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THE NERVOUS SYSTEM
Transcript of THE NERVOUS SYSTEM
The brain is protected by:
(1) the skull or cranium;
(2) meninges — series of three
membranes that surround brain and
spinal cord ;
(3) cerebrospinal fluid — fills spaces
in meninges to create a cushion.
Unit 7 Nervous System
PERIPHERAL NERVOUS SYSTEM
It consists of the autonomic nervous system and the somatic nervous system.
The autonomic nervous system is not consciously controlled and is divided into: (a) sympathetic nervous system; (b) parasympathetic nervous system.
Sympathetic nervous system: It sets off a “fight or flight” response to deal with an immediate threat. When stimulated, heart rate and breathing rate increases and blood sugar is released by the liver. It can be good and bad — test anxiety.
Acetylcholine is a primary neurotransmitter of somatic nervous system and parasympathetic nervous system. It can have excitatory or inhibitory effects — for example, it stimulates skeletal muscles and inhibits cardiac muscle.
THE NERVOUS SYSTEM
Cells, tissues, organs and ultimately organ systems must maintain a biological balance despite changing external conditions.
Homeostasis is the state of internal balance so critical to existence. It represents a dynamic equilibrium displaying constant interactions and checks / balances both within organisms and between organisms and their environment. Structure and function of the nervous system.
The Nervous system is comprised of two Divisions/Sections :
Central Nervous System (CNS): brain and spinal cord. The CNS receives sensory information and initiates motor control.
Peripheral Nervous System (PNS): includes nerves that lead into and out of the CNS. It consists of the autonomic nervous system and the somatic nervous system.
1) Autonomic Nervous System: The autonomic nervous system is not consciously controlled and is made up of the sympathetic and parasympathetic nervous systems.
2) Somatic nervous system: Made up of sensory nerves that carry impulses from the body’s sense organs to the CNS. It consists of motor nerves that transmit commands from CNS to the muscles and deals primarily with the external world and the changes in it. It is able to control subtle reflexes such as blinking which does not require a conscious decision.
The spinal cord is protected by:
(2) cerebrospinal fluid.
The brain has three main parts:
(3) brain stem.
The brain is divided into regions that control specific functions.
Cerebrum : the part of the brain where all information from our senses is sorted and interpreted. Voluntary muscles that control movement and speech are stimulated from this part of the brain. Memories are stored and decisions are made in this region. It is the center of human consciousness and separates us from every other animal on the planet. The Cerebrum is divided into two Hemispheres (Left and Right) and 4 lobes: Frontal, Parietal, Occipital and Temporal.
Cerebellum: controls muscle coordination. It contains 50% of the brain’s neurons but only makes up 10% of brain volume. It controls our balance. We do not have to think about certain skills as we get older because they are controlled by the cerebellum.
Medulla Oblongata: attached to the spinal cord at the base of the brain and has a number of functions all related to a particular structure. The cardiac center controls heart rate and force of contractions. The vasomotor center adjusts blood pressure by controlling blood vessel diameter. The respiratory center controls rate and depth of breathing. It also contains reflex centers for hiccupping, vomiting , coughing and swallowing. Damage to this part of the brain is usually fatal.
Thalamus: the sensory relay center. It receives information such as touch , pain , heat , cold as well as information from the muscles. Mild sensations are relayed to the appropriate part of the cerebrum (conscious part of the brain). If sensation is strong, the thalamus triggers a more immediate reaction while transferring the sensations to the homeostatic control center, the hypothalamus.
Hypothalamus A complex bundle of tissue that acts as the main control center for the autonomic nervous system. It enables the body to respond to external threats by sending impulses to various organs via the sympathetic nervous system. After the threat has passed, it re-establishes homeostasis by stimulating the parasympathetic nervous system.
Midbrain A short section of brainstem between the cerebrum and the pons. It is involved in sight and hearing.
Pons Contains bundles of axons traveling between the cerebellum and rest of CNS. It works with the medulla to regulate breathing rate and has reflex centers involved in head movement.
Corpus callosum is a series of nerve fibers that connect the left and right hemispheres of the brain.
THE SPINAL CORD
Extends from the base of the brain all the way down the vertebral column.
Surrounded by the vertebral column that protects it.
Consists of White and Grey matter. Grey matter is comprised of nonmyelinated neurons. White matter contains myelinated neurons.
Contains a ventral and dorsal root for each nerve. Dorsal root receives sensory impulses and transfers them to the brain. The Ventral root sends out motor impulses from the brain to the effectors (glands or muscles)
Reflex Acts and Arcs
Reflex act: An automatic involuntary response to a stimulus – a reflex.
Reflex arc: It is a nerve path that leads from a stimulus to a reflex action.
Let's take a quick break from our notes and explore a couple of unique areas of our brain!
When Broca's area is damaged, a person is able to understand and process information. But unfortunately they are unable to produce language properly whether it be spoken or written. This can clearly lead to a lot of frustration by the person afflicted because they are completely aware of their deficits .
When Wernicke's area is damaged, the person afflicted is unable to understand language written or spoken. When they speak, they think they know what they are saying, but a bunch of random words just come out. The patient is usually unaware of the .problem
(A) Sensory neuron receives a stimulus (such as heat or pain) that triggers nerve endings in hand. The nerve endings are dendrites of a sensory neuron. A strong stimulus is required to activate the neuron.
(B) An impulse travels along this neuron and goes to the spinal cord where the signal is passed on to interneurons — (link between sensory and motor neurons).
(C) A motor neuron is stimulated and transmits an impulse along its axon. The motor neuron triggers contractions of muscles in your arm and you pull your hand away.
(D) While this happens, other interneurons in the spinal cord transmit a message to your brain making you aware of what has happened.
Parasympathetic Nervous System: It has the opposite effect of the sympathetic nervous system. When threat has passed, nerves of this system slow heart rate and breathing rate.
➢ We associate the PNS with “rest and digest”. It causes the pupil of the eye to contract, promotes food digestion and slows the heartbeat.
( nerve cells )
Neurons are the structural and functional unit of the nervous system. Neurons are cells that send and receive electro-chemical signals to and from the brain and nervous system. There are about 100 billion neurons in the brain. Unlike most other cells, neurons cannot regrow after damage. Fortunately, there are about 100 billion neurons in the brain. They can transmit nerve signals to and from the brain at up to 200 miles/h (or 267 km/h). There are many type of neurons. They vary in size from 4 microns (0.004 mm) to 100 microns (0.1 mm) in diameter. Their length varies from a fraction of an inch to several feet.
Points of Interest : PNS consists of nerves or numerous neurons held together by connective tissue. CNS is also made up of neurons and contain 90 % of body’s neurons.
Neurons consist of three basic parts :
(1) Cell body ; (2) Dendrites ; (3) Axon.
Dendrites are the primary site for receiving signals from other neurons. Depending on the neurons function, the number can range from one to thousands.
Cell body has a large centrally located nucleus and a large nucleolus. It’s cytoplasm contain many mitochondria along with a Golgi complex and rough ER.
Neurons are capable of surviving for over 100 years since many do not undergo cell division after adolescence
Axon is a long cylindrical extension of the cell body than can range from 1mm to 1m. It transmits waves of depolarization along its length and at the end of the axon are structures that release chemicals
Axon terminal is the bulb like ends of the axon, (end brushes or terminal ending).
Schwann Cells insulate cells around an axon. It speeds up waves of depolarization. It is covered with a fatty layer called myelin sheath. Each Schwann cell is separated by a gap called the node of Ranvier.
A myelinated neuron enables nerve impulses to jump from one node of Ranvier to the next speeding up the wave of depolarization to about 120 m/s — (many signals only move at 2 m/s). This is called Saltatory Conduction
Schwann cells also have an important function. Most mature neurons do not reproduce. If the outer layer of Schwann cell ( neurolemma) is present in a neuron , the cell is capable of regenerating if the damage is not too severe. If a neuron is cut, the severed end of the axon grows a number of extensions or sprouts and the original axon grows a regeneration tube from its neurolemma If one of the sprouts connects with the regeneration tube , the axon can re-form itself. * CNS neurons do not regenerate.
(1) Sensory neurons take information from a receptor (pain) to CNS. They require a strong stimulus.
(2) An interneuron is a nerve cell that acts as a
link between a sensory neuron and a motor neuron. It receives information from other interneurons and from sensory neurons.
(3) Motor neurons take information from the CNS to an effector, such as a muscle or gland.
HOW A NEURON WORKS
Resting Potential/Resting Neuron
When a Neuron is at rest it normally has a positive (+) charge on the outside of the membrane while having a negative (-) charge on the inside. There is a voltage difference of -70 mV referred to as the Resting Potential or Threshold level that exists in this condition.
How is Resting Potential Achieved?
The outside has high concentrations of sodium ions and lower concentrations of potassium ions.
Chlorine which has a negative charge is also outside the membrane. Inside the cell there are proteins, amino acids and phosphates and sulfates which have a negative charge. The positive charges inside the membrane are caused by a high concentration of potassium and a lower concentration of sodium.
The membrane has specialized channels for the movement of sodium, potassium and chlorine, but proteins and amino acids (larger anions) are trapped inside the cell.
At rest, the membrane is 50 times more permeable to potassium than to sodium which means that while sodium is moving into the cell there is more potassium diffusing out of the cell. When this happens this causes the inside of the cell to become more negatively charged. Although the increasing negative charge within the cell attracts both the sodium and the potassium the sodium – potassium pump found in the cell membrane offsets this attraction.
Sodium Channels OPEN
Sodium Channels CLOSE
Potassium Channels OPEN
Potassium Channels CLOSE
SODIUM POTASSIUM PUMP ACTIVATES
Neurons do not touch each other. They have tiny gaps between them called synapses. The presynaptic neuron carries wave of depolarization toward the synapse. The postsynaptic neuron receives the stimulus.
How does the wave cross the synapse?
(1) Wave of depolarization reaches the end of presynaptic axon.
(2) It triggers opening of special calcium ion gates.
(3) Calcium triggers the release of neurotransmitter molecules (by exocytosis).
(4) Neurotransmitter is released from special vacuoles called synaptic vesicles.
(5) Neurotransmitter diffuses into the gap between the axon and the dendrites of the neighbouring postsynaptic neurons.
(6) Neurotransmitter attaches to receptors on dendrites and excites or inhibits the neuron.
NOTE : An excitatory response opens the sodium gates and triggers a wave of depolarization. Inhibitory response makes the postsynaptic neuron more negative on the inside which raises the threshold of the stimulus.
Neurons can stimulate more than other neurons - muscles and glands are also stimulated in the same way. Muscles contract. Glands secrete substances such as hormones.
The pump uses active transport to pull three sodium cations from the inside of the cell to the outside and in exchange two potassium cations are pulled from the outside to the inside and thereby increasing the difference in the charge. The slight difference in charge is due to the unequal distribution of cations and anions. The difference in charge is about -70 mV and is referred to as the resting potential.
1. Noradrenaline (also called norepinephrine) is the primary neurotransmitter of the
sympathetic nervous system — (fight or flight).
2. Glutamate is a neurotransmitter of the cerebral cortex that accounts for 75 % of all excitatory transmissions from the brain.
3. GABA (gamma aminobutyric acid) is the most common inhibitory neurotransmitter of the brain.
4. Dopamine elevates mood and controls skeletal muscles.
5. Seratonin is involved in alertness, sleepiness, thermoregulation and mood.
NOTE : Cells within the nervous system require enormous amounts of energy to function. This energy is provided with the processing of glucose and the production of ATP within these tissues, requiring an adequate supply of carbohydrates and oxygen. ATP energy is required to operate the sodium potassium pump which converts cellular chemical signals into electrical signals along a nerve cell and between them (synapse).
DISORDERS OF THE NERVOUS SYSTEM
(1) Multiple Sclerosis (MS)
It affects nerve cells surrounding brain and spinal cord. The myelin becomes inflamed or damaged and disrupts impulses. Some symptoms are: blurred or double vision, slurred speech, loss of coordination, muscle weakness, tingling or numbness in arms and legs, and seizures. MS attacks occur in episodes where symptoms become worse alternating with periods where the symptoms improve. Many people have MS which is rapid and severe. MS is believed to be an autoimmune disorder where the immune system attacks the myelin sheaths of the body’s own nerve cells. At present there is no cure, but treatment involves medication to suppress the autoimmune reaction.
(2) Alzheimer’s Disease
It causes impairment of brain’s intellectual function such as memory and orientation. The brain gradually deteriorates causing memory loss, confusion, and impaired judgement. Alzheimer’s results from protein deposits called amyloids that distort the communication paths between brain cells. Also, acetylcholine levels begin to drop causing further breakdown of communication. There is no present means of prevention, but cholinesterase inhibitors are given to increase the levels of acetylcholine and to improve intellectual function.
(3) Parkinson’s disease
It is a chronic movement disorder caused by the gradual death of cells that produce dopamine. Remember that dopamine carries messages between the areas of brain controlling body movements. The symptoms begin as tremors in one side of the body and as disease progresses, the tremors spread to both sides of the body causing the limbs to become rigid, body movements to slow and an abnormal gait to develop. By the time the first symptoms develop, 70 - 80 % of the brain cells that produce dopamine have already been lost. Treatments involve medication to boost dopamine levels — (sadly, long term use can impair mental abilities). Also, small lesions in the brain can be created by surgeons or implanting electrodes in parts of brain that are overactive.
It is a viral or bacterial infection of the meninges. Viral meningitis is common in children and usually clears after 7-10 days. If not treated immediately, the more serious bacterial meningitis is usually fatal. Symptoms include: headache, fever, and a stiff neck, light sensitivity, drowsiness, and vomiting. It is diagnosed by testing the cerebrospinal fluid that surrounds the spinal cord for the presence of the bacteria or immune system activity (spinal tap or lumbar puncture). Vaccines are available for bacterial meningitis, but it can have severe long-lasting effects such as hearing impairment. Fatality rates are 10%.
(5) Huntington’s Disease (or Huntington’s Chorea)
It is a fatal autosomal dominant disorder in which the nerve cells in certain parts of the brain deteriorate. It causes major progressive decreases in mental and emotional abilities and loss of control over major muscle movements. Each child of a parent with Huntington’s has a 50 % chance of inheriting the disorder. There is no cure at present and no way to slow its progression. Symptoms include: memory loss, dementia, involuntary twitching, clumsiness, chorea (jerky movements), and personality changes.
The brain and spinal cord make up the central nervous system (CNS).
o Meninges are protective membranes that wrap around both spinal cord and brain.
o The spaces between the meninges are filled with cerebrospinal fluid, which helps to protect and cushion the CNS.
o The brain has hollow cavities that are connected to the central canal of the spinal cord called ventricles.
o These hollow cavities are a reservoir for cerebrospinal fluid.
➢ When your body thinks you’re in danger, the SNS accelerates the heartbeat and dilates the bronchi, in order to supply active muscles with enough glucose and oxygen.
➢ The SNS also inhibits your digestive tract because if you’re in immediate danger, digestion is not important.
Conditions Necessary for a Nervous Response? These four things must occur — (1) Sensory receptors to detect a stimulus (skin, eye, ear); (2) Method for impulse transmission (neurons); (3) Interpretation of analysis of impulses (brain and spinal cord); (4) Response carried out by an effector (muscle, gland).