102 Neurophysiology

An electrochemical communication system

The human nervous system controls everything from breathing and producing digestive enzymes to memory, intelligence and consciousness.

• It receives sensory messages from the external environment
• Organises information and integrates it with already stored information
• It uses integrated information to send out messages to muscles and glands
  • producing organised movement and secretions
• Provides the basis for conscious experience

• The CNS is divided into two parts:
  • The brain
  • The spinal cord

• Cerebral Cortex - functions:
  • Thought
  • Voluntary movement
  • Language
  • Reasoning
  • Perception
• Frontal lobe
  • Reaches full maturity around age 25 (cognitive maturity associated with adulthood)
  • Modifies emotions to fit socially acceptable norms & important in retaining long term memories
• Pareietal lobe
  • Integration of sensory information from various parts of the body, knowledge of numbers and their relations
• Occipital lobe
  • Visual processing centre of the brain
• Temporal lobe
  • Involved in speech, memory and hearing

Sensory and Motor neurons - everything except the brain and spinal cord

Somatic Nervous System - Voluntary

• Input - sense organs
• Output - skeletal muscle

Autonomic Nervous System - Involuntary

• Input - internal receptors
• Output - smooth muscle & glands

Voluntary Nervous System

• Single motor neuron travels directly to the skeletal muscle without the mediation of ganglia.

Consists of:

• Peripheral nerve fibres that send sensory information to the CNS
• Motor nerve fibres that project to skeletal muscle
• Reflex arc - neural circuit automatic link between a sensory input and a specific motor output

Acetylcholine - excitatory only

Sensory neurons - range in size

Motor neurons - all large neurons are myelinated

• Function is to maintain homeostasis by automatic control
  • Does so without instructions/interference from conscious mind
• Functions in an involuntary, reflexive manner
• Exerts influence by rapid transmission of electrical impusles via nerve fibres
  • ANS and endocrine system coordinate the regulation and integration of the bodys functions
• Coordinates cardiovascular, respiratory, digestive, urinary & reproductive functions
  • By adjusting internal bodily fluid constituents
  • e.g. nutrients, water, electrolytes & dissolved gas concentrations
• Visceral effectors do not depend on the ANS to function
  • Only to adjust to the body's changing needs
• The ANS has two branches
  • Sympathetic nervous system - 'fight or flight' response - adrenergic
  • Parasympathetic nervous system - 'rest & repose - relaxed' - cholinergic

Sympathetic nervous system neurons

• Short preganglionic fibres
• Long postganglionic fibres
• Prepares body for heightened levels of somatic activity
  • Fight or flight

Parasympathetic nervous system neurons

• Long preganglionic fibres
• Short postganglionic fibres
• Rest & repose

Most organs are innervated by both the parasympathetic and sympathetic division of the ANS at the same time - dual innervation.

Consider the heart:

• Parasympathetic (vagal) innervation ==> decreased contraction
• Sympathetic innervation ==> increased contraction
• However, vagal innervation controls heart rate
  • Parasympathetic stimulation

• Stimulation of preganglionic neurons
  • Release acetylcholine (ACh) at synapses with ganglionic neurons = excitatory
  • Release neurotransmitters at specific target organs
• Most sympathetic ganglionic neurons release noradrenaline (NA)
  • Adrenergic
  • Some release ACh
    • e.g. brain, skin, skeletal muscle
• Effects of NA at the postsynaptic membrane persist for a few seconds
  • Significantly longer than duration of ACh effects (20 msec)

• All parasympathetic postganglionic fibres release acetycholine - cholinergic
• Synthesised from acetyl coenzyme A - acetyl CoA and choline
• Two major receptors for acetylcholine:
  • Nicotinic (nAChR)
  • Muscarinic (mAChR)
• Effects of ACh are short lived
  • As most is inactivated by acetylcholinesterase (AChE)
  • Thus effects are localised & last few seconds at most

• Acts to oppose/balance the actions of sympathetic division
• Does not discharge as a complete system; only affects specific organs (e.g. stomach, eye) - no significant unpleasant symptoms upon activation
• In non-emergencies - allows us to 'rest' and 'digest' 
• Generally dominant over sympathetic system at rest
  • saves energy
• Essential for life

• Structure: Iris SNS: Pupil dilation PNS: Pupil constriction
• Structure: Salivary glands SNS: Saliva production reduced PNS: Saliva production increased
• Structure: Oral/Nasal Mucosa SNS: Mucus production reduced PNS: Mucus production increased
• Structure: Lung SNS: Bronchial muscle relexed PNS: Bronchial muscle contracted
• Structure: Stomach SNS: Peristalsis reduced PNS: Gastric juice secreted; motility increased
• Structure: Small intestine SNS: Motility reduced PNS: Digestion increased
• Structure: Large intestine SNS: Motility reduced PNS: Secretions and motility increased
• Structure: Liver SNS: Increased conversion of glycogen to glucose PNS: Glycogen synthesis
• Structure: Kidney SNS: Decreased urine secretion PNS: Increased urine secretion
• Structure: Bladder SNS: Wall relaxed, Sphincter closed PNS: Wall contracted, Sphincter relaxed

• Heightened mental alertness
• Increase metabolic rate
• Reduced digestive & urinary functions
• Activation energy reserves
• Increase respiratory rate & bronchodilation
• Increase hear rate & blood pressure
• Activation of sweat glands

• Constriction of pupils
• Decreased metabolic rate
• Decreased heart rate & blood pressure
• Increased secretion by salivary & digestive glands
• Increase motility & blood flow in digestive tract
• Stimulation of urination & defecation
• Sexual arousal & stimulation of sexual glands

• Both are efferent division that carry out motor commands

ANS

• Controls visceral effectors
• Motor neurons of CNS synapse on visceral motor neurons in autonomic ganglia
• These ganglia control visceral effectors

Somatic NS

• Controls skeletal muscle
• Motor neurons of CNS exert direct control over skeletal muscle

• Baroreceptors - pressure
• Chemoreceptors - chemical
• Mechanoreceptors - mechanical stress or strain
• Nociceptors - pain
• Photoreceptors - light
• Proprioceptors - sense of position
• Theremoreceptors - temperature, heat/cold/both
• Gustatory receptors - chemicals in food

Neurons are similar to other cells because:

• They are surround by a cell membrane
• Have a nucleus that contains genes
• Contain cytoplasm, mitochondira and other organelles
• Carry out basic cellular processes such as protein synthesis and energy production

Neurons differ from other cells because:

• Have specialised extenstion called dendrites and axons
• Communicate with each other through an electrochemical process - via electrical impulses called action potentials (AP)
• Contain some specialised structures, for example, synapses, and chemicals, for example, neurotransmiters.

Tree-like extensions at the beginning of a neuron

Help increase the surface area of the cell body and are covered with synapses

Receive information from other neurons and transmit electrical stimulation to the cell body

Where the signals from dendrites are joined/integrated and passed on

The cell body and the nucleus do not play an active role in the transmission of the neural signal

They serve to maintain the cell and keep the neuron functional

The Axon

• Transmits the neural signal away from the cell body 
• May or may not have a myelin covering

Schwann cells

• Function is to support the axon and keep it healthy
• Wraps around the axon and creates the myelin sheath

The axons of many neurons are encased in a fatty myelin sheath (made of Schwann cells)

Their function is to speed up the rate of electrical conduction

Where the sheath of one Schwann cell meets the next, the axon is unprotected

The sodium channels of myelinated neurons are confined to these gaps (nodes of Ranvier)

When things go wrong.. Multiple Slerosis (MS)

• The body's defences attack its own nerves
• The protective myelin covering of the nerve fibres in the CNS is damaged
• Slower transmission of impulses along the nerve fibre pathways = typical symptoms common in MS

Sensory neuron

• Afferent neuron
• Receptor ==> signal arrives at brain or spinal cord

Motor neuron

• Efferent neuron
• Signal exits the brain or spinal cord ==> muscles and organs

Interneuron (relay neuron)

• Sensory neuron ==> motor neuron
• Make up the brain and spinal cord

Approximately 90% of cells in the CNS are not neurons

Glial cells commonly called neuroglia or simply glia do not initiate or conduct nerve impulses

They are responsible for:

• Providing support and nutrition
• Maintaining homeostasis
• Removing dead neurons
• Destroying pathogen
• Forming myelin

• Astrocytes
  • Numerous projections that anchor neurons to their blood supply
• Oligodendrocytes
  • Coats axons in CNS with their cell membrane to form myelin
  • Producing the myelin sheath
  • Schwann cells are a type of glial cell in the PNS
• Microglia
  • Specialised macrophages capable of phagocytosis
  • They are mobile within the brain and multiply when the brain is damaged
• Ependymal cells
  • Line the cavities of the CNS and make up the walls of the ventricles

ANS is a motor system that requires information from periphery on the body's affairs

Consider blood pressure:

• Decreased blood pressure
• Baroreceptors in the heart send less information to the brain

Results

• Reflex response
• Sympathetic output to the heart and blood vessels
• Decreased parasympathetic output to the heart and increased blood pressure

Humans are unable to generate new neurons with ages and this leads to cognitive decline.

Dementia: 

• a chronic or persistent disorder of the mental processes caused by brain disease or injury and marked by memory disorders, personality changes and impaired reasoning.

The impairment is over and above what is to be expected form normal ageing.

• Most common cause of dementia, first described by the german neurologist Aloi Alzheimer
• Progressive disease of the brain that causes dementia
• Gradually destroys a persons memory, ability to learn, reason, make judgements and communicate
• Most cases develop in people aged 65+ years
  • 1 in 20 over 65
  • 1 in 1000 below 65
  • Risk of developing Alzheimer's increases with age
  • By age 85, 1 in 2 will have the disease.
• Women have a slightly greater chance of developing the disease

Symptoms can vary greatly from person to person

Generally include:

• Memory problems (normally short term memory first)
• Problems with speech and language
• Confusion
• Mood and behaviour changes

Classed as a fatal disease

• Actualy cause of death is usually another disease in addition to Alzheimer's - e.g. pneumonia
• On average patients die within 8 years of first experiencing symptoms although it can range from 3 - 20 years.

Mild

• Gradual loss of brain function
• Minor memory problems - 'forgetfulness'

Moderate

• Confabulation - invent events/conversations to fill in gaps in memory
• Disorientation
• Language problems - inability to find the correct word - dysphasia
• Can result in alarm & frustration/mood changes

Severe

• Severe disorientation & confusion
• Violence
• Hallucinations & paranoid delusions
• Ignoring personal hygiene, incontinence

Number of changes occur in the structure and functioning of the cells in the brain

The exact cause of Alzheimer's is unknown, it is unlikely to be a single cause

Risk factors that increase likelihood:

• Age - after 65 risk doubles every 5 years
• Family history - higher is parent/sibling have it
• Genes - rare, early onset
  • Down's syndrome
• Severe head/whiplash injury
• Same risk factors for CAD

Could not be diagnosed with complete certainty while patient is alive

• only during post-mortem
• although recent research shows this may now be possible

Normally diagnosed from most common symptoms e.g. memory loss

Eliminating other causes, a diagnosis can be made with 90% accuracy

Common form of dementia which affects more than 111,000 people in the UK

Cerebrovascular disease

• Caused by reduced blood flow to the brain - ischaemia - because there is a problem with the blood vessels e.g. atherosclerosis
• Parts of the brain become damaged and eventually die from a lack of oxygen and nutrients

Many cases of vascular dementia can be prevented, e.g. by managing high blood pressure, losing excess weight and stopping smoking

• Rare condition where motor neurons are damaged/destroyed - causes progressive weakness, usually with muscle wasting.
• Motor neurons control important muscle activity:
  • Gripping, walking, speaking, swallowing, breathing
  • As the condition progresses, surfferers find these activitites difficult and eventually impossible
• Affects every 2 in every 100,000 people in the UK each year
• Generally develops in those ages 60+ years, slightly higher incidence in men 
• Researchers believe the cause may be a combination of genetic and environmental factors that build up throughout life
• No single test to diagnose MND and diagnosis is based mainly on the opinion of a neurologist
• There is no cure and it is severely life-shortening.

Parkinson's Disease - reduction in dopamine production

Depression

Manic depression

Bi-polar disorder

Schizophrenia