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Nervous-Controlin-Humans
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Uploaded: 7 years ago
Category: Biology
Type: Lecture Notes
Tags: impulses, neurons, neuron, impulse, spinal, across, postsynaptic, synaptic, messages, reflex, neurotransmitter, presynaptic, synapses, nervous, sensory
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Transcript
Nervous Control in Humans
The Nervous System:
Central Nervous System (CNS):
Brain – Controls the whole nervous system by receiving messages from and sending messages to different parts of the body
Spinal Cord – A great number of nerve cells grouped together
Peripheral Nervous System (PNS):
Cranial nerves – connected to the brain and carry messages directly to and from the brain
Spinal Nerves – connected to the spinal cord and carry messages from the receptors to the spinal cord and the brain and from the spinal cord and the brain to the receptors.
Neurons:
Neurons are nerve cells specially adapted to conduct messages in the form of electric signals called nerve impulses
Main Parts:
3424555227965Sensory Neuron
00Sensory Neuron
Cell Body – contains the cell organelles: nucleus, mitochondria, ribosome’s, endoplasmic reticulum
36556953937000Dendrites - carry impulses to cell body
Axon – carries impulses away from cell body
Types of Neurons:
Sensory Neurons:
Function:
Carry impulses from receptors to the CNS
Properties:
Single long Dendron
351282025400Motor Neuron
00Motor Neuron
365569528448000Relatively short axon
Motor Neurons:
Function:
Carry impulses from the CNS to the effectors such as muscles and glands
Properties:
A number of short dendrites
A single long axon
Relay Neurons (Interneurons)
Function:
Carry impulses inside the CNS from a sensory neuron either to a motor neuron or to another relay neuron.
Properties:
A number of short dendrites
A short axon
Detailed Structure of a Motor Neuron
Long axon
Can be up to one meter in length
Carries impulses away from the cell body and to the effector, such as a muscle
Myelin Sheath
Surrounds the axon
Consists of Swann Cells
Myelin is a fatty substance that insulates the axon and speeds up the transmission of an electrical impulse along the axon.
Nodes of Ranvier
Gaps in the Myelin Sheath between Swann cells
No insulation at the gaps
The impulse jumps from one gap to the next
Increase the rate of transmission of the electrical impulse
Dendrites
Short
Unmyelinated
Carry impulses from other neurons to the cell body
Reflex Actions:
Characteristics:
The simplest responses to stimuli
Inborn – cannot be learned, but are there from birth
Rapid, Automatic and involuntary responses to specific stimuli
Each reflex always follows a specific pattern and gives the same response
Spinal Cord:
Protected by backbone
Path taken by an impulse:
Sensory Nerve Receptor Cells in Spinal Cord Relay Neuron Motor Neuron Effector Muscle
The relay neuron allows the impulse to be transmitted to other motor neurons and different effectors
Reflex Arc:
The pathway a nerve impulse travels along to bring about a reflex action
Simplest reflex arc (two neuron spinal reflex arc) involves only sensory and motor neurons
Knee jerk reflex:
The hit on the tendon is the stimulus
Special stretch receptor cells in the thigh muscle detect this stimulus
A nerve impulse is sent along sensory neurons to the spinal cord
In the spinal cord, the nerve impulses from the sensory neurons pass to the motor neurons
The motor neurons carry the nerve impulses from the spinal cord to the same thigh muscles
Most reflex arcs involve relay neurons and trigger both voluntary and involuntary actions.
Synapses:
32289753746500The junction of two or more nerve cells
Between any two communicating neurons is a synaptic gap
Nerve impulses are transmitted across the synaptic gap by means of a chemical substance which is released.
Neurons on either side of the synapse are described as presynaptic and postsynaptic neurons.
The presynaptic neuron communicates with the postsynaptic neuron by releasing a, chemical messenger, a neurotransmitter such as acetylcholine, across the synapse.
742950952500
Transmission across a synapse:
The vesicles of the axon terminal release a neurotransmitter into the synaptic gap
The neurotransmitter diffuses across the gap and combines with receptor molecules on the postsynaptic membrane
This triggers a nerve impulse in the postsynaptic neuron
The neurotransmitter in the postsynaptic membrane is then broken down by enzymes
Types of Synapses:
Excitatory – They transmit a nerve impulse
Inhibitory – They block a nerve impulse
The type of synapses depends on the kind of receptor molecules of the postsynaptic membrane
One Way Transmission
The synapses, not the neurons are responsible for the one way transmission of impulses
Impulses travel from the axon of the presynaptic neuron, across the synapses to the dendrites of the post synaptic neurones
Only the presynaptic neurons can produce the neurotransmitter and only the post synaptic neurons have the receptors for them.
Synapses and Integration
One neuron may synapse with many other neurons
e.g. A motor neuron in the spinal cord may have hundreds of synapses with other neurons
The dendrites of the postsynaptic neurons need to sort-out and integrate all the incoming messages from the axons of the presynaptic neurons to ensure the appropriate response.
Effect of opiate drugs on the synapses
Opiate drugs contain opium, or a derivative, morphine.
Opiates work by blocking the release of neurotransmitters at the synapses.
This prevents impulses from travelling across the synaptic gap and reaching the effectors.
Effects:
Numbness + Sleepiness
Reduces pain
Uses:
Medicinally as a sedative, to encourage relaxation and sleep
As painkillers
Mimic the activity of endorphins, naturally occurring painkillers which are released by the body
Caffeine and Nicotine have the opposite effect of opiate drugs:
They act as stimulants
Enable impulses to pass more easily across the synapses.
Nervous and Hormonal Control
The nervous and endocrine systems share the same basic function of controlling and co-ordinating the majority of activities in the body.
Nervous System
Endocrine System
Sends electrical impulses along high speed nerve cells
Hormones travel more slowly from one place to another through the blood stream
Carry message to one particular part of the body
Information is precise and controlled with a high degree of accuracy
Effects of hormones are widespread and diffuse
Only the hormone’s target cells will respond to its stimulus
The effect of a nerve impulse is immediate and final.
Any further stimulation or activity will require another impulse from the nervous system
Effects of hormones are long lasting
They exert their effect over their target cells for several hours or even days
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