ATAR Human Bio - Nervous System #2

Propagation

the travel of a nerve impulse along an action potential

Myelinated Transmission

- the nerve impulse "jumps" between the nodes of ranvier (saltatory conduction)

- this is possible because the axons are covered by myelin sheath

Unmyelinated Transmission

- the nerve impulse travels along the neuron, stimulated by the flow of sodium ions, depolarizing the area

- transmission is slower, due to the need to repolarise at each node

all or none response

neurons have a threshold potential of +15 mV, if a stimulus is able to provide a voltage equal or higher to this (so the overall membrane potential is -55 mv), than an action potential is initiated

Depolarisation

Depolarisation causes an influx of Na+ ions into the intercellular environment due to voltage gated sodium channels opening. This creates an overall positive charge, continuing until the membrane potential reaches +30 mV.

Repolarisation

Voltage gated sodium channels close and voltage gated potassium channels open. K+ rushes out of the cell (down the concentration gradient) as membrane potential decreases. This creates an overall negative charge relative to the outside.

Hyperpolarisation

- K+ channels open for too long, resulting in membrane potential reaching -90 mV.

- the Sodium-potassium pump restores the resting membrane potential by pumping out 3NA and in 2K until the resting potential of -70 mV is re-established

Sodium Potassium Pump

The sodium potassium pump maintains a negative sodium charge in the cell, relative to the outside.

It uses ATP (active transport) to move 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) in.

Sodium Potassium Pump role in Nervous and Endocrine System

Role in Nervous System

- Neurons rely on the sodium-potassium pump to maintain the resting membrane potential

- Without this pump, a neuron can't reach threshold, depolarise, or send action potentials.

role in Endocrine System

Some hormones change pump activity to alter cell excitability

e.g. thyroid hormone increases pump rate to raise metabolism and heat production

aldosterone increases pump rate in the kidney to reabsorb more water

Synaptic Transmission

1. an incoming action potential research the axon terminal and causes depolarisation in the presynaptic knob

2. voltage gated calcium channels open; an influx of calcium in the synaptic gap enters the cell and binds to neurotransmitter vesicles

3. these vesicles release neurotransmitters into the synapse via exocytosis

4. neurotransmitters diffuse across the synaptic gap & bind to receptor sites on the dendrites of the other neuron. this triggers ligand-gated ion channels to open

5. the influx of sodium can initiate a new action potential in the postsynaptic neuron

bones (protection of brain)

- cranium protects the brain

- vertebral canal protects spinal cord

- hard bone provides a strong, rigid barrier. (but can cause issues when the brain swells, as the swelling has nowhere to go)

meninges

- connective issues that cover surface of brain & spinal cord

dura mater: outer layer, tough & fibrous

arachnoid mater: middle layer, loose mesh of fibres

pia mater: inner layer, thin and delicate. contains blood vessels.

cerebrospinal fluid

- clear watery fluid containing few cells, glucose, protein, urea and salts

- occupies a space between arachnoid and pia mater (sub arachnoid space)

- acts as a shock absorber & provides mechanical support to keep the brain buoyant

Sensory Neurons

- afferent neurons

- receives nerve impluses from CNS to body (e.g. move hands away after touching a hot stove)

- contains dorsal root ganglion

motor neurons

- efferent neurons

- sends nerve impulses from the CNS to body (e.g. instructions to start walking, speaking, swallowing)

- contains ventral root ganglion

somatic nervous system

controls 5 senses & voluntary movement

cranial nerves

12 mixed nerves arsing from the brain & carrying sensory fibres to and from the brain.

spinal nerves

31 pairs of spinal nerves arise from the spinal cord. includes ventral and dorsal root.

autonomic nervous system

involuntary bodily functions (e.g. heart rate, respiration, digestion)

sympathetic nervous system

- activates body in response to a threat (fight or flight)

- nerve endings release noradrenaline

examples of sympathetic responses

- dilation of pupil

- acceleration of heart rate

- dilation of bronchi

- inhibits stomach & pancreas

- relaxes bladder

- stimulates ejaculation

parasympathetic nervous system

- maintains homeostasis & normal levels of arousal (rest & digest)

- nerve endings release acetylcholine

Cerebral Cortex

outermost brain layer, involved in higher cognitive processes. (e.g. thinking, reasoning, memory, consciousness)

Corpus Callosum

connects the right and left hemispheres and allows communication between them (contralateral control)

Cerebrum

responsible for posture, balance and coordination of voluntary muscle movements

Hypothalamus

maintains homeostasis (e.g. digestive system, body temperature) and motivated behaviour (e.g. thirst, sex drive) and controls mood.

Spinal Cord

provides a pathway for communication between muscles/glands and the brain, allows for reflex responses

Medulla Oblongata

regulates the heart, breathing & diameter of blood vessels under the influence of hypothalamus.

Cardiac Centre

regulates rate and force of heartbeat

Respiratory Centre

regulates rate and depth of breath

Vasomotor Centre

regulates diameter of blood vessels

cerebrum (grey and white matter)

- Is the biggest structure in the brain

- consists of:

○ grey matter (cerebral cortex): neuron cell bodies, dendrites and unmyelinated axons

○white matter (corpus callosum): myelinated axons (below the cerebral cortex)

Spinal Cord (inc tracts)

- a structure extending from the foramen magnum

- nerve fibres are arranged in ascending & descending tracts.

ascending tract - sensory neurons that carry impulses upwards towards the CNS

descending tract - motor neurons that sends impulses downwards away from the CNS