Psychology Chapter 2 - Nervous System Functioning

What is the nervous system and its main roles?

The body's information processing system that allows us to receive, interpret and respond to stimuli.

What are the two main divisions of the nervous system

Central NS - brain and spinal cord

Peripheral NS - nerves (outside CNS) and sensory receptors in the body

Full hierarchical diagram of the nervous system

Neuron diagram

3 types of neurons

Afferent (sensory) - carry signals towards the CNS

Efferent (motor) - carry signals away from the CNS

Interneurons (relay/association) - connect afferent and efferent pathways (found only in CNS)

Neural pathway

A neural route for connection within brain and between the brain and other areas of the NS

3 main roles of the Central NS (and what they mean we can do)

- Receive bodily messages

- Process information

- Send messages to the body

Means that we can perceive and respond to the world, experience consciousness and have subjective mental experiences

Brain

Command centre for almost all NS responses. The cerebral cortex is the only part in the NS that has consciousness.

3 main roles of the spinal cord

- Convey bodily sensory information (from PNS) to the brain

- Convey motor information to the body's muscles, glands and organs (via PNS)

- Spinal reflex

How does paralysis occur?

When the spinal cord is damaged, sometimes nerves below damaged area can be disconnected from the CNS, leading to paraplegics and quadriplegics.

2 main functions of the Peripheral NS

To carry sensory/afferent information from the body's muscles, organs and glands to the brain, and to carry motor/efferent information back.

Somatic NS

Peripheral NS subdivision that connects skeletal muscles and sensory organs with the brain

2 main functions of the Somatic NS

- Detect external stimuli and transmits it to the brain

- Allows voluntary movement of skeletal muscles

Autonomic NS

Peripheral NS subdivision that connects the CNS to cardiac muscles, smooth muscles and glands

The 4 different types of muscles

Skeletal - involved in voluntary movement functions

Visceral - involved in autonomic functions in organs like heart and stomach and can generate and maintain activity without brain's input.

Cardiac - involved in heart functions

Smooth - involved in autonomic functions in hollow organs like intestines, blood vessels and bladder

2 main functions of the Autonomic NS

- Detect internal stimuli, mostly in the cardiac and digestive system

- Facilitates and REGULATES (not initiates) involuntary movement of cardiac and smooth muscles needed for heartbeat and digestion, as well as the release of hormones from glands

Where is information processed from the Somatic NS and Autonomic NS

Somatic information is mainly processed in the in the cerebral cortex (sensory and motor cortices) where conscious awareness is experienced.

Autonomic information is mainly processed in the hypothalamus (homeostasis) and brainstem.

How autonomous is the autonomic nervous system?

While many of its organs, glands and processes are self-regulating and aren't under voluntary control, it is linked to the cerebral cortex, meaning we can choose to control some of its functions with conscious effort.

Three Autonomic NS subdivisions

- Sympathetic

- Parasympathetic

- Enteric

How do the sympathetic and parasympathetic NS function in relation to each other?

They counterbalance each other's activities without conscious effort, with one system usually dominant at any given time.

Somatic NS vs Autonomic NS

Somatic:

Afferent = five senses

Efferent = skeletal muscles

= conscious perception of external environment and voluntary movement

Autonomic:

Afferent = Internal organs

Efferent = cardiac and smooth muscles, glands

= self-regulating internal systems.

A conscious response involves...

1. awareness of environmental stimuli received through sensory systems

2. formation of an intention to respond

3. a response involving deliberate physical movement

Sensory receptors

Where external stimuli is detected so an afferent signal can be generated

Effector sites

Where an efferent signal attaches to a muscle or gland so a function can be executed

5 steps of a conscious response

1. Sensation - Stimulation from environment is detected in sensory receptors in somatic NS

2. Afferent signal - Sensory signal generated and sent via afferent pathways in somatic NS to CNS

3. Processing - Brain receives signal and consciously perceives it, making a decision and initiating a motor signal

4. Efferent signal - Motor signal travels via efferent pathways in the SNS to motor organs

5. Response - Motor signal received at effector site causing motion

Unconscious responses and 2 locations they occur

External or internal actions carried out in response to environmental stimuli that involve little/no awareness. Two places they occur are in autonomic NS and in somatic NS with spinal reflex.

An unconscious response usually involves

1. Internal stimuli without our awareness or a threatening external one

2. Feedback in lower parts of brain to regulate bodily functions

3. Involuntary cardiac or smooth muscles movement or hormone secretion from a gland in order to stimulate or supress bodily functions.

Sympathetic Nervous System

Autonomic NS subdivision that increases physiological arousal, putting the body in a state of biological and behavioural preparedness.

Roles of sympathetic NS

- increase heart rate, dilate pupils, increase oxygen intake and glucose release

- Suppress parasympathetic NS functions (i.e. functions not related to immediate survival)

- Stimulates adrenaline release from adrenal medulla above kidney

- Stimulates noradrenaline release in brainstem to increase threat perception

- Triggers fight-or-flight response

Fight-flight-freeze response

Automatic physiological reaction to a perceived threat. Flight and fight responses are caused by sympathetic NS dominance and noradrenaline release, and are adaptive responses, but freeze response occurs due to parasympathetic NS dominance that 'locks' with sympathetic NS functions, leading to immobility and high alertness.

Bodily (physiological) stress response

Sympathetic NS and HPA axis

Adrenaline

Released from adrenal medulla above kidney in response to perceived threat (sympathetic NS). Increases physiological arousal.

2 main functions of adrenaline

- Stimulates functions that increase our ability to physically exert ourselves, mainly aids sympathetic NS with its functions.

- Suppresses functions unrelated to physical exertion, by relaxing bladder, decreasing salivation and slowing digestion.

Parasympathetic Nervous System

Autonomic NS subdivision that decreases physiological arousal and stimulates life-sustaining functions.

Roles of Parasympathetic NS

• Returns body to homeostasis

• Decreases physiological arousal and allows life-sustaining functions to occur: e.g. digestion, salivation, reproduction, tears and pupil constriction in light environments

• Contributes to 'freeze' response

Table comparing Sympathetic vs Parasympathetic functions

Where is the Enteric Nervous System found

Embedded in walls of gastrointestinal tract (hollow digestion organs like mouth, oesophagus, stomach, intestines and rectum) is a mesh-like system of neuron clusters (ganglia). They also connect with non-hollow digestion organs like liver, pancreas and gallbladder.

Enteric NS roles

- Detect physiological state of gastrointestinal tract and controls its movement and functions.

- Works with CNS to control digestive system

SOME FUNCTIONS INDEPENDENT OF PARASYMP, SYMP and CENTRAL NS, even some memory of past gut movements

Spinal reflex

Involuntary, nearly instantaneous motor response initiated by spinal interneurons in response to sudden pain or intense heat. The shorter pathway due to the spinal reflex arc increases efficiency and leads to pain perception after a response has already occurred. Brain still will make conscious decision about further movements afterwards.

Spinal reflex examples

- Patellar reflex (can be used to test NS functioning)

- Withdraw reflex

- Reflexes involving muscle length and tension during movement

Do all reflexes happen through the spinal cord?

No, some occur in the base of the brain such as the startle reflex

Which division of the peripheral nervous system is involved in the spinal reflex - SNS or ANS?

Both, but majorly the Somatic NS.

Steps in the spinal reflex

1. Sensation - Sensory receptors in SNS detect intense heat, pain, etc.

2. Afferent signal - Afferent neurons carry signal towards CNS

3. Processing - SPINAL INTERNEURONS in the CNS relay the message to a motor neuron to initiate reflexive movement

4. Efferent signal - Efferent neuron in SNS carries message to effector site

5. Response - Withdrawal reflex occurs involuntarily

6. Extra step - Message simultaneously conveyed to brain; conscious awareness of stimuli follows response

Dendrites

Branch-like structures reaching out from neuron. Contains neurotransmitter receptor sites, and stimulation received determines if neuron will 'fire'

Axon

Portion of neuron that conveys signal towards the next cell. Passes electrical potential along the axon from the dendrites and soma to the synapse.

Myelin/myelin sheath

Fatty axon coating made of other cells that insulates the signal to speed up transmission. Prevents 'crossed wires' and means that the electrochemical signal can 'jump' more efficiently between nodes (gaps in myelin). Signal 'jumps' from each myelin node to another, triggering the next like dominoes.

Axon terminals

At the ends of the axon, with small swellings called terminal buttons. Release chemicals (neurotransmitters).

What is an Action Potential (AP)?

The technical term for when a neuron 'fires', i.e. an electrochemical signal is propagated along the axon

How is electrical (action) potential created?

Ion channels 'gates' between myelin allow charged particles in and out, creating an electrical potential (a difference between the charge inside and outside the neuron).

Neuron's voltage (resting is at around -70mV) is changed by other neuron's stimulation. If threshold if -55mV is reached, cell depolarises (has a sharp spike in positive charge) and an AP is 'fired'. The neuron then resets its voltage to rest.

Why is action potential an electrochemical signal?

Signal is 'electrochemical' because it involves the transfer of electrical charge via the flow of charged ions.

The 'all-or-none' principal

Action potentials either happen or they don't, there's no such thing as a big or small AP. If threshold is reached, AP will fire every time, otherwise it won't fire at all.

Neurons are...

The basic building blocks of the nervous system, specialised to communicate information around the body

Synaptic gap/cleft

Physical gap between the axon terminal of one neuron and the receptors of the next neuron, at the synapse

Pre-synaptic neuron

Neuron passing on the signal

Post-synaptic neuron

Neuron receiving the signal

Neural Synapse

Typical point of communication between neurons, includes pre-synaptic axon terminal (and its terminal buttons), synaptic cleft and post-synaptic dendrite with neurotransmitter receptor sites

Neurotransmission

Communication between neurons, more than the action potentials within the neuron but the events that occur at the synapse that change the outcome.

Neurotransmitter

Chemical substance produced by a neuron that carries a message to other neurons or cells in other tissue.

Lock-and-key process

Keys - Neurotransmitters with unique molecular structure (shape and size)

Locks - Post-synaptic receptor sites, which needs to have a complementary chemical structure (shape and size) to bind with a neurotransmitter.

This 'binding' process changes the post-synaptic neuron, making it more or less likely to fire AP.

Reuptake

Neurotransmitter that doesn't bind to receptors in the post-synaptic neuron are absorbed back into the presynaptic terminal buttons.

What are the two different effects a neurotransmitter can have?

Excitatory - stimulates or activates postsynaptic neuron to perform functions/ increases voltage closer to threshold and makes it more likely to fire

Inhibitory - blocks or prevents postsynaptic neuron from firing/ decreases voltage and makes it less likely to fire

What determines the effects of a particular neurotransmitter?

The location and properties of the receptor the neurotransmitter binds with, along with the actual chemical itself

Summative neurotransmitter effects

A single neuron can have both excitatory and inhibitory receptor sites, therefore the excitation and inhibition adds together, and the overall charge determines if the cell reaches the threshold. Excitatory and inhibitory effects only last for a short time.

Do neurons only communicate through neurotransmitters?

No, neurons can also communicate electrically by transmitting electrical signals directly between different parts of neurons.

What are the 3 types of neurochemicals?

Neurotransmitters, neuromodulators, neurohormones

Neurohormones

Some neurotransmitters can also occur as hormones, like such as noradrenaline which is secreted as a hormone by the adrenal glands and as a neurotransmitter from neurons

Glutamate

The main excitatory neurotransmitter in the CNS. (2nd most abundant NT in the brain after GABA). Serves to depolarise post-syn neuron. It is involved in most normal brain functions including learning, memory, perception, thinking and movement. Its excitatory effects promote the growth and strengthening of synaptic connections, representing memory (synaptic plasticity).

GABA

Gamma-amino butyric acid

Primary inhibitory neurotransmitter in the CNS. Serves to further polarise post-syn neuron. Helps to fine tune neurotransmission in the brain to ensure it is at an optimal level. Without it seizures would be overly prevalent, and low levels are related to phobia and anxiety.

Neuromodulator

Modulatory neurotransmitter able to influence the effects of other neurotransmitters by modifying and regulating signal transmission. Affects responsiveness of neurons.

Some ways neuromodulators can influence other neurotransmitters' effects include...

- Changing receptor reactivity to particular neurotransmitters to enhance their responses

- 'Teaming up' with another neurotransmitter in the synapse to make it more or less potent

- Controlling amount of neurotransmitter released

- Increasing likelihood of neurotransmitter being received

Where do neuromodulators release their chemical messengers?

Instead of releasing into one synapse like other neurotransmitters, during neuromodulation broad areas of 100,000 or more neurons or synapses can be affected. This may be a whole neural tissue, brain area, pathway or pathways. Effects also take longer to establish and are more long-lasting

Dopamine

Modulatory neurotransmitter with important roles in voluntary movements, pleasure, motivation, appetite, reward-based learning and memory.

Dopamine effects as neurotransmitter vs neuromodulator

NT - Primarily excitatory

NM - Excitatory or inhibitory depending on location and receptors present

Mental conditions implicated by dopamine

Parkinson's, depression, addiction, schizophrenia

What are the brain's two main dopamine pathways?

- Nigrostriatal pathway

- Mesolimbic (and mesocortical which overlaps) pathways

Forms dopaminergic system

Nigrostriatal pathway

Dopaminergic pathway originating in midbrain structure called substantia nigra. Dopamine produced by neurons in this structure carry messages to allow smooth voluntary muscle movement coordination.

What happens if the substantia nigra is damaged?

Brain structures linked to movement receive less dopamine messages as a result of lower dopamine production in nigrostriatal pathway, leading to extreme muscle rigidity (like in Parkinson's disease)

Mesolimbic pathway

Dopaminergic pathway that, along with mesocortical pathway, forms dopamine reward system responsible for pleasure and reward-based learning (with help from limbic system). Originates in ventral tegmental area in midbrain.

Reward-based learning

Pleasurable consequences make us more likely to repeat behaviour. Even the anticipation or thought of receiving a rewarding stimulus can release dopamine.

What causes schizophrenia?

High levels of dopamine in the mesolimbic pathway are associated with hallucinations and delusions

Serotonin

Modulatory neurotransmitter with important roles in mood stabilisation, wellbeing, emotional processing, sleep onset, reproductive drive, appetite and pain perception.

Serotonin effects as neurotransmitter vs neuromodulator

NT - Inhibitory and excitatory depending on location

NM - Only inhibitory

Mental conditions implicated by serotonin

Depression, anxiety disorders and sleep disorders

Where is serotonin produced in the body?

10% in brain, mostly in Raphe Nuclei comprising clusters of cell soma in the brain stem.

Over 80% is in the enteric NS within the gastrointestinal tract.

Makes up serotonergic system

What chemical process is linked to depression?

Because dopamine is a mood stabiliser, low levels are associated with depression (and seasonal affective disorder and OCD) leading to prolonged hopelessness and decreased reward from pleasurable experiences.

Antidepressants

SSRIs - Serotonin-Specific Reuptake Inhibitors block serotonin reuptake to increase its availability in the synapse.

Serotonin and the sleep-wake cycle

Helps regulate when, how and how much we sleep with the help of other neurotransmitters and the hormone melatonin. The brain uses serotonin to produce melatonin, explaining insomnia in those with depression.

Serotonin Syndrome

Excessive serotonin can cause fever, high heart rate, hallucinations, seizures and unconsciousness or death. Often caused by illegal drugs

Neuroplasticity

Ability of neurons in the brain to change

Synaptic Plasticity

Involves functional and structural changes at the synapse that alter communication between neurons, resulting in increased (LTP) or decreased (LTD) communication.

2 types of neuroplasticity (different to last year)

Developmental plasticity - Change due to physical maturation. Includes synaptogenesis, synaptic pruning and myelination

Adaptive plasticity - Changes due to environmental conditions. Includes learning (LTP and LTD) and recovery from brain trauma (rerouting and sprouting)

Learning

Relatively permanent change in behaviour that occurs as a result of experience

Memory

Processing, storage and retrieval of information acquired through learning. If we don't remember, we haven't learnt

How are memories stored in the brain?

A memory is a pathway in the brain, created through synaptic plasticity (learning)

Hebb's rule

Neurons that fire together, wire together.

Repeated transmission and joint activation between two neurons causes change in the synapse, increasing the chance of firing together again and leading to more forcibly and efficiently transmitted signals in the future

What are the fundamental mechanisms of learning and memory formation?

Long term potentiation, long term depression, sprouting, rerouting and pruning

LTP

Long-term potentiation. Long-lasting strengthening of synaptic connections occurring through repeated co-activation of the relevant neurons. Frequently used pathways get stronger.

LTD

Long-term depression. Long-lasting weakening of synaptic connections occurring through repeated sub-threshold activation. Little-used pathways grow weaker.

What changes at the synapse during LTD and LTP?

- Amount of neurotransmitter produced

- Number of post-synaptic receptors available to the neurotransmitter

- Amount of depolarisation that occurs in the post-synaptic neuron when stimulated

The role of glutamate in LTD and LTP

As glutamate is the brain's main excitatory neurotransmitter, accounting for 90% of the connections in the brain, it's the main chemical involved in synaptic plasticity. This is because it causes increased chances of threshold-level charges, causing neurons to ‘fire together’ and ‘wire together’. LTP and LTD take place in glutaminergic synapses in the hippocampus, cerebral cortex and other brain areas.