Explore Introductory Mammalian Physiology with these specialized notes crafted for Year 1 students at University College London. Immerse yourself in the intricacies of the Nervous System, where discussions unfold on the autonomic nervous system, anatomy, reflexes, neuronal structure and function, r...
Neuronal Structure and Function
Structure of the Nervous System
• Objectives
o Understand and name anatomical divisions of the nervous system
o Name and state the function of the main cells of the nervous system – neurons and glia
o Describe the structure of neurons and projections
• Basic anatomical organisation of the nervous system
o Divisions
▪ Brain
▪ Spinal cord
▪ Peripheral nerves
▪ Autonomic nervous system
▪ Enteric nervous system
• Controls activity of the gut
o Overall structure
▪
• CNS – central nervous system – integration system
o Brain + spinal cord
o Afferent signals enter CNS → processed in spinal cord or brain → efferent
signals leave CNS
• PNS – peripheral nervous system
o Input (afferent) system
▪ Somatic senses
▪ Special senses
▪ Visceral senses
o Output (efferent) system
▪ Somatic
▪ Autonomic
• Sympathetic
• Parasympathetic
o Spinal cord
▪ Cross section of the spinal cord
• Grey matter
o Cell bodies
• White matter
o Myelinated axons
▪ Where axons travel in the spinal cord to the brain or out to the
target cells
▪ Afferent system
• Brings signals from the periphery via the dorsal (posterior) horn
▪ Efferent system
• After processing in the spinal cord or brain – efferent fibres carries signals away via
the ventral route
,Neuronal Structure and Function
• Cells of the nervous system – neurons & glia
o Nerve cells (neurons)
▪ Have thread-like extensions
• Dendrites = extensions from the input end
• Axons = extensions from the output end
▪ Different types
• Motor
• Sensory
• Interneurons
▪ Cell bodies are found in grey matter of CNS and in autonomic ganglia
▪ Main structural features
•
o Cell body (soma)
▪ Metabolic engine for the neuron
o Dendrites
▪ Extensions from the input end
o Dendritic spines
▪ Protusions from dendrites that receive input rom a single axon at
the synapse
o Axon hillock
▪ Important trigger zone for action potential
o Axon
▪ Extensions from the output end
o Axon terminals
▪ Synaptic knobs
▪ Makes synaptic connections with other neurons
▪ Types – according to number of processes extending from the cell body
• Multipolar neurons – many processes
• Bipolar neurons – 2 processes
• Unipolar neurons – 1 process
o Pseudo-unipolar neurons
▪ 1 process – typical of sensory neuron
o Glial cells (neuroglia)
▪ Support cells of the CNS
▪ 4 main types
• Astroglia
o Attach to blood vessels
and neurons
o Provide nutrients to
neurons
• Oligodendrocytes
,Neuronal Structure and Function
o Myelin cells
• Microglia
o Important for protecting neurons from infection
• Ependymal cells
o Types
▪ Schwann cells – in PNS
• Myelin cells
▪ Macrophages
o Line fluid filled spaces – ventricles of the brain
▪ Myelin cells
• Schwann cell / oligodendrocyte – myelin sheath
o Schwann cells in PNS
▪ Only covers a single axon
o Oligodendrocytes in CNS
▪ Form myelin around neurons over several axons
• Myelin wraps around axon → forming sheath
• Nerve trunk structure
o Axons
▪ Convey information from neuron to neuron
▪ Organised in bundles – fascicles
▪ Anchored in the connective tissue by the
epineurium
▪ Each bundle is covered by perineurium
▪ Endoneurium – covers individual nerve fibres
• Summary
o Neurons are supported by glia cells
▪ Function of neurons is to transmit information to other neurons or to the neuromuscular
junction via its axon
▪ Neurons are excitable cells – separation of electrical charge across the membrane
• Capable of producing large rapid electrical signals = action potentials
o In PNS – axons are surrounded by schwann cells – which run in peripheral nerve trunks alongside
blood vessels
o Nerves (bundles of axons) can be sensory, motor or mixed
o Peripheral nerves can also contain sympathetic postganglionic fibres – which innervate muscles and
glands
o Power of a neuron depends on its connections
▪ Fast signal transmission – enabled by action potentials
Action Potential
• Objectives
o Describe action potential in terms of:
▪ Ionic basis
▪ Threshold and refractory period
▪ Propagation
• Action potential
o Signal transmission in neurons – structure = function
▪ Axon – has many voltage gated ion channels
▪ Dendrites – has many ligand gated ion channels
o Axon
▪ Electrical transmission down axons
▪ Chemical transmission at synapse – when neuron reaches target cell
, Neuronal Structure and Function
o Action potential
▪ Phases of an action potential
• Resting potential
o Negative membrane
potential
• Depolarisation
o Membrane potential is less
negative than resting
potential
• Repolarisation
o Membrane potential returns from positive resting potential to resting
potential
• Hyperpolarisation
o Membrane potential is more negative than resting potential
▪ Neurons require minimum depolarisation before generating action potential = threshold
▪ Changes in ionic permeability underlie changes in membrane potential
▪ Each action potential is similar in amplitude and duration = all-or-none
▪ Refractory period
• After an action potential has passed along an axon – it cannot conduct another for a
certain period of time
▪ Frequency of action potentials code for stimulus strength
o Fast and slow spiking neurons
▪ Purkinje neuron
• In the cerebellum
• Very short duration = rapid spiking
▪ Pyramidal neuron
• Broader width action potential
• Longer duration spiking
▪ Dopamine neuron
• Long duration
o Threshold for activation
▪ Generation of an action potential
• Requires an adequate stimulus
o Minimum strength required = threshold to open
enough ion channels to bring the membrane
potential to threshold
• Subthreshold = below threshold → no action potential
• Suprathreshold – above threshold → generates action
potential
o Magnitude of the action potential is the same as the
threshold stimulus = all-or-none response
o Rate coding determines stimulus intensity
▪ Strength of stimulus is coded by frequency of action potential
• Ionic basis of the action potential
o Action potentials are fast + unidirectional + simple + effective
o Amplitude is sodium dependent
▪ Less Na+ in extracellular environment = less depolarisation
• Action potential is smaller in amplitude + longer in duration
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