Nervous-Controlin-Humans

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