5.2.1 Nervous system

Overview of the nervous system

• CNS = brain + spinal cord

• PNS = neurones that connect CNS to rest of body

Difference between somatic and autonomic nervous system

Somatic NS	Autonomic NS
Voluntary actions	Involuntary actions
Carries impulses to skeletal muscles	Carries impulses to endocrine glands, smooth muscle + SAN in RA of heart
SNS neurones fully myelinated	ANS only myelinated from ganglion to CNS
1 neurotransmitter released – Acetyl Choline	2 NTs released – Ach + noradrenaline
Doesn’t divide any further	Divides into sympathetic + parasympathetic

 

differences between parasympathetic + sympathetic NS

structure of human brain

function of cerebrum/cerebral cortex in the brain

1. cerebrum

• has 2 hemispheres —> Left hemi receives sensory inputs from receptors on RHS of body

• surface covered by cell bodies

• extensively folded

• controls conscious thought, language, memory, emotional responses + intelligence

function of cerebellum

• controls non-voluntary movements + muscle coordination

function of medulla oblongata

• controls autonomic (involuntary) activities e.g heart rate, blood pressure, peristalsis (relaxing/constriction of gut muscle)

function of hypothalamus + pituitary gland in brain

hypothalamus:

• controls thermoregulation, osmoregulation, homeostasis, secretion of hormones

pituitary gland:

• posterior lobe: stores + secretes hormones made by hypothalamus

• anterior lobe = makes + secretes hormones

what are the 3 general structures of neurones

1. cell body

• contains nucleus

• has lots of ER + mitochondria for production of neurotransmitters

2. Dendrons

• short extensions of cell + cytoplasm from cell body —> increases SA to receive nerve impulses from other neurones

• divides into smaller branches = dendrites —> transmit electrical impulses TOWARDS cell body

3. Axon

• single, elongated nerve fibre that extends from cell body

• transmits electrical impulses AWAY from cell body

• surrounded by plasma membrane + can be myelinated

sensory neurone

carries impulse from receptor to relay neurone

relay neurone

carries impulse from relay (in CNS) to motor neurone

motor neurone

carries impulse from motor to effector (muscle to contract, gland to secrete hormones)

what’s the difference between an axon and a dendron

• an axon always goes AWAY from the cell body + to the effector

• dendron always goes TO cell body from a receptor

structure + function of myelin sheath

• made from many layers of plasma membrane synthesised by Schwann cells

• these layers wrap around axons/dendrons

• they insulate the axon, making the myelinated parts impermeable to Na+/K+

• small gaps between Schwann cells = Nodes of Ranvier —> sites of depolarisation that allows movement of Na+/K+

• enables salutatory conduction

what’s the importance of the myelin sheath

• increase lengths of local currents —> when Na+ enter the axon during depolarisation, they generate local electrical currents that spread along the axon and trigger the next node of ranvier to be depolarised —> if local currents can travel further, fewer action potentials need to be generated + depolarisation only occurs at the nodes of ranvier

• significance: faster conduction of nerve impulses + less energy is used as fewer Na+'/K+ pumps needed to restore resting potential

what happens if there’s no myelin sheath

• no electrical insulation of axon

• axon becomes permeable to Na+/K+ ions

• no saltatory conduction

• shorter local circuits - depolarisation happens along whole membrane, more APs need to be generated along the axon

• slower transmission of electrical impulses

what is an action potential

the change in electrical potential associated with the passage of an impulse along the membrane of a neurone

what stages are in an action potential

1. Ressting potential

2. depolarisation

3. repolarisation

4. hyperpolarisation / refractory period

what happens in depolarisation

• occurs when a neurone is stimulated

• Na+ channels in axon membrane open

• Na+ diffuse into the membrane, down the electrochemical gradient

• inside of axon becomes LESS negative

• if membrane potential reaches -50mV (threshold level), this will stimulate more voltage gated Na+ channels to open —> more Na+ diffuse into cell

what happens after depolarisation (propagation along axon)

• the Na+ ions diffuse sideways along axon

• this triggers Na+ channels in the next region (next node of ranvier) of axon to open

• Na+ diffuse into axon + membrane becomes LESS negative

• this causes wave of depolarisation to travel along the axon

what happens in repolarisation (peak of AP)

• all voltage gated Na+ channels close

• voltage gated K+ channels open

• K+ diffuse out of the axon, down the electrochemical gradient

• inside of the membrane becomes MORE negative

• Na+/K+ pumps restores ionic balance

what happens during hyperpolarisation

• K+ channels are slow to close, so diffusion of K+ out of the axon continues until the inside of the membrane becomes more negative than the resting potential (-70mV)

• voltage gated K+ ion channels close + action of Na+/K+ pumps restores membrane to resting potential

What is resting potential

when the neurone is not transmitting an impulse but it’s still active

what happens in a resting potential

• active transport of 3 Na+ OUT of axon, and 2K+ IN via proton pumps

• inside of membrane is MORE negative than outside

• so Na+ slowly diffuses back in to membrane and K+ diffuses out via non-voltage gated channels

what is the refractory period

delay between one AP and the next to prevent the overlapping of impulses

importance of the refractory period

• ensures action potential travels in only 1 direction - if more depolarisation occurs during 1 AP, this can cause the AP to travel backwards - never reaching target cell

• ensures impulses aren’t sent too quickly - this limits frequency of APs

• prevents axon becoming depolarised after an AP so no impulses overlap

how does a larger stimulus affect the AP

• larger stimulus increases the frequency of APs

• but it does NOT change the size of the AP

what is the all or nothing response

an AP can only occur if the threshold level is reached

structure of a synapse

• synaptic knob = lots of mitochondria + SER to produce NT

• synaptic vesicles = contain NT - fuses with presynaptic membrane, allowing NT to be released into synaptic cleft via exocytosis

How does a nerve impulse cross a synapse

• axon potential reaches end of presynaptic neurone

• causes depolarisation of presynaptic membrane

• Ca2+ ion channels open + C2+ diffuses into presynaptic knob

• influx of Ca2+ activates the synaptic vesicles to fuse with the presynaptic membrane

• causes NT to be released via exocytosis into synaptic cleft

• NT diffuses across cleft, down the conc gradient

• NT binds to specific, complementary receptors on post synaptic membrane

• stimulates Na+ channels (on post sm) to open —> Na+ diffuse in to Post neurone

• if sufficient Na+ enters that overcomes the threshold value, an AP is generated

• impulse is propagated along post-synp neurone

what are the 2 types of neurotransmitters

1. excitatory - causes depolarisation of postsyn neurone

• if threshold is reached, AP is generated

• AcH, glutamate

2. Inhibitory - causes hyperpolarisation of postyn membrane

• prevents AP being generated

• GABA, glutamate (rod cells only)

What happens to the neurotransmitter once an AP has been generated in post synaptic membrane

• must be broken down to prevent stimulus being maintained

• AcH is hydrolysed by acetylcholinesterase into choline + acetate (ethanoic acid)

• Ach is released from receptors on postsyn membrane

• Na+ ion channels close

• choline + acetate reabsorbed back into synaptic knob by endocytosis

• Ach is reformed by aerobic respiration using ATP

• Ach repackaged into vesicles —>NT are recycled

types of postsynaptic potentials

roles of synapses

1. ensure impulses travel in 1 direction - NT receptors only found on post syn membranes so diffusion of impulses can only occur from pre - post

2. synaptic divergence -

what’s the nature of a reflex arc and how does this compare to a normal reaction

• rapid, involuntary responses to stimuli to prevent harm to body —> not taught

• different to normal reaction as it’s INVOLUNTARY

what types of reflex arcs are there

• blink reflex - rapid closing of the eye in response to stimulation of the cornea e.g bright lights/dust in eye

• plantar reflex - used as a diagnostic tool to assess NS damage/consciousness —> sole of foot stimulated with blunt object —> NORMAL response = foot flex down, ABNORMAL = foot flexes up

• iris reflex - involuntary changing of pupil size in response to different light intensities (protects retina) —> NORMAL = pupils constrict to same degree, ABNORMAL = constrict differently

what are the 2 types of brain damage

1. traumatic - severe head injury e.g car crash, fall

2. non-traumatic - not caused by head injury e.g stroke, infection

what techniques are used to assess brain + spinal cord damage

• MRI

• functional MRI

• CT scans

• PET scans

• EEGs

why are brain scans important

• detects location and extent of injury

• necessary treatments can be carried out quickly

• needed to diagnose brain damage

effect of brain + spinal cord damage

what drugs can be used to treat brain damage

effect of recreational drugs

drug dependency

how can CT scans be used to detect the location/extent of injury in nervous system

uses X-rays to build up a detailed 3D image of brain/spinal cord—> shows areas with bleeding/poor blood supply

how can MRIs/fMRIs be used to detect the location/extent of injury in nervous system

MRIs

• uses magnetic fields to detect swelling/inflammation/areas of demyelination

• shows difference between healthy and damaged areas

RISKS: ionising radiation, not suitable if pt has metal implants, pt must be still

fMRIs:

• shows difference between healthy and damaged areas

• detects changes in blood flow

• no ionising radiation used, non invasive

RISKS: pt must be still