Nervous System

Question 1

Describe the different anatomical axes and planes.

Answer 1

There are two sets of axes - those relative to the organism (rostral-caudal, dorsal-ventral, and medial-lateral), and those relative to 3D space (superior-inferior and anterior-posterior).
The rostral-caudal axis travels from the top of an organism’s tract (rostral) to the bottom (caudal). For example, the spinal cord extends caudally away from the brain, or food travels caudally from the GI tract. [Important to note, this corresponds to anterior-posterior in quadrapoedal organisms, but to superior-inferior in bipoedal organsisms].
The dorsal-ventral axis goes from the back aspect (dorsal) to the frontal aspect (ventral) of an organism [think dorsal horn].
The medial-lateral axis goes from the mid-line of a bilaterally symmetrical organism outwards.
The superior-inferior axis goes from up to down, and the anterior-posterior axis goes from front to back.
A horizontal plane is observing a cross-section from a bird’s-eye view.
A sagittal plane is observing a cross-section from a side-on view.
A coronal plane is observing a cross-section from a front-on view.

Question 2

Describe the gross anatomy of the human brain.

Answer 2

The human brain contains 4 main components: cerebrum, cerebellum, brain stem, and spinal cord.
The cerebrum is the largest, and is located rostrally. It is split into two hemispheres which are connected by the corpus callosum.
The human brain is gyrencephalic (grooved), giving it a greater surface area. The ridges are known as gyri and the grooves as sulci.
The cerebellum is located caudally. The brainstem is caudal and inferior in humans. It is the most primitive part of the brain, and is composed of the midbrain, the pons, and the medulla.
The spinal cord is continuous with the brainstem, and descends caudally and inferiorly down the spinal tract.

Question 3

Describe the key landmarks of the cerebral cortex.

Answer 3

The cerebral cortex is split into four main lobes: the frontal lobe (rostral-most), the parietal lobe (dorsal and caudal), the occipital lobe (inferior and caudal to the parietal), and the temporal lobe (inferior and lateral).
The precentral and postcentral gyri are found at the boundary between the frontal and parietal lobes, with the central sulcus lying between them.
The lateral sulcus (AKA sylvian fissure) separates the frontal and parietal lobes from the temporal lobe.
The parieto-occipital sulcus is located between the parietal and occipital lobes - you can only see part of it from a lateral view (clearer from a mid-sagittal view).
The calcarine sulcus is found deep within the occipital lobe.

Question 4

Describe the trends in white and grey matter within the brain and spinal cord, and what these symbolise.

Answer 4

Grey matter is made of neuronal cell bodies and dendrites, and white matter is made of myelinated axons.
Grey matter is found on the surface of the cerebral cortex, and white matter is found deeper in the brain. The corpus callosum is a band of white matter connecting the two brain hemispheres.
In the spinal cord, the opposite trend is seen. Grey matter is found in the centre of the spinal cord in a “butterfly” shape, and white matter is found around it.

Question 5

Describe the key features of the cerebellum and brainstem.

Answer 5

The cerebellum is densely packed with neurons (more than the entire cerebrum). It is composed of three lobes - anterior, posterior, and flocculondular. It has a massive surface area, as it is made up of tightly folded leaf-life structures called folia.
The brainstem is split into the midbrain, pons, and medulla oblongata.
The midbrain is a relay centre for sensory information, the pons is important in breathing, and the medlla has control centres for the autonomic nervous system.

Question 6

Describe the ventricular system and meninges.

Answer 6

There are four cavities in the brain (ventricles) which are interconnected. Two of these are lateral ventricles (one in each hemisphere), a third ventricle sits within the midline from a sagittal view, and the fourth ventricle is located caudally, near the brainstem.
These are filled with CSF which is produced by the choroid plexus in the ventricles.
CSF acts as a shock absorber to protect the brain, and surrounds the outer surfaces of the brain and spinal cord.
The meninges are a three-layer membrane which surround the brain and spinal cord. The outermost layer is the dura mater. It is tough, and makes contact with the skull and vertebrae.
The middle layer is the arachnoid mater, and the innermost is the pia mater (sits in contact with the CNS). The arachnoid and pia maters are not in direct contact with one another, there is a gap called the subarachnoid space which is filled with CSF.

Question 7

Describe the anatomy of the spinal cord.

Answer 7

The spinal cord is divided into 30 segments, named after the bones of the vertebral column where they are located. There are eight cervical segments (C1-8), twelve thoracic segments (T1-12), 5 lumbar segments (L1-5), and 5 sacral segments (S1-5). Each segments has its own pair of spinal nerves.

Question 8

How can the white matter of the spinal cord be subdivided?

Answer 8

Can be subdivided into dorsal columns (ascending sensory axons), lateral funiculi (ascending and descending sensory axons), and ventral funinculi (descending axons).

Question 9

Describe the 5 criteria by which neurons can be classified.

Answer 9

1) Number of neurites (axons and dendrites): unipolar neurons have one neurite (an axon, and no dendrites). Bipolar have a single dendrite and a single axon. Pseudounipolar are the same, but the cell body is not in between the dendrite and axon. Multipolar neurons have many dendrites as well as their axon.
2) Dendritic tree structure: Pyramidal neurons have triangular cell bodies with very long, branched apical dendrites and basal dendrites (spiny dendrites). Stellate neurons have spherical cell bodies with short dendrites (can be spiny or aspinous)
3) Axon length: Projection neurons (Golgi type I) have very long axons. Local circuit neurons (Golgi type II) have short axons - they rely on interneurons to pass along information
4) Neurotransmitter: Many neurotransmitters can be released at the synapses of neurons (e.g. acetylcholine, glutamate, GABA, noradrenaline, dopamine, histamine, serotonin).

5) Connections: Sensory neurons relay sensory information from the body’s sensory surfaces towards the CNS.
Interneurons form connections between neurons to form neural circuits.
Motorneurons carry nerve impulses away from the CNS towards muscles.

Question 10

How are resting membrane potentials established?

Answer 10

The resting potential of the typical neuron is -70mV, meaning that it is 70mV more negative on the inside than the outside. This potential is held constant by the membrane which is impermeable to charged particles.
The resting potential will only be significant across the membrane, but will remain constant elsewhere, as the charged particles will line up by the the membrane since they “want” to equilibriate. Extracellular sodium and intracellular potassium ions produce the majority of the resting potential, but negatively charged intracellular proteins and some other metal ions contribute as well.
The membrane is significantly (25-30 times) more permeable to potassium than it is to sodium, since its ion channels are kept open at rest.

Question 11

Explain the concepts of EPSPs, graded and action potentials, and summation.

Answer 11

EPSPs are excitatory potentials from a synpase which become graded potentials. Graded potentials are small depolarisations which occur from a single ligand-gated sodium channel, when it is stimulated to open. A rapid influx of sodium ions occurs, and there is small depolarising effect on the local region surrounding the ion channel. The sodium ions move outwards from here down their electrochemical gradient in order to equilibriate, the ligand will dissociate, and the channel will close. As the ions move outwards, their effect will quickly dimished, as they become further apart and their concetrantion decreases.
Summation occurs either spatially or temporally, when many of these graded potentials occur in close proximity to one another, or when the same signal is repeated several times in a short time-span, respectively. Summation of these EPSPs may be sufficient to pass a threshold potential (usually -55mV), which will result in the opening of voltage-gated ion channels, causing rapid depolarisation (action potential). This depolarisation will drive the membrane potenntial to around +35mV, and this will be sensed by voltage-sensing domains (VSDs) on voltage-gated potassium channels, which will result in potassium ions rapidly leaving the cell to repolarise it. A temporary hyperpolarisation will bring the membrane potential to around -90mV very briefly before the resting potential is restored, and voltage-gated potassium channels close.

Question 12

How are action potentials propagated down axons (including myelinated)?

Answer 12

Action potentials arise on localised areas, while adjacent sections are still at resting potential. Subsequent regions VSDs recognise the depolarisation of the previous section, and voltage-gated sodium channels open. This results in a sequential wave of depolarisation down the axon while each previous section repolarises behind the impulse due to the sodium/potassium pump restoring equilibrium.
Myelin sheaths encase axons at lengths of roughly 1mm. nodes of Ranvier between these segments are densely populated with voltage-gated ion channels. The myelin draws positively charged ions from the intracellular environment of a depolarised node, towards the negative environment of the subsequent node. This results in the opening of voltage-gated channels at the adjacent node, propagating the action potential 50 times faster than in continuous conduction. This is known as saltatory conduction, because the impulse “jumps” from node to node, skipping over the myelinated sections of axon.

Question 13

Explain the process of synaptic transmission.

Answer 13

There are two kinds of synpase - chemical synapses and gap junctions.
Gap junctions are direct connections between two neurons, whereby ions are passed straight from one to the next for the continuation of the action potential (electrically coupled by channel pores spanning from the cytoplasm of one cell to the other).
Chemical synapses rely on diffusion of chemical neurotransmitters across a synaptic cleft from the axon terminal of one neuron into the receptors of another (dendrites or soma). Neurotransmitters stored in presynaptic vesicles are released when the action potential reaches the axon terminal - this occurs via voltage-gated calcium channels, which associate with ions to form a complex which drives the diffusion of the vesicles within the presynaptic membrane such that they exocytose into the synaptic cleft. Once they cross the cleft, neurotransmitters bind to receptors on the post-synaptic neuron, inducing EPSPs or IPSPs. The sum of all excitatory and inhibitory signals will determine if the post-synaptic neuron fires its own action potential. The neurotransmitter then dissociates from the receptor and is released into the synaptic cleft. This is then cleared via a specific mechanism. These mechanisms often involve a secondary protein, found adjacent to the postsynaptic receptors. These break down toxic neurotransmitters and the components are taken back up into the presynaptic cell, reassembled, and packaged up into vesicles again (recycled).

Question 14

Describe the functions that the main cortical areas are associated with.

Answer 14

The frontal lobe can be subdivided into the prefrontal cortex, the motor cortex (premotor, primary, and supplemtary motor cortices), and Broca’s area. The prefrontal cortex is responsible for executive functions such as problem solving, complex planning, and personality. The motor cortex is reponsible for planning, control, and execution of voluntary movements (premotor prepares and guides, supplementary plans movement sequences, primary executes these voluntary movements - precentral gyrus, M1). Broca’s area is involved in the production of speech (usually only found in the left hemisphere).

The parietal lobe can be divided into the postcentral gyrus and the posterior parietal cortex. The postentral gyrus is the primary somatosensory cortex, and it processes tactile sensation (exhibits cortical amplification - sensory homunculus). The posterior parietal cortex (AKA somatic sensory associated area) integrates sensory information, and has a role in spatial perception and attention.

The occipital lobe can be divided into the visual cortex and the visual association area. The primary visual cortex is located around the calcarine sulcus, and it receives visual information from the thalamus (organised into 6 layers).

The temporal lobe can be divided into the auditory cortex, the auditory association area, and Wernicke’s area. The auditory cottex is located just underneath the primary somatosensory cortex, and is involved in processing sound.
Wernicke’s area is important in language comprehension (usually only found in the elft hemisphere).

Question 15

Describe the sensory and motor cranial and spinal nerves.

Answer 15

There are 12 pairs of cranial nerves (emerge from the brain), and 31 pairs of spinal nerve (emerge from the spinal cord).
Cranial nerves I, II, and VIII are special sense nerves: I is the olfactory nerve, II is the optic nerve, VIII is the vestibulocochlear nerve.
Cranial nerves III, IV, VI, XI, and XII are motor cranial nerves: III, IV and VI all regard the movement of the eyes, XI is movement of neck muscles, and XII innervates the muscles of the tongue.
Cranial nerve V has a sensory component which innervates the face and a motor component which innervates the muscle of mastication.
Cranial nerve VII innervates the muscles of the face, and senses the anterior of the tongue and soft palate glands.
Cranial nerve IX is the glossopharyngeal nerve, its sensory component is the tongue, tonsils, and pharynx, and its motor component controls the muscles involved in swallowing.
Cranial nerve X is the vagus nerve - its motor component works the heart, lungs, and GI GI tract (and more), and its sensory component does the same.

There is one pair of spinal nerves for each spinal cord segment (cervical x8, thoracic x12, lumbar x5, sacral x5) as well as a pair of coccygeal nerves. Each nerve pair is a mixed fibre - sensory axons end in sensory receptors, and motorneuron axons end in effectors. Each spinal nerve applies to sensation of a single area of skin called a dermatome (with the only exception being C1 - there are 30 dermatomes).

Question 16

Why are reflex arcs important? What are the monosynaptic and polysynaptic arcs? Give examples of each.

Answer 16

Redlex arcs are the simplest circuits in the nervous system - they start with sensory receptors and end in effectors. These allow us to quickly respond to harmful stimuli such as sharp objects, with no higher processing required. This is important in protecting the body from harm quickly, as higher processing requires more time.
Monosynaptic somatic reflexes involve a sensory input which synapses with a motor output. Polysynaptic reflex arcs involve interneurons between the sensory input and motor output.
Monosynaptic = knee jerk
Polysynaptic = withdrawal reflex

Question 17

Describe the path of the nerve impulse in the knee jerk reflex. What type of reflex is this?

Answer 17

When the patella tendon is tapped, it stretches, and a muscle spindle senses the stretch. This results in an afferent action potential which arrives in the spinal cord via the dorsal horn, where it splits and synapses onto two different motorneurons (one excitatory, and one inhibitory). The efferent impulses then leave the spinal cord via the ventral horn, the excitatory pathway causes the contraction of the quadricep muscles (extensors), and the inhibitory pathway causes relaxation of the antagonistic muscle, the hamstring. This is an example of reciprocal innervation. It is a monosynaptic reflex because there are no interneurons (only one synapse); it is a stretch reflex because the sensory receptor is a stretch receptor; it is an extension reflex because the extender muscle is contracting; and it is a control (closed-loop) reflex, because the motor output is directly acting upon the stimulus (muscle stretch, but contracting it).

Question 18

Give an example of a crossed-extensor reflex. Describe the pathway the nerve impulse takes (including receptors and effectors).

Answer 18

Crossed-extensor reflexes are an extension of flexion withdrawal. When a harmful stimulus is stepped on, and the leg withdrawn, the opposite leg must extend to hold our entire weight - this is an example of a crossed-extensor reflex.
The pain receptor on the foot travels afferently to the spinal cord, and enters via the dorsal horn. It then branches into 5 different parts - one branch synapses with an excitatory interneuron, which synapses with an ipsilateral hamstring motorneuron, causing contraction of the hamstring (flexion); the second synapses with an ipsilateral inhibitory interneuron which synapses with a quadricep motorneuron, causing the quadricep to relax (reciprocal innervation); the third synapses with an excitatory interneuron which decussates and synapses with a contralateral quadricep motorneuron, extending the opposite leg’s quadricep; the fourth synapses with an inhibitory interneuron which decussates and synapses with a contralateral hamstring interneuron, relaxing the hamstring of the opposite leg (reciprocal innervation). The last branch ascends up the spinal cord for sensation and postural adjustment.

Question 19

Explain the divisions of the nervous system, and state their key similarities and differences.

Answer 19

The nervous system can be divided into the somatic and autonomic divisions. The somatic nervous system involves the conscious and unconscious control of movement (skeletal muscle). It usually involves a single myelinated neuron which travels from the CNS to a skeletal muscle, and uses acetylcholine as a neurotransmitter. Receptors in skeletal muscle are nicotinic.
By contrast the autonomic nervous system unconsciously regulates internal body conditions to maintain homeostasis and important processes. It innervates smooth muscle, cardiac muscle, and glands, and uses two neurons in its effector pathway, separated by autonomic ganglion (a collection of neuronal cell bodies - called a ganglion in the PNS and a nucleus in the CNS). There is a preganglionic neuron which synapses with a postganglionic neuron in the ganglion. preganglionic neurons tend to be myelinated, but postganglionic neurons don’t (they are typically situated close to the target tissue). preganglionic neurons typically use acetylcholine as a neurotransmitter, postganglionic typically use acetylcholine or noradrenalin. Receptors in ganglia are nicotinic, and in target tissues are muscarinic or adrenergic.
The autonomic nervous system can be further subdivided into the sympathetic, parasympathetic, and enteric systems. The enteric NS is the separate innervation of the GI tract. The sympathetic and parasympathetic divisions work antagonistically, like an accelerator and a brake pedal.

Question 20

Describe the modes of action of the sympathetic and parasympathetic nervous systems. State their similarities and differences.

Answer 20

The sympathetic nervous system facilitates the bodies response to stress/danger. It does this by altering the physiology in a number of ways - it boosts alertness, dilates blood vessels leading to the heart and muscles, and constricts those to the skin and abdomen, inhibits digestion by reduces peristalsis, dilates bronchial smooth muscle to increase airflow to the lungs, retracts urethral sphincter, mediates kidney secretions of renin (mediates fluid volume in blood), increases glucagon output from the liver, increases adrenaline secretions from the adrenal medulla, increase sweat secretions and erector pili in the skin, and causes pupil dilation to accommodate far vision.
The parasympathetic nervous system slows or reverses the action of the sympathetic. It mostly does this by having the opposite effects on the same tissues as the sympathetic (e.g. inhibitory where sympathetic is excitatory). However, in the parasympathetic NS, there is no innervation of blood vessels, kidneys, liver, adrenal medulla, or skin - these are mediated by the action or lack thereof from sympathetic.
Both systems are always active, a balance is found to maintain homeostasis.

In the sympathetic NS, the preganglionic neuron is shorter, and the postganglionic is longer than in the parasympathetic NS. The sympathetic typically uses noradrenalin as a neurotransmitter, acting upon alpha/beta-adrenoreceptors, whereas the parasympathetic using acetylcholine acting on muscarinic receptors. However, both use acetylcholine on nicotinic receptors at their respective ganglia.

The parasympathetic preganglionic neurons originate from cranial nerves III, VII, IX, and X, as well as spinal cord levels S2-4 (called craniosacral outflow). Those from III, VII, and IX synapse to ganglia in the head, but X and the sacral nerves synapse to ganglia close to target organs (this is why they have longer preganglionic neurons and also why parasympathetic pathways are faster).

The sympathetic preganglionic neurons originate from spinal cord levels T1-L2 (specifically from the lateral horn - this is called thoracolumbar outflow). Sympathetic ganglia form chains alongside the spinal cord on either side, as well as pre-vertebral ganglia (midline structures located anterior to the aora - including celiac, superior mesenteric, and inferior mesenteric ganglia). This arrangement is to distribute neurons across the body. The adrenal medulla is also a modified sympathetic ganglion, as well as a target organ.

Question 21

What are stimulus-driven and goal-directed pathways? Why are they important?

Answer 21

Stimulus-driven pathways are ascending, afferent pathways which originate from a sensory stimulus. Goal-directed pathways are descending pathways which feed back to the sensory receptors. This is important in order to mediate the amount of information let in by the sensory receptor, as well as the immediate attention given to it (to prevent the brain from being overloaded).

Question 22

How can sensory receptors be categorised?

Answer 22

Can be categorised based on three criteria - modality, distribution, and origin of stimuli.
Modality is the specific type of stimulus that a receptor responds to (chemoreceptors, thermoreceptors, etc).
Distribution is the division of sensory receptors between somatosensory and special (those in the head).
Origin of stimuli divides receptors into three categories - interoceptors (internal stimuli), proprioceptors (body position and movement), and exteroceptors (external stimuli).

Question 23

Describe the key differences between somatosensory and special receptors. What implications do these differences have on their specificity?

Answer 23

Somatosensory receptors are free nerve endings, meaning they are high distributed. Contrary to this, special sensory receptors synapse directly or indirectly (via interneurons) onto ganglion cells which transmit their action potentials. This allows for a much greater degree of specificity in the special systems.
All afferent somatosensory signals fully decussate, meaning they cross to the contralateral side of the brain for processing, whereas special senses only partially decussate (half the fibres follow the ispilateral and half cross contralaterally), with the exception of the olfactory and gustative systems, which do not decussate at all.

Question 24

Describe the process of olfaction.

Answer 24

At the top of the nasal cavity, there is a Cribriform plate (part of the ethmoid bone). On the side of the nasal cavity, olfactory sensory neurons (bipolar) sit in the epithelium, with dendrites which project into the mucus layer, and axons which ascend through the Cribriform plate. Odorants bind to G-coupled protein receptors on olfactory cilia in the mucus layer, and depolarisation of olfactory receptors occurs (graded potentials). If threshold potential is reached, APs will fire down the ascending axons, which cross the Cribriform plate, converge to join the olfactory bulbs and synapse to dendrites of specific mitral cells at glomeruli. The mitral cell axons then converge to form the olfactory tract (CrN I) which afferently carries the signals to the primary olfactory cortex.
The primary olfactory cortex is composed of the enterohinal cortex, the piriform cortex, and the amygdala (all part of the limbic system).

Question 25

Describe the process of gustation.

Answer 25

Gustative papillae contain taste buds, which each contain 50-150 taste receptor cells. Microvilli at the surface of each taste receptor (projecting through the epithelium) bind to specific tastants. Afferent action potentials will be transmitted down the gustatory axons, and take one of three different ascending pathways from the tongue to the brain - the facial nerve (CrN VII), the glossopharyngeal nerve (CrN IX), and the vagus enrve (CrN X). These nerves all reach the primary gustatory area of the cerebral cortex (located in the insula, between the frontal and parietal lobes) via the solitary nucleus of the medulla, and then the ventral posterior medial nucleus of the thalamus.
Disproportional cortical amplification occurs, meaning that sweet and salty stimuli are represented more, and sour represented the least.

Question 26

Describe the process of retinal processing.

Answer 26

Two types of photoreceptors, rods and cones (further categorised as S, M, L-cones based on wavelength of light they respond to) transduce light to electrical impulses. Rods use rhodopsin as a photopigment, whereas cones use opsins. Rods respond to low light, and only detect grey shades, whereas cones respond to intense light and are responsible for colour vision.
Photoreceptors are hyperpolarised by light, and release glutamate as a response. Bipolar interneurons then depolarise and fire graded potentials, and they synapse to ganlgion cells which fire action potentials, whose axons converge to form the optic nerves.
Partial decussation occurs at the optic chiasm, then fibres project to either of the two lateral geniculate nuclei of the thalamus, and then to the primary visual cortex (V1). There are other pathways as well as this, such as those which project into reflex pathways, the suprachiasmatic nucleus, and the superior colliculus.

Question 27

Describe the processes of hearing and equilibrium.

Answer 27

Sound enters the external ear through the acoustic canal and vibrates the eardrum. The eardrum transduces the mechanical vibrations to the auditory ossicles, which then transduce and amplify these vibrations to the fluid-filled vestibule via the oval window. Stereocilia on mechanoreceptors of the cochlea are embedded in the tectorial membrane of the organ of Corti, and move under the influence of specific frequencies which vibrate the endolymph fluid within the vestibular and cochlear environment (further into the cochlea = lower frequencies). These movements are transduced to APs which travel down the cochlear nerve, which merges with the vestibular nerve to form the vestibulocochlear nerve (CrN VIII).
The vestibulocochlear nerve projects into the cochlear nucleus of the brainstem, then partially decussates, and the two bilateral pathways project into the medial geniculate nucleus of the thalamus, then the primary auditory cortex (temporal lobe, near Wernicke’s area, where language is processed).

The vestibular (balance/equilibrium) system works very similarly to this, but the hair cells are located in the vestibular and the semicircular canals of the inner ear (also filled with endolymph). Head position and unilateral movement is sensed by the atricule and the saccule of the vestibule, more complex movements/acceleration is sensed in the semi-circular canals - these signals are sent to the brainstem and cerebellum, and then the primary vestibular cortex (many small regions which make up the vestibular system).