UNIT 4: Homeostasis
Chapter 11: The Nervous System
pg. 514
11.1: The Role of the Nervous System
pg 516 - 521
Organisms need to senses heir environments to make appropriate
adjustments and survive. The nervous system and the endocrine system are
responsible for these adjustments.
The nervous system allows organisms to sense and respond to external
environments and control their internal environment.
Neural Signaling
Neuron – is a nerve cell that is capable of conducting nerve impulses.
Neural signaling – is the reception, transmission, and integration of nerve
impulses by neurons, and the response of these to these impulses.
Afferent neuron – is a neuron that carries impulses from the sensory
receptors to the central nervous system; also called a sensory neuron.
Interneuron - is a local circuit neuron of the central nervous system that
relays impulses between afferent (sensory) and efferent neurons (motor).
Efferent neuron – is neuron that carries impulses from the central nervous
system to skeletal muscles; also known as a motor neuron.
Dendrite – is a projection of cytosol that carries signals toward the nerve cell
body.
Axon – is an extension of cytosol that carries nerve signals away from the
nerve cell body.
The neuron is the functional unit of the nervous systems. It is responsible for
sensing and responding to internal and external stimuli. There are four
components to a nervous system that allow neural signaling; reception,
transmission, integration, and response.
Reception is the detection of a stimulus, performed by neurons and
specialized sensory receptors, such as; eyes and skin.
Transmission is the movement of the neural signal along one neuron to
either another neuron or a muscle or gland.
Integration is the sorting and interpretation of multiple neural messages and
the determination of the appropriate response.
The response is the output or action resulting from the integration.
Neural signaling requires three functional classes of neurons; afferent neuron,
interneuron and efferent neuron. The afferent neuron is also known as the
sensory neuron is responsible for transmitting the stimuli received by the
sensory receptor, to the interneuron.
The interneuron will integrate the information to determine the appropriate
response. The interneurons are located in the spinal cord and the brain. The
efferent neurons deliver the response signal to the effectors (muscles or
glands). The muscles that move our skeleton are called motor neurons.
Summary:
1.
2.
3.
4.
Stimulus reception by sensory receptors on afferent neurons.
Message transmission by afferent neurons to interneurons.
Integration of neural messages in interneurons.
Response by the transmission of neural messages by efferent neurons
to effectors.
Neurons consist of enlarged cell body and two types of extensions. The cell
body, which contains the nucleus and most organelles in the cell perform the
cellular functions, producing neural proteins, carbohydrates and lipids.
The projections from the cell’s body are responsible for conducting
electrochemical signals. Ions and concentration gradients are responsible for
producing the electrochemical signals.
A dendrite is highly branched projections from the cell’s body that receives
the signals and transmits them to the cell body. The axons are projections
from the cell’s body that carry the electrochemical signals away to another
neuron or an effector. An axon is usually a single branch that arises from a
junction called an axon hillock. The axon will branch at its end, with each
branch ending in a small button like swelling called an axon terminal. The
axon terminals are the points where the signal is enabled to be transmitted
from one neuron to another other or to an effector.
Figure 3: the structure of a neuron (nerve cell). The arrows indicate the direction of the
electrical impulse that travels along the neuron.
Axons are usually bundled together to form nerve fibres or commonly
known as nerves. The nerves branch out extensively to relay signals
throughout the periphery of the entire body.
The neuron circuit consists of the dendrite of one cell interacting with the
axon of an adjacent cell. A typical neuronal circuit contains an afferent
neuron, many interneurons, and efferent neuron.
Figure 4: Axons are
bound together to form
nerve fibres similar to the
way small fibres are
bundled together to form a
fibre-optic telephone
cable.
Neuron Support System
Glial cell – is non-conducting cell that is important for the structural support
and metabolism of nerve cells. Provide nutrition and support.
One type of glial cell is the Schwann cell which forms a tightly wrapped
layer of plasma membrane called the Myelin sheath.
Myelin Sheath – is an insulated covering over the axon of a nerve cell.
These cells consist of high lipid content, acting as electrical insulator,
making sure the nerve impulse travels along the axon and accelerates the
rate of which the electrical impulse travels.
Node of Ranvier – is a regularly occurring gap between sections of myelin
sheath along the axon.
Figure 5: The myelin sheath formed by Schwann cells act like an electrical insulator. As
many as 300 overlapping layers of Schwann cell plasma membrane wind around an axon,
like a jelly roll.
The Structure and Organization of the Human Nervous System
Central Nervous System (CNS) – is the body’s coordinating centre for
mechanical and chemical actions; made up of the brain and spinal cord.
Peripheral Nervous System (PNS) – are all the parts of the nervous system,
excluding the brain and spinal cord; relays information between the central
nervous system and other parts of the body.
Afferent system – is the component of the peripheral nervous system that
receives input through receptors and transmits the input to the central
nervous system.
Efferent system – is the component of the peripheral nervous system that
carries signals away to the effectors (muscles and glands).
Somatic system – is a subdivision of the efferent system (within the PNS);
composed of efferent (motor) neurons that carry signals to skeletal muscles
in response to external stimuli.
Autonomic system – is a subdivision of the efferent (within the PNS);
regulates the internal environment.
Sympathetic division – is one of two subdivisions of the autonomic nervous
system; increases energy consumption and prepares the body for action.
Parasympathetic division – is one of two subdivisions of the autonomic
nervous system; stimulates body activities that acquire and conserve energy.
The nervous system is made up of two functional subsystems. The two
subsystems are the Central Nervous System; consisting of the brain and the
spinal cord, and the Peripheral Nervous System; made up of the afferent
(carrying towards) and the efferent systems (carrying away).
The efferent system is also subdivided into two sub systems; the Somatic
System (communicates to the skeletal muscles) which is a voluntary system,
and the Autonomic System (communicates with the muscles and glands)
which is an involuntary system.
The Autonomic system is also subdivided into two more systems; the
sympathetic division and the parasympathetic division.
The sympathetic system is involved in situations that involve stress, danger,
excitement, or strenuous physical activity, such as; increasing the force and
heart rate.
The parasympathetic system is involved in situations that are quiet, lowstress situations, such as; relaxation.
These systems are always active and oppose each others affect on organs
and enabling precise control over an organ.
Figure 6: Overview of the human nervous system, comprising the central and peripheral
nervous systems (CNS and PNS).
Neural Circuits and the Reflex Arc
Neural circuit – is the coordination of the receptor, afferent neuron,
interneuron, efferent neuron, and effector in response to a stimulus.
Reflex Arc – is a neural circuit that travels through the spinal cord but does
not require the coordination of the brain; allows for reflex actions.
A reflex arc occurs when you accidentally come in contact with a stimulus,
your reaction is instantaneous. You do not think about the reaction, it just
occurs. The afferent neuron transmits the impulse to the interneurons in the
spinal cord, which in turns relays a response to the efferent neurons, motor
neurons for a rapid withdrawal from the stimulus. This reaction is
involuntary and occurs in a fraction of a second, and without conscious
thought.
This is an example of a neural circuit, consisting of five components;
receptor, afferent neuron, interneuron, efferent neuron, and final the motor
neuron, also known as the reflex arc.
The interneurons in the spinal cord also send a signal to the brain, so you are
made aware of the situation and the cause of the reflex arc.
Figure 7: The withdrawal reflex is the result of a series of steps that nervous system uses
to integrate incoming information and respond appropriately. It happens sp rapidly that
the experience of pain occurs after the hand is already withdrawn from the stimulus, such
as; the hot stove element.