chapter 2 (nervous system functioning)

2.2 Sub-divisions of Central and Peripheral Nervous Systems

…central nervous system…

• Comprised of brain and spinal cord
• Process information received from the body's internal and external environment
• activate appropriate responses

…brain…

• Process information received through neural pathways from the body and in directing actions within the body
• Neural pathway:
• comprises one or more circuits of interconnected neurons that form a communication network
• connect the brain to other parts of the nervous system and body

…spinal cord…

• Long thin bundle of nerve fibres that extends from the base of the brain to the lower back
• encased in vertebrae
• two major functions:
• receives sensory information from the body (via peripheral nervous system) and sends the messages to the brain for processing
• receives motor information from the brain and sends it to relevant parts of the body (via peripheral nervous system)
• Ascending tracts: carry sensory information up to the brain
• descending tracts: leaves the brain and travels down the spinal cord to exit via the spinal nerves

…peripheral nervous system…

• The entire network of nerves located outside the CNS
• carries information to the CNS from the bodies muscles organs and glands and sensory organs
• carries information from the CNS to the body's muscle organs and glands
• two divisions - somatic nervous system and autonomic nervous system

… somatic nervous system (SNS)…

• Carries sensory information to the CNS
• received that sensory receptor sites in the body and carried along sensory neural pathways by sensory neurons
• called afferent information because sensory information is coming into the CNS
• carries motor information from the CNS
• Carried along motor neural pathways by motor neurons to skeletal muscles
• skeletal muscles attached to bones and initiate change or stop movement
• efferent information as motor information is leaving the CNS
• Voluntary movement is controlled through the coordinated actions of both motor and sensory information

… autonomic nervous system…

• connect the CNS to the body's internal organs and glands providing feedback to the brain about their activities
• Self-regulating - occurs without any conscious effort and is not usually under our voluntary control
• Functions continuously and keeps the vital organs and systems of our body functioning thereby maintaining our survival
• regulates the activity of the visceral muscles organs and glands
• three subdivisions
• sympathetic nervous system > increasing activity
• parasympathetic nervous system > decreasing activity
• enteric nervous system > dedicated to the gastrointestinal tract and helps regulate digestion

…sympathetic nervous system…

• activates internal muscles organs and glands to prepare the body for vigorous activity or to deal with a stressful or threatening situation
• Enhances survival by providing an immediate response to any emergency
• Emergency is perceived the sympathetic nervous system activates specific organs and glands to respond (fight or flight)
• e.g adrenal glands (above kidneys) > release hormones into bloodstream activating an energising various muscles organs and glands in preparation for dealing with a threat

…parasympathetic nervous system…

• Helps to maintain the internal body environment in a steady balance day of normal function (homeostasis)

• counterbalances the activities of the sympathetic nervous system and restores the body to a state of calm

• dominates the sympathetic system

• takes longer to return the body to its normal state compared with the sympathetic system's immediate activation

  

…enteric nervous system…

• embedded within the walls of the gastrointestinal tract and is dedicated to its functioning

• mouth, oesophagus, stomach, small intestine, large intestine, rectum and anus

• composed of thousands of small clusters of neurons (ganglia) and nerve fibres

• detects the physiological condition of the gastrointestinal tract

• integrates information about its state

• provides outputs to control gut movement

• other functions – e.g. nutrient management, regulating gastric acid secretions, changing local blood flow, interacting with the parts of the immune and endocrine systems that are in the gut

• two-way connections with CNS > controls the digestive system in the context of local and whole-body physiological demands

• can functions independently of the brain and carry out some of its own functions in the digestive process without communicating

• activity is influenced by:

• external factors affecting hunger eating and digestion

• microbiota - bacteria that lives in the gastrointestinal tract which may influence how the ENS functions to regulate the digestive process

• regulates the response of the food and drinks that are taken in

• sometimes referred to as ‘second brain’ because

• degree of anatomy

• reliance

• types of neurons and neurotransmitters

  

2.3 Conscious and Unconscious Responses to Sensory Stimuli

…conscious responses…

• Reaction that involves awareness
• usually voluntary intentional reactions
• likely to be goal directed and purposeful
• Ability to exercise some degree of control over it
• maybe response to an internally sourced stimulus (e.g. stomach ache)
• Depending on its severity the best response will be decided (e.g. ignore or go to nurse)

.. unconscious response…

• reaction that does not involve awareness
• regulated by the ANS and occur automatically without conscious effort
• many are reflexive responses (autonomic reflexes)
• help us to avoid danger and minimise harm
• awareness is not part of the reflex itself but comes after the reflex action has been completed

.. spinal reflexes…

• unconscious automatic response controlled solely by neural circuits in the spinal cord

• often referred to as reflex arc

• response to stimulus is automatically reflected without initial input from the brain and before brain processes (e.g. withdrawal reflex)

• Immediate response enables faster reaction time

• spinal reflex involving a withdrawal reaction is believed to be an adaptive response

• adaptive - save time in situations that may be very harmful

• because reflexes are predictable, they can provide useful information about the functioning of the nervous system and assist in diagnosis of neural disorders

• damage or disease can cause reflex to be absent or abnormal

• demonstrate that a response can have both a conscious and unconscious component

• Small number of neurons convert a sensory stimulus into action

  

2.4 Role of Neurotransmitters

.. synaptic gap…

• Tiny space between the terminal buttons of 1 neuron which releases neurotransmitters and the dangers of another
• 500 times thinner than a strand of hair
• component of neural synapse

.. neural synapse…

• sites where communication typically occurs between adjacent neurons
• Communication also involves:
• terminal buttons of presynaptic neuron (sending)
• dangers of postsynaptic neuron (receiving)

.. neurotransmitter…

• Chemical substance produced by a neuron

• carries a message to other neurons or cells in muscles organs or other tissue

• attaches itself to receptor sites of postsynaptic neurons that are specialised to receive that specific neurotransmitter

• reuptake

• Once the newer transmitter is received any additional ones left in the signups are reabsorbed by the presynaptic neuron

• Many medications work by affecting the processes of reuptake (increase or decrease)

• two possible effects:

• Excitatory effect

• inhibitory effect

• Effects may also be due to the receptor to which the neurotransmitter binds

• properties of the receptor at the synapse where it is released

• receptor location in the brain

• Number of neurotransmitters that a neuron can manufacture varies

• communication can be:

• electrical (e.g. Axons transmit messages directly two other axons or cell bodies)

• Chemical

• can also occur as

• neurohormones

• Neuromodulators

  

.. reuptake…

• If it does not bind to the receptors, it is absorbed back into the terminal buttons of the presynaptic neuron

.. excitatory effect…

• stimulate or activate postsynaptic neurons to perform their functions

.. inhibitory effect…

• block or prevent postsynaptic neurons from firing

.. neurohormones…

• secreted by the adrenal glands into the blood and as a neurotransmitter from neurons

.. neuromodulators…

• neurotransmitters that can influence/modulate the action of other neurotransmitters

.. glutamate…

• main excitatory neurotransmitter in the CNS
• enhances information transmission by making per synaptic neurons more likely to fire
• involved in learning, memory, perception, thinking, and movement
• Crucial roles in synaptic changes which occur during learning and memory
• promotes the growth and strengthening of synaptic connections between neurons within neural pathway (synaptic plasticity)
• too much or too little may be harmful to neurons and brain functioning as a whole
• glutamatergic communication - right concentration
• high concentration results in - over excitation of receiving neurons leading to neuronal damage and or death (Damaging neural networks)

.. gamma-amino butyric acid (GABA)…

• Primary inhibitory neurotransmitter in the CNS

• makes postsynaptic neurons less likely to fire

• fine-tunes neurotransmission in the brain and maintains neurotransmission at an optimal level

• without it postsynaptic neurons might get out of control

• can cause seizures and anxiety symptoms (phobia)

  

2.5 Role of Neuromodulators

.. neuromodulators…

• Neurotransmitters that modulate or influence the effects of other neurotransmitters

• can work together with other newer transmitters in a synapse to make the others more or less potent

• they do not release their chemical messengers into a synapse

• They are released into broader areas to affect many neurons at once

• They exert their influence over a slower time period then excitatory and inhibitory neurotransmitters at synapses with fast acting receptors

• Take longer and last longer

• Thought to convey global control of brain states that underlie different behaviours

  

.… dopamine…

• modulatory neurotransmitter/neuromodulator known to have multiple functions depending on where in the brain it acts

• e.g. Voluntary movement, experience of pleasure, motivation, appetite, reward based learning and memory

• Implicated in mental conditions such as Parkinson's disease, addiction, and schizophrenia

• Can have either excitatory effect at one location or inhibitory effect at another depending on the type of receptor

• the brain has distinct dopamine producing areas in neural pathways creating the dopaminergic system

• Nigrostriatal

• Mesolimbic

• Mesocortical

  

.… dopamine - nigrostriatal…

• Dopamine pathway that takes part in coordinating movement
• carries messages that allow smooth coordinated function of the body's muscles and movements
• origins in the midbrain structure called substantia nigra
• when substantia nigra is deceased or damaged amount of dopamine available along this pathway is reduced
• results in extreme muscle rigidity or stiffness as occurs in Parkinson's disease

.… dopamine – mesolimbic and mesocortical…

• both overlap and are strongly associated with rewarding behaviour through the experience of pleasure
• form dopamine reward system
• includes both healthy and harmful behaviours (loss of impulse control and addiction)
• mesolimbic – reward/feeling of pleasure and possibly euphoria
• originates in the ventral tegmental area deep within the midbrain
• high levels of dopamine in this area is linked to schizophrenia
• connections to limbic system structures and cortical areas work together to produce rewarding effects and increase the likelihood of rewarding behaviours being repeated

.… serotonin…

• modulating neurotransmitter that has a wide range of functions depending on where in the brain it acts

• produced in brain stem

• linked with mental conditions:

• depression

• anxiety disorders

• sleep disorders

• only has inhibitory effects - does not stimulate brain activity

• helps counterbalance excessive excitatory effects of other neurotransmitters

• described as a mood stabiliser - low levels associated with mood disorders

• drugs commonly used to treat depression target serotonin an increase it's availability at synapses

• reduced levels have been associated with anxiety disorders (e.g. OCD)

• important role in the regulation of the sleep wake cycle

• associated with the amount of serotonin in different brain areas

• melatonin - a hormone that has a crucial role in sleep and wakefulness created by serotonin in the pineal gland also contributes to the cycle

• serotonin and melatonin relationship may also contribute to insomnia

• too much serotonin can lead to serotonin syndrome which can be life threatening

• serotonin syndrome - collection of symptoms that include fever, elevated heart rate, restlessness, agitation, confusion, hallucinations, delirium, and seizures

• illegal drugs can also increase serotonin to a toxic level

  

2.6 Neural Mechanisms of Memory Formation and Learning

.… learning…

• The acquisition of new information behaviour or ability through experience
• occurs with or without conscious awareness and is evidenced by change
• new knowledge skill must be retained in memory for learning to occur again

.… neural plasticity…

• In physical changes that take place at synapses when neurotransmission occurs
• multiple neurons interconnect from neural pathways

.… synaptic plasticity…

• The ability of the synapse to change in response to experience
• Controls how effectively two neurons communicate with each other
• strengthens or weakens connections
• strengthening - continue use of synaptic connexion or growth of new additional connections
• weakening - this use of synaptic Connexions resulting in the decay or elimination of a synapse
• allows brain to recognise and reassign its neural connections and pathways based on which parts of it are overused or underused
• remodelled by experience
• Donald Hebb
• Enables neurons to restore or compensate for a lost function following a brain injury and/or maximise remaining functions

.… Donald Hebb…

• idea that learning involves the establishment and strengthening of neural Connexions at the synapse
• creation of cell assemblies (interconnected groups of neurons that form networks or pathways that function as a unit)
• same neurons may be involved in learning different things or in producing different patterns of behaviours depending on which combination of neuron is active
• presynaptic and postsynaptic neuron are active at the same time which changes the structure or chemistry of the synapse strengthening the connections between these two neurons
• more likely to fire together again and to transmit their signals more forcibly inefficiently
• not firing together weakens connections
• known as ‘Hebb’s rule’ or ‘Hebbian learning’
• neurons that fire together, wire together

.… sprouting…

• Creation of new extensions on a neuron to allow it to make new connections with other neurons
• Growth of nerve ending (sprouts) on axons or dendrites

.… rerouting…

• Occurs when new connections are made between neurons to create alternate neural pathways
• May involve existing synaptic connections and or new connections from the sprouts

.… pruning…

• elimination of weak ineffective or unused synapses
• refines/finetunes neural connections to maintain efficient brain functioning

.… long-term potentiation (LTP)…

• Long lasting enhancement of synaptic transmission due to repeated strong stimulation

• enables postsynaptic neurons to be more easily activated

• become more responsive to the presynaptic neurons due to repeated stimulation by neurotransmitter

• also causes changes in the presynaptic neuron

• e.g. terminal buttons release more glutamate after the potentiation

  

.… long-term depression (LTD)…

• Long lasting decrease in the strength of synaptic transmission and neuronal response
• lack of stimulation of presynaptic and postsynaptic neurons or prolonged low-level stimulation
• postsynaptic neuron becomes less responsive to the neurotransmitter

.… similarities: LTP and LTD…

• Both are activity dependent
• involve glutamate
• occur at glutamate synapses
• involve changes in excitability
• long lasting effects
• forms of long-lasting neural plasticity