Week 3 The Nervous Sytem Flashcards
Neuroscience
Neuroscience is the study of the nervous system
• Includes structure, function, health and disease
• Understanding the structure and function of the nervous system requires integration of the knowledge of various scientific disciplines
Neurones
Fundamental units of the nervous system
• Human brains contain ~86 billion neurons
• Neuron development ceases before birth
• Under influence of various factors
• How do they connect with each other and other parts of the nervous system
Neurons - Structure
typical neuron has three regions:
1) Cell body (Soma)
Contains organelles and cell processes radiate out from the cell body.
2) The dendrites = Neuron’s receivers. Receives impulses and carries the impulse towards cell body
3) The axon = Neuron’s transmitter. Conducts electrical impulses away from cell body.
Neurons - Function
• Neurons are responsible for sending and receiving information around the body
•Depending on location, neurons can be:
Sensory
Motor
Interneuron
Sensory (afferent) neurons
Make up sensory division of the PNS
Motor (efferent) neurons
Connect to muscle fibres at distal end of axon Can have Upper and Lower neurons
Upper from brain
Lower arise from spinal cord an attach to muscles
Motor Unit
Motor neuron + muscle fibres
Interneurons
between motor and sensory neurons
Nerve Impulses
Neurons are excitable tissue converting a stimulus to nerve impulses, similar to electricity travelling through wires.
• Resting Membrane potential (RMP)
• Cell membrane of a typical neuron at rest has negative electrical
potential ~ -70mV (Inside v Outside)
• This electrical potential difference = RMP
• Caused by uneven separation of charged ions (↑ conc. K+ inside membrane & ↑ conc. Na+ outside) = polarised
• Imbalance is maintained by i) differences in cell membrane permeability & ii) sodium-potassium pump
Polarisation
Depolarisation:
• If the inside of cell becomes less negative in relation to outside = ↓ in potential difference across membrane = membrane less polarised.
• Occurs when charge difference is more +ve than RMP (- 70mV)
• Repolarisation:
• Membrane potential returns to its resting state after an AP towards a more –ve value (cell membrane more polarized), but continues past RMP
• Hyperpolarisation:
• Charge moves back to RMP (-70mV)
Graded potentials
Localised change in membrane potential of depolarization or hyperpolarisation.
• Not ‘all or none’
• Local event and depolarisation doesn’t spread along neuron – need action potential.
Slight change in membrane potential
Action Potentials
• Action Potential = Rapid (~1ms) & substantial neuron membrane depolarisation.
• Membrane potential changes from RMP (- 70mV) to +30mV and back again.
All Action Potentials begin as Graded Potentials
• If GP does not reach depolarization threshold, nothing happens.
• If threshold ~15-20mV reached (mV from RMP of -70 to ~ -55mV), AP will occur
• ‘All-or-none’ principle
• When an axon is in the process of generating an action potential – unable to respond to another stimulus – absolute refractory period. Won’t be able to respond to anothing else
• Once an axon has dealt with a stimulus, it can deal with a new one if the new stimulus is of a greater magnitude than the one it is dealing with – relative refractory period can react but stimulus has to bigger than the one its dealing with
Synapse
A synapse is the place where neurons connect and communicate to continue transmitting APs
Each neuron has between a few to hundreds of thousands of synaptic connections Site of AP transmission from axon terminals of 1 neuron to dendrites of another.
• Synapse between 2 neurons includes:
i) Axon terminals of presynaptic neuron
ii) Receptors on postsynaptic neuron
iii) Spacebetweenthetwo structures = synaptic cleft
Neurotransmitters
Endogenous chemicals that allow neurons to communicate
• Carry messages from one neuron to the next nerve or effector organ cell
• Located in axon terminal in sacs called synaptic vesicles
• Neurotransmitters released from vesicles into synaptic cleft to carry messages across
• More than 50 neurotransmitters have been identified
Neuromuscular junction
Junction of motor neurone and the muscles fibres it innovates
Axon terminals protrude to motor end plate
Neurotransmitters released from motor neurone a on terminals bind to receptors on plasma
One-Way Signals
Presynaptic terminals contain saclike structures = synaptic vesicles containing neurotransmitters.
When never impulse reaches presynaptic axon terminals, vesicles release neurotransmitters into synaptic cleft
Organisation of the nervous system
Human nervous system divided into 2 parts:
i) CNS (Central Nervous System) - Includes the brain & Spinal Cord.
ii) PNS (Peripheral Nervous System)
—somatic
—autonomic nervous system
— sympathetic NS — parasympathetic NS — enteric NS
Peripheral nervous system
PNS further subdivided into Sensory (afferent) or Motor (efferent)
• Sensory = Sends signals from periphery to CNS afferent
• Motor = Send signals from CNS to effector organs. Efferent
Peripheral nervous system - Motor
Sensory (afferent) and Motor (efferent)
Motor nerves can also be divided into Somatic (voluntary) or Autonomic (Involuntary) control
• Somatic = Conscious processes
• Autonomic = Regulates bodily processes at subconscious level
Sensory-Motor Integration
How does a sensory stimulus result in a motor response?
Sensory-Motor Integration
PNS relays sensory info to CNS
CNS interprets info and sends out appropriate motor signals
Sensory Input
Sensory input from receptors in the body can terminate at varying levels in the CNS The area where sensory impulses terminate = Integration Centre
• Spinal cord – response is typically a simple motor reflex
• Lower brain stem – result in subconscious motor reactions of a
more complex nature e.g. postural control during sitting
• Cerebellum – subconscious control of movement (smoothing)
• Thalamus – Enter conscious level & person can distinguish various sensations
• Cerebral cortex – This part of conscious brain allows us to be aware of our surroundings & our relationship to them.
Degree of movement complexity increase I
V
Peripheral nervous system
Autonomic system further subdivided
Into:
Sympathetic
Parasympathetic
Enteric Nervous Systems
The Central Nervous System
The CNS consists of the brain and spinal cord.
• Brain
• Has 4 major regions:
• i)Cerebrum
• ii)Diencephalon
• iii)Cerebellum
• iv) Brain Stem
CNS - Cerebrum
Right & Left hemispheres connected by fibre bundles = corpus callosum
Cerebral cortex = outer layer of cerebral hemispheres
• Houses the Primary Motor Cortex & Basal Ganglia
Consists of 5 lobes:
Frontal Lobe: General Intellect & motor control
Temporal Lobe: Auditory input & interpretation
Parietal Lobe: General sensory input & interpretation
iv) Occipital Lobe: Visual input & interpretation
v) Insular Lobe: Diverse functions linked to emotion &
self-perception
CNS - Diencephalon
Composed of thalamus & hypothalamus
• Thalamus – All sensory input (except smell) comes here to be relayed to appropriate area of cortex.
Regulates what sensory input reaches conscious brain
• Hypothalamus – Directly below thalamus & maintains homeostasis Neural centres here assist in control and regulation of BP, HR, Breathing and Digestion
CNS - Cerebellum
Located behind brain stem & crucial in coordinating timing of motor activities, progression from one movement to the next & smoothing out movements
• Compares expected/intended movement with actual changes in the body and initiates corrective adjustments
• PMC decides on movement & relays decision to cerebellum
• Cerebellum notes desired action and compares to actual movement based on sensory feedback If actions are different, cerebellum sets in process corrective action
PRIMARY motor cortex -> cerebellum
CNS – Brain Stem
• Connects brain to spinal cord
• Sensory & motor neurons pass through brain stem
• Made up of midbrain, the pons & medulla oblongata
• Houses a specialised group of neurons: the reticular formation
• They are involved in many vital functions including:
i) Control CV & respiratory function
ii) Coordinate skeletal muscle function & maintain muscle tone
iii) Influence arousal & consciousness
iv) Circadian rhythm
v) Pain control system
CNS – Spinal Cord
Joins onto the lowest part of the brain stem (medulla oblongata)
• Nerve fibres in spinal cord allow 2-way conduction of nerve impulses
• Sensory (afferent) fibres carry signals from the sensory receptors such as those on the skin and muscles to the upper levels of the CNS
• Motor (efferent) fibres carry signals from the brain and upper spinal cord to the end organs (e.g. muscle)
