The Nervous System (HL IB Biology)

The structure of the brain

• The brain alongside the spinal cord is part of our central nervous system
• The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours, both conscious and unconscious 
• Within the brain are different regions that carry out different functions
  • The cerebral cortex: this is the outer layer of the brain which is divided into two hemispheres. It’s highly folded and is responsible for higher-order processes such as intelligence, memory, consciousness and personality
  • The cerebellum: this is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement
  • The brainstem: this relays messages between the cerebral cortex, the cerebellum and the spinal cord. A key part is the medulla which controls unconscious activities such as heart rate and breathing
• Two important glands of the brain that are integral the endocrine system are
  • The pituitary gland: This gland is responsible for producing many hormones including those involved in controlling the menstrual cycle (FSH and LH)
  • The hypothalamus: This region of the brain is involved in regulating body temperature, it also producing hormones which control the pituitary gland

Structures of the brain diagram

The brain is made up of several regions

The role of the brain

• The brain coordinates and processes information received
• Interactions within the brain are responsible for learning and memory 
• The brains requires several receptors in order to receive information (this is input of information)
  • At a conscious level information is received by
    • Photoreceptors located within the retina of the eye for visual information
    • Chemoreceptors found in the tongue for tasting
    • Thermoreceptors located in the skin for detection of temperature changes
    • Mechanoreceptors located in the inner ear which are sensitive to sound vibrations 
  • At unconscious level input of information is via
    • Osmoreceptors located in the carotid arteries and hypothalamus which detect the water content of the blood
    • Baroreceptors, also located in the carotid arteries and the aorta, these sense pressure changes of the blood
    • Proprioceptors which are located in muscles and joints and provide information on balance and movement

• The spinal cord is part of the central nervous system (CNS), along with the brain 
• It is a neural pathway between the body and the brain, yet it can also process information independently from the brain 
  • This information is processed at the unconscious level and involves some reflex reactions
    • The spinal cord can be described as an integration centre for unconscious processes
  • Note that information processed at conscious level means that the cerebrum of the brain is also involved
• The spinal cord is responsible for bringing sensory information to the CNS from the body and enables motor (muscular) information to be sent out
• There are two types of tissue in the spinal cord
  • White matter contains mainly the axons only of neurones that carry information to and from the brain
  • Grey matter contains the neurones and synapses involved in spinal cord integration processes which create a reflex response 
    • Sensory information enters the spinal cord along sensory neurones, this is immediately processed and sent out as motor information along motor neurones; this pathway is called a reflex arc
    • Because the brain is not involved in this pathway this is unconscious control directed by the spinal cord alone

Reflex arc diagram

The spinal cord of the central nervous system acts as an integration centre for unconscious processes

• A neural pathway begins with a receptor
• A receptor is a specialised cell that can detect changes in the environment that cause a stimulus
• Receptor cells are transducers – they convert energy in one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
• Receptor cells are often found in sense organs (e.g. light receptor cells are found in the eye)
  • Some receptors, such as light receptors in the eye and chemoreceptors in the taste buds, are specialised cells that detect a specific type of stimulus and influence the electrical activity of a sensory neurone
  • Other receptors, such as some kinds of touch receptors, are just the ends of the sensory neurones themselves
• When receptors cells are stimulated they are depolarised
  • If the stimulus is very weak, the cells are not sufficiently depolarised and the sensory neurone is not activated to send impulses
  • If the stimulus is strong enough, an action potential is initiated in the sensory neurone and the impulse is transmitted to the CNS, specifically the spinal cord and the cerebral hemispheres

An example of the sequence of events that results in an action potential in a sensory neurone

• The surface of the tongue is covered in many small bumps known as papillae
• The surface of each papilla is covered in many taste buds
• Each taste bud contains many receptor cells known as chemoreceptors
  • These chemoreceptors are sensitive to chemicals in food and drinks
• Each chemoreceptor is covered with receptor proteins
  • Different receptor proteins detect different chemicals
• Chemoreceptors in the taste buds that detect salt (sodium chloride) respond directly to sodium ions
• If salt is present in the food (dissolved in saliva) being eaten or the liquid being drunk:
  • Sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptor cells
  • This leads to depolarisation of the chemoreceptor cell membrane
  • The increase in positive charge inside the cell is known as the receptor potential
  • If there is sufficient stimulation by sodium ions and sufficient depolarisation of the membrane, the receptor potential becomes large enough to stimulate voltage-gated calcium ion channel proteins to open
  • As a result, calcium ions enter the cytoplasm of the chemoreceptor cell and stimulate exocytosis of vesicles containing neurotransmitter from the basal membrane of the chemoreceptor
  • The neurotransmitter stimulates an action potential in the sensory neurone
  • The sensory neurone then transmits an impulse to the brain

Diagram to show the sequence of events initiated by activated chemoreceptors in the taste buds

Sensory neurons convey messages from receptor cells to the CNS

• Once an action potential has been transmitted from a sensory neurone to the CNS
• The cerebrum within the brain uses the information to process movements within the body as needed; the part of the cerebrum responsible for this is called the motor cortex

The role of motor neurones

• Motor neurones are used to carry action potentials to muscles to initiate the movement required
• All motor neurones terminate within a muscle within a neuromuscular junction (also known as motor end plates)
  • There are multiple neuromuscular junctions spread across several muscle fibres within the muscle
    • Neuromuscular junctions work in a very similar way to synapses
    • They are located between the terminal branches of a motor neurone and a muscle cell

Transmission across the neuromuscular junction

• When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane, the action potential causes calcium ions to diffuse into the neurone
• This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the presynaptic membrane
• The ACh that is released diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma (surface membrane of the muscle fibre cell)
• This stimulates ion channels in the sarcolemma to open, allowing sodium ions to diffuse in
• Influx of sodium ions depolarises the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre

Action potentials stimulate muscle contraction

• Action potentials travel down the T-tubules and trigger the opening of voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum
• Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils
• Calcium ions bind to troponin molecules, stimulating them to change shape
• This causes the troponin and tropomyosin proteins to change position on the thin (actin) filaments
• The myosin-binding sites are exposed to the actin molecules
• The process of muscle contraction (known as the sliding filament model) can now begin

Muscle contraction diagram

Action potentials from the motor neurone cross the neuromuscular junction to trigger the events leading to muscle contraction

• Information is sent through the nervous system as nerve impulses – electrical signals that pass along nerve cells known as neurones
• Nerves are made up of bundles of sensory neurones or motor neurones
  • These may be myelinated or unmyelinated 
• The different structures of these neurones are considered in more detail here
• Myelination is covered in more detail here

Structure of a Nerve Diagram

A nerve is made up of a bundle of individual neurone cells

Transverse and Cross Section of Myelinated Neurone Diagram

Each Schwann cell wraps its plasma membrane concentrically around the axon to form a segment of myelin sheath about 1mm long