NERVOUS SYSTEM
CENTRAL AND PERIPHERAL SYSTEMS
Central Nervous System (CNS):
- the brain and spinal cord in vertebrate animals
- protected by bone (skull and vertebrae)
- information processing
Peripheral Nervous System:
- nerves and ganglia that lie outside the central nervous system.
- projects towards and away from the central nervous system.
- carries impulses to and from the CNS
The peripheral system is divided into two groups of nerves
i) Somatic Nervous System:
- nerves that serve the musculoskeletal system (voluntary)
- nerves that serve the exterior sense organs
- gives you information about the external environment and allows you to respond to it
ii) Autonomic Nervous System:
- Controls the internal organs automatically and without “awareness”
BRAIN PARTS AND FUNCTIONS
Medulla Oblongata:
- Unconscious part
- Closest to the spinal cord
- centers for heartbeat, breathing
- vomiting, coughing, sneezing, hiccoughing, swallowing
- Control of internal organs
Cerebrum
- Responsible for consciousness
- largest portion of the brain (human)
- Outer layer: Cortex
Thalamus:
- “Gatekeeper” to cerebrum
- Last stop before the cerebrum
- Central relay station for sensory impulses (sorts out messages to cerebrum)
- Channels impulses to appropriate regions of the brain
Cerebellum:
- Resembles a butterfly
- 2nd largest portion of brain
- lets body have smooth and graceful motions (muscle coordination)
- maintains body position in space (proprioception)
- in charge of balance
Hypothalamus:
- Homeostasis
- hunger, sleep, thirst, body temperature, blood pressure
- link between nervous and endocrine systems (controls pituitary gland)
- Control of internal organs
Corpus Callosum:
- Horizontal tract, first nerve connection between L. & R. hemispheres
- Connects the two cerebral hemispheres to share information
Pituitary gland:
- link between nervous system and Endocrine system
HYPOTHALAMUS / PITUITARY INTERACTION
The Hypothalamus is the part of the brain that is concerned with homeostasis (maintaining a constant internal environment). The pituitary is a small gland (2 parts, anterior and posterior lobes) that produces a large number of hormones, many of which control the release of hormones from other glands in the body. For this reason it is often called the “master gland”.
i) Posterior Pituitary
The posterior pituitary gland releases the hormones that are actually made in the Hypothalamus, but are stored in the Post. Pit. The hormones are transferred and stored in special hollow nerve fibres that run from the hypothalamus to the post. pit.
eg. ADH and Oxytocin
ii) Anterior Pituitary
The anterior pituitary gland makes and releases its own hormones. It is stimulated to release its hormones by releasing hormones from the hypothalamus. There is a blood connection between the hypothalamus and the ant. pit.
Hormones made in the Ant. Pit. include:
Growth hormone
Prolactin
FSH & LH
Thyroid Stimulating Hormone (TSH)
Adrenal Cortex Stimulating Hormone (ACTH)
Melatonin
The level of all the Pituitary hormones in the blood are monitored by the hypothalamus and are controlled by a negative feedback system (except Oxytocin!), so the level of these hormones remains relatively constant. The sex hormones FSH & LH vary in the female.
NEURONS - entire system is built from nerve cells - neurons
Functions
- dendrite: conducts message towards the cell body
- axon: conducts message away from the cell body
- synapse: message jumps from one neuron to another
- myelin sheath: insulates the nerve fiber electrically
- node of Ranvier: speeds up transmission of nerve impulses
- cell body: conducts normal functions (respiration, etc)
SENSORY, MOTOR AND INTERNEURONS
a) Motor neuron - connected to an effector (muscle, fiber or gland)
- short dendrite, long axon
- signal causes effector to react (eg. muscle to contract)
- cell body is inside CNS
- message is traveling away from the brain
b) Sensory neuron - starts with a sensory receptor (pressure, heat, light etc)
- message travels towards CNS
- cell body is outside CNS in Ganglia
- long dendrite, short axon
c) Association (inter) neuron
- smaller than a & b above
- entirely within CNS
- both long and short axons and dendrites
- conveys messages between system parts in CNS
Three types of neurons
REFLEX ARC
- A reflex action (eye blink, hand jerking away from a hot object) in which a stimulus causes a response, without the brain being involved in making a decision.
Label this diagram
- The interneuron bypasses the brain
- Another neuron also goes to the brain, but by the time the impulse reaches the brain, the motor neuron has already contracted the muscle.
1: Receptor makes the message
2: Sensory Neuron
- Takes message to Central Nervous System
3: Interneuron
- Passes message to motor neuron
4: Motor neuron
- takes message away from Central Nervous System
5: Effector
- receives message
- reacts, muscles contract
Written Questions
1. Contrast parts and functions of the central nervous system and the peripheral nervous system. (4 )
2. What is the function of each of the following structures: medulla oblongata, hypothalamus, thalamus, cerebrum, cerebellum, corpus collosum and pituitary gland. Label them on the diagram below. (7)… The diagram will not be marked.
3. What are the three layers of the meninges? What is the function of the meninges? (4)
4. What are the three classes of nerve cells? How are they alike? How are they different? (5)
5. What is myelin? Where is it located? What are two functions of myelin? (4)
6. Outline the path of a nerve impulse through a reflex arc. (5)
Nervous System II
IMPULSE GENERATION
1. Nerve transmission
- the nerve impulse is electrical in nature.
i) normal (no impulse), at rest.
Axomembrane
Na+ Na+ Na+ Na+ Resting potential = -60 mV.
K+ K+ K+ - The inside is negative with respect to
------ the outside.
large organic (-) ions - The ion differential is maintained by
K+ K+ K+ active transport (energy required)
Na+ Na+ Na+ Na+ called the sodium-potassium pump
The negative charge inside the axomembrane is brought about by the fact that K ions diffuse back outside the membrane faster than Na diffuses back in, so there are always more + ions outside the membrane than in.
The Na / K pump is always active because the Na and K are always leaking back across the membrane.
ii) action (an impulse)
Two phases
a) Na+ pours inside the axon due to sodium gates in the fiber membrane opening
Na+ Na+ Na+ Na+ Action potential = +40 mV
K+ K+ K+ - The inside is now + with respect to the
------outside depolarization
K+ K+ K+
Na+ Na+ Na+ Na+
b) K+ pours outside the axon due to potassium gates opening
Na+ Resting potential = -60mV
K+ Na+ K+ Na+ K+ - the inside is again - with respect to
------the outside
- after a very brief recovery period, the
K+ Na+ K+ Na+ K+ fiber is now ready to transmit another
Na+ impulse
- repolarization
Many impulses can pass down a nerve because only a small fraction of the Na+ and K+ actually move.
Action potential graph
+40
Membrane potential (milli - volts)
0
-60 Recovery period
Time (milli seconds)
1 - Resting potential (sod.-pot. pump)
2 - Action potential, Na+ gates opening, Na+ pouring in - Depolarization
3 - Action potential, K+ gates opening, K+ pouring out - Repolarization
4 - Resting potential, fiber is ready to conduct again (after rec. period)
After a time however, a nerve can lose its ability to make an action potential because too much Na+ and K+ have traded places. The nerve needs a resting period, and the Na+ and K+ are actively pumped back across the membrane.
Movement of the impulse
The impulse moves down the fiber, because reversal at one point of the fiber (an action pot.) stimulates the sodium gates to open at the very next point. The gates that have just opened and closed cannot be restimulated for a very brief period of time, (Recovery period) so the impulse moves in one direction only.
- +++++++++++++ + - ++++++++++++ ++++ - ++++++++
+ ------+ ------+ ------
+ ------+ ------+ ------
- +++++++++++++ + - ++++++++++++ ++++ - ++++++++
ONE WAY
- an action potential can only be initiated at the dendrite end of a nerve fiber (receptor or synapse) Thus the impulse can only travel away from the receptor towards the cell body. It never goes in the opposite direction (although it can be made to do so artificially)
ALL OR NONE
- action potentials, once stimulated, are all the same (-60 to +40 mv). It is either there, or not.
MYELIN SHEATH
In myelinated cells, the action potential cannot change under the myelin, so they “jump” from one node of Ranvier to another. This way they travel approx. 200 X faster.
- Myelinated nerve cells are covered by a myelin sheath
- has white appearance
- interrupted by gaps at intervals: nodes of Ranvier.
- Outside the central nervous system
Non myelinated fibers: Impulse goes point to point (slower)
Myelinated fibers: Impulse goes node to node. (faster)
THE SYNAPSE - Junctions between nerve cells
-The transmission of the impulse across a gap is not electrical, but chemical.
SYNAPTIC TRANSMISSION
As the impulse moves into the synaptic ending, it stimulates the synapse vesicles to move to the pre synaptic membrane of the axon. (This involves Ca+2 ions). The Ca ions move into the bulb, causing the microtubules to constrict, pulling the synaptic vesicles towards the presynaptic membrane. These fuse to the membrane which then empty their contents (neurotransmitter substance) into the synaptic cleft. This substance diffuses across the cleft to the receptor sites in the post synaptic membrane of the dendrite. These can either:
i) increase action potentials in the dendrite
ii) decrease action potentials in the dendrite
iii) initiate an action potential in the dendrite (open Na+ gates)
- The impulse can only go one way across the gap because only the axon has the vesicles and the dendrite the receptors.
Neurotransmitters
- Different nerve cells can have different neurotransmitters.
Some examples are: Acetylcholine
Noradrenalin (norepinephrine)
Serotonin
Dopamine
NEUROTRANSMITTER BREAKDOWN
- Enzymes exist in the synaptic cleft that break apart the neurotransmitter. This clears out the gap, so that a new impulse can be rapidly transmitted. It also allows reuse of the neurotransmitters as the parts diffuse back into the axon to be rebuilt into new neurotransmitter substance.
Example: Acetylcholinesterase (destroys Acetylcholine into choline and acetic acid)
SYMPATHETIC AND PARASYMPATHETIC SYSTEMS
1 Sympathetic System:
- Fight or flight
- dilates pupils, accelerates heartbeat and breathing rate, inhibits digestive tract.
- Neurotransmitter is Noradrenalin
- Postganglionic is longer than preganglionic
- Come from middle of spinal cord
2 Parasympathetic System:
- Normal activity (opposite to Sympathetic system)
- maintains a “relaxed” body state
- Neurotransmitter is Acetylcholine
- Preganglionic is longer than post ganglionic
- Come from brain, lower part of spinal cord
Both Systems - function automatically, and activate internal organs
- have 2 motor nerves and 1 ganglia
- arise in different areas of the spinal cord and have quite different structure and chemistry.
ADRENAL GLAND
Adrenalin is a hormone produced by the medulla (inner layer) of the adrenal glands (one on top of each kidney). It is responsible for maintaining the “fight or flight” response. It is secreted in times of emergency or stress. Some affects of adrenalin are - dilated pupils
- increased heart rate
- decreased blood flow and peristalsis in digestive tract
- increased blood flow to the CNS and skeletal muscles
It continues the affect of sympathetic nerve impulses, and is used as a neurotransmitter substance in this system as well.
Written Questions
1. What is resting potential and how is it maintained? (2)
2. What are the 2 phases of an action potential, and how are they brought about? What is the refractory period and why is it important? (6)
3. Discuss the structures involved and steps that occur for an impulse to move from 1 nerve cell to another. (lots)
4. What is a Neurotransmitter substance and how do they work? How are they destroyed? Name 2 important neurotransmitters. (5)
5. What is the autonomic nervous system and what are its 2 main divisions? List several differences between these two divisions. (5)
6. Explain the Neuroendocrine function of the nervous system. (2)
7. Describe the body`s response to increased stimulation by the sympathetic nervous system. How does this prepare the body to respond to emergency situations? (3)
8. Where is adrenalin produced? How is it involved in the “fight or flight response”? (2)