Biology SS 1 Week: 6
Topic: Peripheral Nervous System
Introduction
The peripheral nervous system (PNS) is the division of the nervous system containing all the nerves that lay outside of the central nervous system (CNS), that is, the PNS consists of all other nerves and neurons that do not lie within the CNS. The peripheral nervous system is divided into two, the somatic nervous system and the autonomic nervous system.
There are two types of cells in the peripheral nervous system, these include:
Sensory nervous cell: sends information to the CNS from internal organs or from external stimuli.
Motor nervous cells: carries information from the CNS to organs, muscles, and glands.
Peripheral Nervous System Divisions
The peripheral nervous system is divided into the following sections:
Somatic Nervous System – controls skeletal muscle as well as external sensory organs.
Autonomic Nervous System – controls involuntary muscles, such as smooth and cardiac muscle. This is further divided into two;
Sympathetic – controls activities that increase energy expenditures.
Parasympathetic – controls activities that conserve energy expenditures.
The somatic nervous system: The somatic nervous system is responsible for coordinating the body’s movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.This system is said to be voluntary because the responses can be controlled consciously. Reflex reactions of skeletal muscle however are an exception. These are involuntary reactions to external stimuli.
The autonomic nervous system: This controls involuntary muscles, such as smooth and cardiac muscle. This system is also called the involuntary nervous system. The autonomic nervous system can further be divided into the parasympathetic and sympathetic divisions.
The sympathetic nervous system: responds to impending danger or stress (the flight or fight response), and is responsible for the increase of one’s heartbeat and blood pressure, dilate pupils, relax the bladder and the sense of excitement one feels due to the increase of adrenaline in the system.
The parasympathetic nervous system: on the other hand, is evident when a person is resting and feels relaxed, and is responsible for the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, contracting the bladder and the stimulation of the digestive and genitourinary systems. The nerves of the sympathetic division often have an opposite effect when they are located within the same organs as parasympathetic nerves.
Differences between the Somatic and the Autonomic Nervous Systems
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Somatic Nervous System |
Autonomic Nervous System |
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1) |
Impulses speed along motor fibres that extend from CNS to effectors without synapses |
Impulses speed along motor fibres that extend from CNS to where they synapse. |
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2) |
It affects skeletal muscles. |
It affects glands, cardiac muscles and smooth muscles. |
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3) |
It always stimulates effectors. |
It may stimulate or inhibit effectors. |
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4) |
Body activities are mainly voluntary. |
Activities are mainly involuntary. |
Peripheral Nervous System Connections
Peripheral nervous system connections with various organs and structures of the body are established through cranial nerves and spinal nerves. There are 12 pairs of cranial nerves in the brain that establish connections in the head and upper body, while 31 pairs of spinal nerves do the same for the rest of the body. While some cranial nerves contain only sensory neurons, most cranial nerves and all spinal nerves contain both motor and sensory neurons.
The Neurone or Nerve Cell
The nerve cell or neurone is defined as the basic unit of nervous system which is responsible for the transmission of impulses within the body.
The brain is what it is because of the structural and functional properties of interconnected neurones. The mammalian brain contains between 100 million and 100 billion neurones, depending on the species. Each mammalian neurone consists of a cell body, dendrites, and an axon.
Structure of a Neurone
The neurone has a cell body (Soma) which contains the nucleus and cytoplasm. The axon extends from the cell body and often gives rise to many smaller branches before ending at nerve terminals.
Dendrites extend from the neurone cell body and receive messages from other neurons. Synapses are the contact points where one neuron communicates with another. The dendrites are covered with synapses formed by the ends of axons from other neurones.
When neurones receive or send messages, they transmit electrical impulses along their axons. Many axons are covered with a layered myelin sheath, which accelerates the transmission of electrical signals along the axon. This sheath is made by specialized cells called glia. In the brain, the glia that make the sheath are called oligodendrocytes, and in the peripheral nervous system, they are known as Schwann cells.
The brain contains at least ten times more glia than neurones. The glia performs many jobs. Researchers have known for a while that glia transport nutrients to neurones, clean up brain debris, digest parts of dead neurons, and help hold neurons in place.
There are several differences between axons and dendrites:
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Axon |
Dendrites |
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1. |
Take information away from the cell body. |
Bring information to the cell body. |
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2. |
Smooth surface. |
Rough surface (dentritic spines) |
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3. |
Generally only 1 axon per cell. |
Usually many dendrites per cell. |
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4. |
No ribosomes. |
Have ribosomes. |
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5. |
Can have myelin. |
No myelin insulation. |
Different Types Of Neurones
There are different types of neurones. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are found in different parts of the body.
Sensory neurones or bipolar neurones carry messages from the body’s sense receptors (eyes, ears, etc.) to the CNS. These neurones have two processes. Sensory neurone account for 0.9% of all neurones. (Examples are retinal cells, olfactory epithelium cells.)
Motor neurones or multipolar neurones carry signals from the CNS to the muscles and glands. These neurones have many processes originating from the cell body. Motor neurones account for 9% of all neurones. (Examples are spinal motor neurones, pyramidal neurones.)
Intermediate neurones or Pseudopolar (Spelling) cells form all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord; one with either the skin or muscle. These neurones have two processes. (Examples are dorsal root ganglia cells.)
The Transmission of Nerve Impulses
Nerve impulses have a domino effect. Each neuron receives an impulse and must pass it on to the next neurone and make sure the correct impulse continues on its path. Through a chain of chemical events, the dendrites (part of a neurone) pick up an impulse that’s shuttled through the axon and transmitted to the next neurone. The entire impulse passes through a neurone in about seven milliseconds — faster than a lightning strike. Here’s what happens in just four easy steps:
Polarization of the neurone’s membrane: Sodium is on the outside, and potassium is on the inside. Cell membranes surround neurones just as any other cell in the body has a membrane. When a neurone is not stimulated — it’s just sitting with no impulse to carry or transmit — its membrane is polarized, not paralyzed. A polarized membrane means that the electrical charge on the outside of the membrane is positive while the electrical charge on the inside of the membrane is negative. The outside of the cell contains excess sodium ions (Na+); the inside of the cell contains excess potassium ions (K+). (Ions are atoms of an element with a positive or negative charge.)
Resting potential gives the neuron a break: When the neuron is inactive and polarized, it’s said to be at its resting potential. It remains this way until a stimulus comes along.
Action potential: Sodium ions move inside the membrane. When a stimulus reaches a resting neurone, the gated ion channels on the resting neurone’s membrane open suddenly and allow the Na+ that was on the outside of the membrane to go rushing into the cell. As this happens, the neurone goes from being polarized to being depolarized. When the neurone was polarized, the outside of the membrane was positive, and the inside of the membrane was negative. Well, after more positive ions go charging inside the membrane, the inside becomes positive, as well; polarization is removed and the threshold is reached.
Each neurone has a threshold level — the point at which there’s no holding back. After the stimulus goes above the threshold level, more gated ion channels open and allow more Na+ inside the cell. This causes complete depolarization of the neurone and an action potential is created. In this state, the neurone continues to open Na+ channels all along the membrane. When this occurs, it’s an all-or-none phenomenon. “All-or-none” means that if a stimulus doesn’t exceed the threshold level and cause all the gates to open, no action potential results. However, after the threshold is crossed, there’s no turning back. Complete depolarization occurs and the stimulus will be transmitted.
When an impulse travels down an axon covered by a myelin sheath, the impulse must move between the uninsulated gaps called nodes of Ranvier that exist between each Schwann cell.
Repolarization: Potassium ions move outside, and sodium ions stay inside the membrane. After the inside of the cell becomes flooded with Na+, the gated ion channels on the inside of the membrane open to allow the K+ to move to the outside of the membrane. With K+ moving to the outside, the membrane’s repolarization restores electrical balance, although it’s opposite of the initial polarized membrane that had Na+ on the outside and K+ on the inside. Just after the K+ gates open, the Na+ gates close; otherwise, the membrane couldn’t repolarize.
Functions of Neurones
The nerves conduct impulses to the brain (i.e. sensory neurone).
The nerves, e.g. motor n