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Summary Notes on the nervous system and action potentials
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A level notes on the nervous system and action potentials
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The Mammalian Nervous SystemThe communication system must enable:
• detection of changes in the environment
• cell signaling to occur between all parts of the body
• coordination of a range of effectors to carry out responses to the sensory input
• suitable responses
Divisions of the nervous system
Central Nervous System (CNS)
• consists of brain and spinal cord
• much of the brain is composed of relay neurons, which are non-myelinated and make up the
grey matter
• the spinal cord also has many non-myelinated relay neurons, making up the central grey
matter
• the spinal cord also contains many myelinated neurons, making up the outer region of white
matter; the myelinated neurons carry action potentials up and down the spinal cord for
rapid communication over longer distances
• the spinal cord is protected by the vertebral column; between each of the vertebrae,
peripheral nerves enter and leave the spinal cord carrying action potentials to and from the
rest of the body
Peripheral nervous system (PNS)
• composed of sensory and motor neurons, which are bundled together in a connective tissue
sheath to form nerves
• ensures rapid communication between the sensory receptors, the CNS and effectors
,Sensory nervous system
• the sensory fibers entering the CNS are dendrons of the sensory neurons, which conduct
action potentials from the sensory receptors into the CNS
• sensory neurons have their cell body in the dorsal root leading into the spinal cord and a
short axon connecting to other neurons in the CNS
Motor nervous system
• conducts action potentials from the CNS to the effectors
Somatic nervous system
o consists of motor neurons that conduct action potentials from the CNS to the effectors that
are under voluntary control, such as the skeletal muscles
o neurons are mostly myelinated, so that response can be rapid
o there is always 1 single motor neuron connecting the CNS to the effector
Autonomic nervous system
o consists of motor neurons that conduct action potentials from the CNS to effectors that are
not under voluntary control, such as glands, the cardiac muscle and smooth muscle in wall
of blood vessels, the airways and wall of the digestive system
o neurons are mostly non-myelinated, as it does not require rapid responses
o there are at least 2 neurons involved in the connection between CNS and the effector
o these neurons are connected at small
swellings call ganglia
o can be further divide into the
sympathetic system, which prepares
the body for activity, and the
parasympathetic system, which
conserves energy
→antagonistic systems, as the action
of one system opposes the action of
the other
→ changes to internal conditions or
stress, lead to changes in the balance
of stimulation between the 2
systems, leading to an appropriate
response
, Structure and function of neurons
Structure of neurons
o Many are very long so that they can transmit the action potential over a long distance
o The CSM has many gated ion channels that control the entry or exit of Na+, K+ or Ca2+ ions
o Na+/K+ pumps use ATP to actively transport Na+ ions out of the cell and K+ ions into the cell
o Neurons maintain a potential difference across their CSM
o A cell body contains the nucleus, many mitochondria and ribosomes
o Numerous dendrites connect to other neurons. The dendrites carry impulses towards the cell body
o An axon carries impulses away from the cell body
o Neurones are surrounded by a fatty layer, composed of Schwann cells, that insulates the cell from
electrical activity in other nerve cells nearby
Motor neurons – carry an action Sensory neurons – carry the Relay neurons – connect
potential from the CNS to an action potential from the sensory and motor neurons
effector (muscle or gland) sensory receptor to the CNS
Difference between types of neurons
• Motor neurones have their cell body in the CNS and have a long axon that carries the action
potential out to the effector
• Sensory neurons have a long dendron carrying the action potential from a sensory receptor to
the cell body, which is positioned just outside the CNS. They then have a short axon carrying the
action potential into the CNS
• Relay neurons connect the sensory and motor neurons together. They have many short dendrites
and a short axon. The number of dendrites and the number of divisions of the axon is variable.
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