situations or
emergency
• accelerate heart rate
• increase breathing rate
• dilation of pupils
• inhibit digestion
o decrease peristalsis
o decrease enzyme secretion
o decrease blood flow to intestines
• neurotransmitter – noradrenalin
• synaptic enzyme - monoamine oxidase
• Promotes the release of adrenalin from the adrenal
gland
Parasympathetic nervous system
• promotes all those internal responses associated with
a relaxed state
• slow heart rate
• slow breathing rate
• contraction of pupils
• digestion of food
o
increase peristalsis
o
increase enzyme secretion
o
increase blood flow to intestines
• neurotransmitter - acetylcholine (Ach)
• synaptic enzyme – acetyl cholinesterase (AchE)
Functions of the Major Parts of the Brain
1.
Spinal cord - relays nerve impulses to and from the brain.
2.
Medulla oblongata - control center for breathing, heartbeat, blood pressure and various
reflex centers such as swallowing. (controls the autonomic nervous system)
3.
Thalamus - relay station for sensory input between the spinal cord and cerebrum. It
“decides” what to pass on the cerebrum and passes this to appropriate areas of cerebrum.
4.
Hypothalamus - controls the pituitary gland (neuroendocrine center) and is mainly
concerned with homeostasis eg. temperature control, water balance etc.
5.
Cerebrum - higher thinking area, responsible for consciousness and voluntary muscle
control. Damage to left hemisphere results in paralysis to right side of body & vice versa.
6.
Cerebellum - muscular coordination and balance.
7.
Corpus callosum - a nerve tract that connects the two hemispheres of cerebrum.
8.
Pituitary gland - “master gland”, releases many hormones that affect other glands
Posterior pituitary gland releases ADH and oxytocin.
Anterior pituitary gland releases FSH and LH (sex hormones) and TSH (thyroid stimulating hormone)
Neuroendocrine control centre
• Hypothalamus acts as part of nervous system as well as endocrine (hormone) system
• Hypothalamus produce ADH and oxytocin which travel through specialized neurons into the
posterior pituitary gland where they are temporarily stored and released into the blood stream.
• Hypothalamus produces releasing hormones which travel through a portal system (capillary
bed) and cause the anterior pituitary gland to produce and release the sex hormones and thyroid
stimulating hormone (TSH) sex hormones: Follicle stimulating hormone (FSH) and luteinizing hormone (LH )
Connections: homeostasis of water balance
• hypothalamus detects low water in blood (low blood volume, concentrated blood)
• hypothalamus causes thirst - drink more water
• hypothalamus produces ADH, released through posterior pituitary gland, which causes
collecting duct of kidney to become more permeable, reabsorb more water (less urine volume)
Connections: homeostasis of low body temperature
• Hypothalamus detects low body temp. and causes vasoconstriction in peripheral regions (ex. skin),
also causes shivering which produces heat.
• Hypothalamus stimulates the anterior pituitary gland to produce TSH (thyroid stimulating hormone which
stimulates the thyroid gland to produce thyroxin (long term cold) which increases metabolic rate.
• Homeostasis of high body temperature - hypothalamus detects high body temp. and causes
sweating and vasodilation of peripheral regions (ex. skin)