Phyl Module 2.1 Nervous System

Parts of the neuron

• soma

• dendrites

• axon hillock

• axon

• axon terminal

soma

the cell body of the nerve cell that contains the nucleus, connects the dendrites and axons which allow for nerve impulses to be transmitted from one neuron to another

dendrites

• arise from the cell body and may contain additional small protrusions, known as dendrite spines

• receive electrical signals from axons of other nerve cells which in turn accumulate in the soma being sent to the axon hillock

axon hillock

the area at which the axon is attached to the cell body

axon and myelin sheath

• single elongated structure that extends from the axon hillock

• specialised, insulating substances known as myelin on its surface that boosts the transmission of nerve impulses

• thicker axons with more myelin conduct action potentials faster than thinner axons with less or no myelin

axon terminal

distal part of the axon that meets other cells

• release of neurotransmitters

sensory neurons and relation to speech path

• cell body and axon in periphery

• activated by external physical or chemical stimuli - signal TO the CNS

• critical to identify and perceive sounds in their environment

motor neurons (relation to speech path)

• cell body in CNS, axon in periphery

• connect the CNS to organs, muscles, glands in the body

• transmit signals FROM the CNS

• control movement of different types of muscles

interneurons

• cell body and axon in the CNS

• connect motor neurons to sensory neurons within the CNS, allowing signalling between the two

• allow a rapid communication between the hearing parts of our brain and the motor parts of our brain that activate speech muscles

ions

• charged molecules: positive or negative

• nerves carry an electrical signals through ions

• sodium is high concentration in ECF; potassium is higher concentration in ICF

ion channels

• membrane proteins

• passive transport of ions across membranes

• voltage gated ion channels crucial for action potential propagation

• ligand gated ion channels crucial for synaptic transmission

Membrane potential

• uneven distribution of ions across cell membranes is maintained by the action of ion channels and ion transporters

• uneven ion concentration => uneequal charge across the membrane = membrane potential

• resting membrane potential = -10 to -90 mV

• membrane potential less negative = depolarisation

• membrane potential more negative = hyperpolarisation

action potential (depolarisation and repolarisation)

• the electrical pulse (signal) that moves along a nerve /muscle cell

• possible because of voltage gated ion channels

• excitable cells are cells generate high action potential

• sodium channels open and sodium flows into cell → depolarisation (goes from negative powers positive

• triggers potassium channels to open: potassium flows out of cell → repolarisation

parts of the synapse

1. the presynaptic terminal (contains packets of neurotransmitter)

2. the synaptic cleft: physical space between the neurons

3. post synaptic neuron: the region of dendrite of a postsynaptic neuron that is rich in ligan-gated ion channels

explain the steps of how neurotransmission works

1. action potential arrives at the nerve terminal => depolarisation of the presynaptic terminal

2. opening of voltage-gated calcium channels => allows Ca2+ to enter terminal

3. the increase in Ca2+ in the terminal allows neurotransmitter vesicles to fuse with the presynaptic membrane

4. neurotransmitter is released from the presynaptic neuron and cross the synapse

5. neurotransmitter binds to post-synaptic receptors on the post synaptic membrane

6. triggering action potential and continuation of the electrical signal along the nerve pathway

excitory vs inhibitory synaptic potential

• excitatory post synaptic potential causes depolarisation → more excitable (causes Na influx)

• inhibitory post synaptic potential causes hyperpolarisation → less excitable (activates potassium cell)

• depends on the threshold hoow excitable the cell is from the resting membrane potential

effect of GABA

GABA activates a chloride channel which produces a negative effect (hyperpolarised)

Myelin

• insulating layer or sheath that forms around nerves

• made up of protein and fatty substances

• allow electrical impulses to transmit quickly and efficiently along the nerve cells (saltatory conduction)

• protect and nourish the nerve

• make the transmission energy efficient → it takes much energy to transmit the signal

Nodes of ranvier

• Nodes of Ranvier: the gaps in myelin where the action potential is

• action potential jumps between the nodes of ranvier to increase efficiency

multiple sclerosis

• targets myelin sheath, exposes nerve and causes inflammation → axonal degeneration

• disrupt nerve function => problems with sensation, movement, vision

• slows/interrupts nerve signalling

• increases changes in ion gradient > energy stress => burnout and death in spinal nerves

• paralysis in muscles (including speech muscle)

• flare ups due to inflammation; nerve repairs then the muscle function comes back

Function of neuromuscular

• produce movement (locomotion, balance, fine motor control, maintain posture and body position, stabilise joints, generate heat as they contract)

macrostructure of a muscle

muscle → fascicles → muscle fibres → myofibrils → sacromere (1 unit)

microstructure of neuromusculature

myofibrils → thin and thick myofilaments

• thick filaments made of myosin

• thin filaments made of actin, tropomyosin, troponin -

Excitation of neuromuscular junction

the neuromuscular junction

• acetylcholine is released from the motor neuron at the junction

• binds to post-synaptic nicotinic receptors: opens sodium channel causing a depolarisation of muscle fibres and triggering an action potential

• action potential travels along the sarcolemma (muscle membrane)

• travels down the t-tubules (part of cell membrane that project into centre of muscle cells)

• muscle contraction is stimulated by calcium

• calcium floods into muscle proteins to stimulate contraction

How does calcium affect neurotransmission?

• stimulates muscle contraction, essential for neurotransmission

• blockage of calcium can shut down nerves talking

• this is how pain drugs are used to relieve pain signals

contraction of neuromusculature

• in a resting muscle myofibre, the binding site between myosin and actin is covered by the troponin and tropomyosin

• calcium ions are released from the sarcoplasmic reticulum

• calcium attaches to the binding site of actin which causes the tropomyosin and troponin to move away from the acting sites on the actin

• actin then binds to myosin to form cross bridges (requiring ATP), thus shortening the sarcomere

• the two ends of the sarcomere are being pulled towards the M-line (middle)

• shortening the muscle and contraction occurs

motor units

• the smallest functional unit in the musculoskeletal system

• a single motor neuron and all the muscle fibres it innervates

• small motor units: small contraction - fine motor control

• large motor control: larger, explosive movement

• to control muscle movement → recruit motor units

• number of motor units and discharge rate (frequency recruited motor units discharge) → increases force of movement

• if lack control of motor units, might have exaggerate muscle movement and inappropriate mouth muscles

Reflexes

• the involuntary, rapid response to a stimulus

• helps maintain posture, control visceral activities (e.g. pulling away from pain, pupil constriction) and exist as safety mechanisms so we don’t get hurt

• somatic reflexes: activate skeletal muscle

• autonomic (visceral) reflexes: activate visceral effectors (smooth or cardiac muscle or glands)

What are the 5 main components of a reflex?

1. receptor: site of stimulus action

2. sensory neuron: transmit afferent impulses to CNS

3. Integration centre (interneuron): either monosynaptic or polysynaptic region within CNS

4. motor neuron: conducts efferent impulses from integration centre to effector organ

5. effector: muscle fibre or gland cell that responds to efferent impulses

Primitive oral reflexes

• feeding reflexes that babies have

• disappear over development

• sucking reflex: sucking when the oral region is stimulated, or an object is inserted into the mouth (2-5mth)

• rooting reflex: mouth turning toward an object, is seen in response to light stroking on the cheek or bringing an object into the patient’s visual field (1mth)

motor nerve neurotransmitter, receptor, permeating ion, effect

• neurotransmitter - ACh

• receptor - nicotinic ACh receptor

• permeating ion: Na

• Effect - depolarisation

excitatory CNS neuron neurotransmitter, receptor, permeating ion, effect

neurotransmitter = glutamate

receptor = NMDAR, AMPAR, ionotropic glutamate receptors

permeating ion = Na

effect = depolarisation

excitatory CNS neuron neurotransmitter, receptor, permeating ion, effect

glycine or GABA; GlyR or GABA R; Cl; hyperpolarisation

sympathetic NS neurotransmitter

noradrenaline

how is neurotransmitter signalling between neurons stopped by (3)

• breakdown of neurotransmitter by an enzyme

• transmitter diffusing away from synapse

• cells around synapse mopping it to be recycled: reuptake