12HBY The Nervous System - Ch 4

Cerebrum

. the largest part of the brain made up of left and right hemispheres

. responsible for higher order functions and conscious thought processes (thinking, reasoning, memory, learning, conscious awareness of surroundings, intelligence)

. interprets sensory information and initiates voluntary muscle movements

Cerebrum: cerebral cortex

. the outer layer of the cerebrum, made up of grey matter (comprised primarily of neuron cell bodies, dendrites and unmyelinated axons)

Cerebrum: describe below the cerebral cortex

. white matter (contains myelinated axons organised into tracts that enable communication within the brain and between the brain and spinal cord)

Cerebrum: Basal ganglia

. the masses of grey matter inside each cerebral hemisphere

. deep within cerebrum

. regions of grey matter involved in motor control

Describe the folded surface of the cerebrum

. cerebral cortex is highly folded into ridges (gyri) or convolutions and grooves (sulci) and fissures

. folds increase the surface area, allowing cerebrum to contain 70% of CNS neurons

What is the cerebrum divided into and by what

. divided into left and right hemispheres, separated by the deep longitudinal fissure and connected by the corpus callosum

What are the 4 lobes on each hemisphere

. frontal, parietal, temporal, occipital

Frontal lobe functions

. controls motor activity, thinking, emotions, personality and language

Parietal lobe functions

. responsible for processing temperature, taste, pain, touch and movement

Temporal lobe functions

. processing memories and linking them with senses, auditory processing

Occipital lobe functions

. the visual processing centre of the brain

. sight perception

Insula

. a part of the cerebrum that’s buried deep inside the brain

. considered a 5th lobe of each cerebral hemisphere

. responsible for recognising different senses and emotions, addiction and psychiatric disorders

Areas of the cerebral cortex

. sensory areas: interpret impulses from receptors

. motor areas: control muscular movements

. association areas: relevant to intellectual and emotional processes

Describe the memory function of the cerebrum

. memory = the process by which the brain encodes, stores and later retrieves info through changes in neural connections within the nervous system

. memories are pathways of nerve cells

. when a memory is stored, new links are made between neurons or existing links between neurons are modified

Corpus callosum

. a thick bundle of nerve (dense white matter structure) fibres that links the 2 cerebral hemispheres

. located at the base of the longitudinal fissure

. consists of commissural fibres that connect the left and right cerebral hemispheres

. function: to enable communication between the hemispheres, allowing sensory, motor and cognitive information to be shared

—> this integration ensures coordinated brain activity across both hemispheres

Corpus callosotomy

. a surgical procedure for severe, drug-resistant epilepsy that cuts the bundle of nerve fibres connecting the brain’s 2 hemispheres

. palliate procedure to reduce symptoms without addressing the cause of the seizure symptoms

Cerebellum

. the part of the brain behind and below the cerebrum, concerned with coordination of movement

. located beneath the posterior region of the cerebrum

. has a highly folded surface that increases its processing capacity

. outer layer consists of grey matter, while inner regions contain white matter that branches extensively

Cerebellum function

. function: that occur at an unconscious level

—> responsible for balance, posture, fine coordination of voluntary muscle movements

—> receives sensory input from inner ear about posture and balance, and stretch receptors in skeletal muscles about the length of the muscles and muscle tone

—> refines and smooths motor activity, not initiates it

Hypothalamus

. the part of the brain lying just below the thalamus (deep within the brain), controlling many homeostatic mechanisms, such as body temp, water balance and heart rate

Hypothalamus function

. homeostatic control centre

. regulation of:

—> autonomic nervous system (HR + BP)

—> body temp

—> food + water intake

—> patterns of waking + sleeping

—> contraction of the urinary bladder

—> emotional responses (fear, anger, pleasure)

—> the secretion of hormones and coordination of parts of the endocrine system, acting through the nearby pituitary gland to regulate metabolism, growth, reproduction, responses to stress

What receptors are in the hypothalamus

. central thermoreceptors

. central osmoreceptors

Medulla Oblongata

. the part of the brain that joins to the spinal cord

. forms lower portion of brainstem

. continuous with the spinal cord

Medulla Oblongata function

. many nerve fibres simply pass through it going to or from the other parts of the brain but has a role in automatically adjusting body functions

. contains:

—> cardiac centre: regulates rate and force of heartbeat

—> respiratory centres: control rate + depth of breathing

—> vasomotor centre: regulates diameter of blood vessels

. also controls blood pressure

. other centres regulate the reflexes of swallowing, sneezing, coughing and vomiting

What is the medulla oblongata influenced by

. all centres in medulla oblongata influenced and controlled by higher centres in the brain, particularly hypothalamus

what receptors are in medulla oblongata

. central chemoreceptors

Protection of CNS by bones

. provide rigid mechanical protection to CNS by enclosing and shielding delicate nervous tissue

. brain protected by cranium

. spinal cord protected by vertebral column

. bony structures act as a hard protective barrier that reduces the risk of injury from external impacts or trauma

. also provide structural support, maintaining the shape and integrity of the CNS

. outermost protective layer

Protection of CNS by meninges

. meninges = 3 layers of connective tissue membranes that surround and protect the brain and spinal cord

. Dura mater, arachnoid mater, pia mater

. function to protect and stabilise CNS

. provide an additional protective covering, help anchor brain and spinal cord within the skull and vertebral column + serve as a framework for blood vessels supplying nervous tissue

. enclose the CSF within the subarachnoid space, contributing to shock absorption and cushioning

. found surrounding the entire brain and spinal cord, forming a continuous protective sheath around CNS

Dura mater

. the outermost fibrous and tough meningeal layer, providing strong protection

. epidural space exists pathologically in the brain (between dura and skull) but is a real space in the spinal cord containing fat and veins

. subdural space is a potential space between the dura and arachnoid

. stuck to inside of skull, but not so close fitting on inside of vertebral canal

. Function:

—> provides a layer of protection for the brain (shock absorber)

—> structural support

—> venous drainage + CSF flow

—> pain sensitivity

Arachnoid mater

. the middle meningeal layer which is a loose mesh of fibres, allowing for the flow of CSF

. has a web-like structure that contributes to cushioning

. subarachnoid space is a real space between the arachnoid and pia mater filled with CSF

. Function:

—> protection + cushioning

—> CSF drainage

—> structural framework for blood vessels

—> barrier (separates subarachnoid from subdural space)

Pia mater

. innermost meningeal layer

. thin (delicate) and highly vascularised (containing many blood vessels), allowing close association with nervous tissue

. sticks closely to surface of brain and spinal cord

. function:

—> delivers oxygen + nutrients to underlying neural tissue

—> maintains structural integrity of spinal cord

—> blood-brain barrier

—> CSF formation

—> protection layer

Meningitis

. inflammation of the meninges

Hematomas

. bleeding can occur in the epidural, subdural or subarachnoid spaces, causing pressure on the brain

Lumbar puncture

. performed in the subarachnoid space (lumbar cistern) to sample CSF

Epidural

. a form of regional anaesthesia and a common, highly effective method for pain relief, most notably used during childbirth

. involves injecting pain medication, such as local anaesthetics and opioids, into the epidural space in the lower back via a small catheter, effectively blocking pain signals from nerves

Cerebrospinal fluid (CSF)

. clear, colourless fluid produced in the cavities of the brain (by the choroid plexus within brain ventricles)

. formed from blood and evenutally re enters blood capillaries

. contains a few cells + some glucose, protein, urea and salts

. fills the brain cavities and surrounds the brain and spinal cord

. circulates through the ventricles of brain, the central canal of spinal cord, and the subarachnoid space surrounding CNS

Protection of CNS by CSF (functions of CSF)

1. Protection:

. shock absorber to protect from impact, cushioning any blows CNS may sustain from sudden movements or impacts

. barrier between blood vessels and CSF is quite tight —> blood-brain barrier —> protects against entry of bacteria + some other particles

2. Support:

. CSF provides buoyancy, effectively reducing the weight of the brain and preventing excessive pressure on the base of the skull

. brain is suspended inside cranium and floats in the surrounding CSF fluid

. supports the fragile brain tissue, cushioning it from contact with the skull and floats it within the cranial cavity

3. Transport and chemical stability:

. CSF assists in maintaining chemical stability + facilitating the transport of nutrients and removal of metabolic waste products to and from cells of brain + spinal cord

Action potential

. the rapid depolarisation and repolarisation of the cell membrane

Resting potential definition

. the membrane potential of unstimulated nerve cells

Resting potential (polarisation) steps

1. a resting potential is when a neuron isn’t stimulated nor transmitting an impulse

2. the resting potential of the neuron is -70mV, due to the inside of the neuron being relatively negative compared to the positive extracellular fluid

3. This is achieved due to the action of the sodium-potassium pump, actively transporting 3 sodium ions out of the neuron into extracellular fluid and 2 potassium ions into neuron’s intracellular fluid

4. Other transport proteins, such as sodium leakage channels and potassium leakage channels are open

5. The voltage gated channels are closed

6. Overtime, this leads to the extracellular fluid having a high conc. of sodium ions

7. And intracellular fluid having high conc. of potassium ions

8. More positive ions leave the neuron than enter

9. The neuron is polarised

Polarised

. the situation when the inside of the membrane of a nerve has a negative electrical charge compared with the outside, creating an electrical potential difference across the membrane

Depolarisation definition

. the process of becoming depolarised

Depolarisation process steps

1. the neuron receives a stimulus, which causes local depolarisation

2. an action potential can only be triggered if threshold is reached, it’s an all or non response

3. Threshold is reached when there’s sufficient stimulation and membrane potential reaches -55mV

4. Threshold may be reached by rapid repeated stimulation in quick succession and/or multiple stimuli acting at the same time

5. Once threshold reached, voltage-gated sodium channels open

6. causing sodium ions to rush into the axon along their concentration gradient

7. causing inside of axon to become more positive, relative to extracellular fluid

8. as result, the membrane potential rises from -70mV to +40mV

9. This change is called an action potential

10. Depolarisation at one region triggers depolarisation in adjacent regions, allowing the impulse to travel along neuron

Repolarisation definition

. the process of becoming repolarised

Repolarisation process steps

1. After a short time at +40mV, the voltage gated sodium channels close, preventing further sodium ions from entering the neuron

2. Voltage-gated potassium channels open

3. Allowing potassium ions to diffuse out of the axon

4. Because there are fewer positive ions in the neuron, the intracellular fluid becomes less positive

5. The membrane potential moves back towards -70mV

6. The axon is repolarising

Hyperpolarisation definition

. the process of becoming hyperpolarised

Hyperpolarisation steps

1. voltage-gated potassium channels remain open longer than required

2. causing the excessive movement of potassium ions out of neuron

3. as a result, the membrane potential becomes more negative than the resting potential, falling to around -80mV (< -70mV)

4. This is called hyperpolarisation

5. Then the voltage gated potassium channels close

6. the sodium potassium pump restores the normal distribution of ions

7. Returning the neuron to its resting potential of -70mV, which is a polarised state

Nerve impulse

. an electrochemical signal that travels along a nerve fibre

. there’s a change in electrical charge across the neuron membrane (potential difference)

. caused by movement of chemicals (sodium and potassium ions)

Electrochemical

. a change in electrical charge across a neuron membrane, caused by the movement of chemicals

Refractory period

. a short period of time following a stimulus during which the neuron cannot be stimulated again

. lasts from when the membrane reaches the threshold of -55mV until it returns to the resting potential of -70mV, as the voltage-gated sodium channels are temporarily unresponsive

. ensures that impulses travel only one direction

What is myelination (in CNS)

. schwann cells produce the myelin sheath in the PNS, spiralling their plasma membrane around axons to provide insulation, nutritional support and increased nerve impulse speed

. each schwann cell wraps a single segment of an axon, with multiple schwann cells required along the length of a nerve

Saltatory conduction

. The conduction of a nerve impulse along a myelinated nerve fibre, where the impulse seems to jump from one node of Ranvier to the next

. action potentials can only be transmitted from node to node as myelination prevents the movement of ions across the membrane

. the nodes of ranvier are unmyelinated, so ions can move across the membrane, meaning an action potential can form

. the propagation of action potentials along myelinated axons from one node of ranvier to the next

Sympathetic division nerves

. nerves arise from the thoracic and lumbar (middle) regions of the spinal cord

Parasympathetic division

. nerves arise from the brainstem (top) and sacral (bottom) regions of the spinal cord

Receptor

. a specialised structure or cell that is able to detect a change in the body’s internal or external environment

. sense organs contain particular receptor cells grouped together