Block 6: the senses Flashcards
Describe the role and anatomy of the olfactory nerve.
The olfactory nerve is a special sensory nerve which conveys sense of smell. It is formed of the bilateral olfactory bulbs and tracts. The olfactory bulbs rests on the ethmoid bone which has small openings (known as cribriform plate foramina). Nerves which form the respiratory mucosa project through these foramina from the upper nasal cavity to the olfactory bulb. The dura keeps these nerves in place. There are two pathways by which information can be carried from the olfactory bulb- the medial and lateral olfactory tracts. The medial olfactory tract projects to the primitive brain regions, such as limbic structures (responsible for emotional aspects of olfaction). The lateral olfactory tract projects to the piriform cortex, amygdala, and entorhinal cortex. These are collectively known as olfactory cortices. These pathways can, in turn, project to the prefrontal cortex and hippocampus.
Describe the functional anatomy and the nervous supply to the parotid, submandibular, and sublingual glands.
There are 3 major bilateral salivary glands- the parotid, submandibular, and sublingual glands. The parotid is the largest, and sits lateral to the mandible (just in front and slightly below the ears- ‘prearicular area’); the submandibular sits just below the mandible; and the sublingual gland sits just below the tongue.
The parotid gland contains mainly serous cells (secrete protein in watery fluid which contribute to saliva). It receives parasympathetic secretomotor innervation. Pregangalionic parasympathetic fibres of the glossopharyngeal nerve constitute the lesser petrosal nerve which projects to the otic ganglion. From here, postganglionic fibres project to the parotid gland via ‘hitchhiking’ with the auriculotemporal nerve, a branch of mandibular division of the trigeminal (CNV3).
The submandibular gland contains a mixture of serous and mucous cells. The sublingual gland contains mostly mucous cells. For both, preganglionic parasympathetic fibres originate from the chorda tympani of the facial nerve, but ‘hitchhike’ with the lingual nerve, a different branch of CNV3, to the submandibular ganglion. From here, postganglionic parasympathetic fibres can either project directly to the sublingual and submandibular glands, or can jump back into the lingual nerve and project as part of it.
Describe the central pathways for taste.
Once taste information is received from the tongue (both anterior and posterior), it travels to the gustatory nuclei of the medulla, and subsequently to the ventral posteromedial (VPM) nucleus of the thalamus. From here, pathways project to the primary gustatory cortex, encompassing the anterior insula and the frontal operculum.
Describe what is known about the higher processing of pain.
It was originally believed that the cortex had little or no role in pain perception, however, modern imaging studies have revealed that the cortex is activated following noxious stimuli. The areas activated include the primary and secondary somatosensory cortices in the parietal lobe, as well as the anterior cingulate cortex and insular cortex. The somatosensory cortices are thought to interpret the discriminative aspects of noxious stimuli (e.g. location, duration, nature of stimulus, intensity). The anterior cingulate and insular cortices are thought to interpret the affective and motivational aspects (unpleasantness from cingulate and emotional response (distress) from insular). Evidence for these comes mostly from lesion studies.
Briefly explain how pain can be mediated by descending pathways.
Electrical stimulation of the brainstem (especially near the periaqueductal grey matter) has been found to cause very effective analgesia in rats. This is known as stimulation-produced analgesia (SPA), and has been demonstrated in humans. This phenomenon is poorly understood, but it is thought to involve neurons in the raphe nuclei of the medulla which release 5-HT in the spinal cord. Opiate drugs administered to the periaqueductal grey matter, raphe, or spinal cord, also facilitate pain relief.
Explain some of the key issues with the traditional pain pathway.
Severe injuries don’t always cause pain- this was documented in WW2 where ~30% of soldier with severe injuries did not report any pain. This was proposed to be due to their high stress levels, however, it was later noted that 28% of civilians who sustained severe injuries from unanticipated incidents also did not report any pain. The reason for this remains unknown.
Another problem with our current understanding is that pain can occur without any obvious injury or disease. Pain is heavily affected by psychological factors- placebos are very effective for treating pain. This strong placebo response has been recognised to thwart development of painkilling drugs (since it is hard to prove the drug is an effective painkiller if the placebo cohort also reports loss of pain).
Acupuncture can reduce pain- nobody knows why.
Severing the “pain pathway” by anterolateral cordotomy (cauterising the spinothalamic tract) does not always reduce pain permanently- it is initially effective but can return, the mechanism for this is not understood.
Explain the theory and applications of gate theory.
It has been widely observed that rubbing the skin near an injury can suppress pain. It is proposed that this is due to inhibitory interneurons branching in the spinal dorsal horn, projecting from low threshold mechanoreceptors onto 2nd order spinothalamic neurons. This is known as the gate theory. This has been used to develop transcutaneous electrical nerve stimulation (TENS), a treatment for pain which uses stimulation of low threshold mechanoreceptive afferents to reduce pain. These larger diameter fibres are more easily excitable than the finer nociceptor fibres, so will become activated at weaker electrical stimuli. This treatment has been found to be effective in some cases but not always. Epidural spinal cord stimulation can also be used to suppress chronic pain, and this is thought to work also through inhibitory low threshold interneurons.
Describe the sensory innervation of skeletal muscle.
About 2/3 of all muscle fibres are sensory (twice as many as motor). Muscle activation and coordinated muscle control depends centrally on the sensory innervation, and good control of movement absolutely requires good sensory information. Your capacity to conduct fine motor tasks is sharply reduced when sensory input is lost (for example, when your hands are cold it is difficult to do your buttons; anaesthesia from the dentist slurs speech). There are three main sensory endings which produce large diameter, myelinated afferent fibres- muscle spindles (Ia and II) and Golgi tendon organs (Ib):
Muscle spindles are large, complex mechanoreceptors. They are fast conducting modified muscle fibres. They give rise to two types of sensory ending- primary (Ia), which are sensitive and respond to stretch and acceleration (rate of change of length) and secondary (II) which respond to absolute length of muscle. The sensitivity of both these endings can be adjusted by an independent motor system- gamma motoneurons, which sit in parallel with alpha motoneurons (the ones which innervate extrafusal fibres to deliver force), regulate the information flow of these afferents to the CNS.
Golgi tendon organs are large mechanoreceptors inserted in connective tissue fascicles which make up the tendon. They give rise to large diameter sensory fibres (Ib), and are very sensitive to active force development (respond to force exerted rather than stretch).
Describe the sensory innervation of joints.
Joint receptors are found within connective tissues which comprise joint capsules and ligaments, and are also associated with fat pads. These are similar in structure to Golgi, Ruffini, and Pacinian corpuscles (skin primary afferents). The are free nerve endings which respond to forces in connective tissue, and there are rapidly and slowly adapting types. They have group II, III, and IV afferents.
What are the differences between group I, II, III, and IV sensory afferent fibres?
Group I sensory afferents detect proprioception from skeletal muscle. They are large diameter, melinated axons.
Group II sensory afferents are almost all LTMs in skin. They are still large (but slightly smaller) diameter, myelinated axons.
Group III sensory afferents detect pain and temperature, and are fine, myelinated axons.
Group IV detect pain, temperature, and itch, and are very fine, unmyelinated axons.
What is proprioception and how can it be conveyed?
Proprioception refers to the signals that help the nervous system control movement and posture, both at conscious and subconscious levels. This allows us to know the position of our body parts with respect to each other and with respect to the 3D space around us. This can be divided into stataesthesis and kinaesthesis- awareness of the relative positions of our body parts in space, and awareness of movement of our joints (speed and direction), respectively.
There are two possible ways by which movement and position could be reported to the brain- corollary discharges, and sensory feedback. Corollary discharges are copies of motor commands within the brain. Essentially, when the brain initiates a descending movement command to skeletal muscle, it may branch into a corollary discharge which feeds this information back to another brain area to inform it of the movement. Sensory feedback could come from signals from proprioceptors within the muscle, skin, joints, etc. The corollary discharge method is most important in assessing, for instance, the heaviness of an object. Here, specific motor commands are important for judging information. Sensory feedback is most important when it comes to evaluating the position of the body in space.
Explain how flexion/extension of a joint is detected by muscle spindles.
Flexion/extension of a joint is detected by muscle spindles, where receptors within opposite muscles detect stretch. They therefore provide complementary information about flexion and extension of joints by relaying opposing information about the contraction (length) of the muscle fibres themselves.
To use the elbow as an example, this information can come from the biceps and the triceps. When the elbow is flexed, the bicep muscle is contracted, and is therefore shorter, reducing the firing rate of muscle spindles in the bicep; whereas the tricep is relaxed, increasing the firing rate of its muscle spindles.
Describe the sensory and motor nerve supply of the larynx.
Sensation just above the vocal folds is supplied by the internal branch of the superior laryngeal nerve; sensation below the vocal folds is supplied by the recurrent laryngeal nerve (branch of vagus, CNX)
All intrinsic muscles of the larynx are supplied by the recurrent laryngeal nerve. There is one exception to this- the cricothyroid is supplied by the external branch of the superior laryngeal nerve (also branch of CNX). The vagus nerve descends alongside the trachea bilaterally. The right recurrent laryngeal nerve branches off the right vagus as it passes the right subclavian artery, and then wraps around the artery and ascends back up to innervate the muscles of the larynx. The left recurrent laryngeal nerve branches into the recurrent which loops around the aorta before ascending (not symmetrical).
Describe the mechanism of anaelgesic action of opioid drugs.
Opioid receptors activate Gi/Go proteins to inhibit adenylate cyclase, reducing intracellular cAMP (agonists at opioids receptors- inhibitory GPCRs). They also activate potassium channels, inhibit calcium channels, and activate the MAPK (mitogen-activated protein kinase) cascade. They therefore have the overall effect of reducing neuronal excitability, and therefore decrease neuronal activity. However, they can also increase neuronal activity by disinhibition of inhibitory interneurons (inhibition of inhibitory cell will increase excitation of subsequent cell).
There are three main opioid receptors classes, as well as an opioid-like receptor class. Mu receptors are expressed for endogenous endorphins, and are the main site of opioid analgesics (utilised by most synthetic opioids). Delta receptors are for enkephalins, and Kappa receptors are for dynorphins. All are expressed throughout the brain and spinal cord, as well as peripheral nerve endings. These receptors rarely operate in isolation, but form complexes with other GPCRs (don’t understand the significance of this yet).
Analgesic effect of opioids is mediated in the periphery, spinal cord, and brain- however, this is predominantly attributed to inhibition of nociceptive afferents within the spinal cord dorsal horn. They are especially effective at treating inflammatory pain. They also inhibit spinal reflexes and transmission of nociceptive impulses through the dorsal horn (both post- and pre-synaptic effects), can cause localised release of endorphins in the brain and spinal cord, and may induce release of 5-HT from the raphe nucleus (would contribute to analgesic effects). Although all opioids exhibit analgesics properties, not all elicit the same broad spectrum of pharmacological effects which is seen in morphine.
Explain the effects of opioid drugs on the CNS.
Opioids are also known to reduce the affective (perceptive) component of pain, reflecting effects on the limbic system (certain opioids such as pentazocine, do not have this effect). The analgesic effects of opioids is mostly mediated via mu receptors (but there is also some evidence of delta and kappa activation). Prolonged opioids use can cause paradoxical hyperalgesia- this is increased sensitivity to pain, but is not associated with tolerance (i.e. not due to sensitisation). It is more likely due to alterations in neural network structures within the CNS.
The feeling of euphoria may also contribute to analgesic effects of opioids, and reduces agitation and anxiety associated with illness of injury. IV administration of these drugs causes a spike in blood concentrations which is not seen from oral administration, meaning that the euphoric effect is much more pronounced. Euphoria is also mostly mediated by mu receptors. Kappa receptors elicit the opposite effect (cause dysphoria and hallucinations), but most opioid drugs have a greater affinity for mu.
Respiratory depression is a major adverse side effect of opioid drugs, including morphine- it is the leading cause of death in addicts. Opioids depress breathing via their effects on mu receptors in the pre-Botzinger complex (a respiratory rhythm generating area of the medulla). It also suppresses the hypercapnic reflex (increased ventilation due to elevated PCO2), preventing rectification. However, opioids do not suppress cardiovascular function.
Other CNS-mediated side effects of opioid use include depression of the cough reflex, nausea and vomiting, and pupillary constriction. The mechanisms underlying cough reflex depression is poorly understood, but they do not correlate with the analgesic effects (some opioids are used as cough medicines). Nausea and vomiting is mediated by the area postrema on the floor of the 4th ventricle- the BBB is particularly weak at this site, and lots of opioid receptors are expressed here, leading to increased chances of vomiting in most patients. Pupillary constriction is caused by activation of mu and kappa receptors in the oculomotor nucleus- it is useful in diagnosis as tolerance to this effect is not acquired.
Explain the effects of opioid drugs on the PNS.
Opioids increase muscle tone and reduce motility throughout the GI tract. This can lead to severe, and delayed, gastric emptying (which can also prevent the absorption of other drugs). These effects are mediated by my and delta receptors which are expressed throughout the enteric nervous system in mural enteric nerve plexuses- these inhibit peristalsis in the small intestine by affecting acetylcholine release. Atropine can be used to abolish this effect.
Other peripheral effects include localised urticaria and itching by release of histamine from mast cells- these are not mediated by opioid receptors, and are not seen with pethidine or fentanyl, yet can be relieved by opioid antagonists (can therefore be deduced that they perhaps act of receptors elsewhere with off-target effects on mast cells. Bronchoconstriction can arise due to increase in systemic histamine (can therefore be problematic in people with asthma). Opioid-induced hypotension can also occur via elevated systemic histamine, and via a central effect on my receptors in the medulla. Long-term opioid abusers have suppressed immune systems, and are therefore more susceptible to infection.
Describe the phenomenon of tolerance and dependence with respect to opioid anaelgesics.
Tolerance is a pharmacological phenomenon whereby an increasing dose is required to elicit the same physiological effect with each subsequent administration. Tolerance to opioid analgesics develops rapidly (within a few days) for analgesia, euphoria, and respiratory depression. However, tolerance to constipation and constriction of pupils is not seen. There is also cross-tolerance in some people, but not all (tolerance to morphine resulting in tolerance to heroine, for instance).
Withdrawal symptoms arise from rebound increases in cAMP production, which can lead to restlessness, shivering, runny nose, diarrhoea. With the rebound in cAMP, and thus in PKA activity, there is enhanced neurotransmitter released at many synapses. Psychological dependence also causes drug craving (addiction) which can last months to years (this rarely occurs in patients who take opioids as analgesics). This effect arises from activation of the dopaminergic reward pathway. Inability to experience pleasure as a rebound constitutes a large portion of dependence.
Name the openings of the orbit, and detail what pass through each opening.
The nasolacrimal canal connects the tear ducts to the nasal cavity (drains tears).
The optic canal (superiomedial) is where the optic nerve (CNII) and the ophthalmic artery pass through.
The superior orbital fissure (superiolateral) is where the superior ophthalmic vein pass through, as well as the LFTSNIA nerves (lacrimal branch of the ophthalmic nerve (branch of the optic nerve); frontal branch of ophthalmic nerve; trochlear nerve; superior branch of oculomotor nerve; nasociliary branch of the ophthalmic nerve; inferior branch of the oculomotor nerve; and the abducens nerve).
The inferior orbital fissure is where the inferior ophthalmic vein passes through.
Explain how the anatomy of the retina, optic nerve, optic tracts, and optic radiations can confer visual field deficits following lesions.
Lesions to different elements of the visual pathway (from optic nerve to optic tract to optic radiation) will affect vision differently. This is due to the anatomy of the projections which arise from the optic nerve.
Injury to the optic nerve will remove all vision from the ipsilateral side.
Injury to the optic chiasm (e.g. by compression from a pituitary tumour) will result in tunnel vision in both eyes (lose vision from lateral visual fields- this is the information which decussates).
Injury to the optic tract will affect lateral vision on the contralateral side and medial vision on the ipsilateral side.
During the optic radiation, lower (inferior) fibres carry information from the upper visual field, and upper (superior) fibres carry information from the lower visual field. Therefore, lesions of the lower fibres of the optic radiation will affect the upper more lateral visual field of the contralateral side, and the upper, medial visual field of the ipsilateral side.
Describe the innervation of the eyelids.
The orbicularis oculi muscle is a muscle of facial expression- it is a circular muscle with two parts- the palpebral and orbital parts. Involuntary (reflex/blinking) closure of eyelids is mostly governed by the palpebral part of the orbicularis oculi, whereas voluntary (tight closure) of eyelids is mostly governed by the orbital part. The orbicularis oculi is innervated by the facial nerve (CNVII).
Additionally, the levator palpebrae superioris muscle opens the upper eyelid voluntarily and involuntarily with smooth and skeletal muscle components. It is innervated by the superiod branch of the oculomotor nerve.
Describe the anatomy and innervation of the lacrimal apparatus.
Lacrimal apparatus allows us to lacrimate (cry). The lacrimal gland, located in the upper lateral corner of the orbit, produces tears. These tears are secreted into the conjunctival sac and continually washed across the eye by blinking of the eyelids. Tears drain through the lacrimal caruncles, small ducts found in the upper and lower eyelid (tear ducts). From here, they go into the lacrimal sac, located within the nasolacrimal duct (runs from the nasolacrimal opening to the nasal cavity- this is why we get a sniffy nose when we cry).
The lacrimal gland is under parasympathetic supply from the facial nerve. Preganglionic parasympathetic fibres from the facial nerve travel within the greater petrosal nerve (hitchhiking) and the nerve of pterygoid canal until they reach the pterygopalatine ganglion (paired ganglion). Postganglionic parasympathetic fibres hitchhike into the zygomatic branch of the trigeminal nerve (CNV2) to the lacrimal gland.
Describe the pathways of the pupillary light and accomodation reflexes.
The pupillary light reflex allows adaptation to different levels of luminance falling on the retina such that vision is maintained. Greater intensities of light cause the pupil to constrict; lower intensities cause it to dilate. This pathway begins at the optic nerve, which carries visual afferent information via the optic tract to the pretectal olivary nucleus. Here, interneurons project bilaterally (splits and some decussate) to the Edinger-Westphal nucleus, where they synapse onto preganglionic parasympathetic fibres of the oculomotor nerve. These fibres project to the ciliary ganglion where they synapse onto postganglionic ciliary fibres which innervate the iris sphincter muscle of the iris to mediate contraction.
Describe the pathways of the pupillary light and accomodation reflexes.
The pupillary light reflex allows adaptation to different levels of luminance falling on the retina such that vision is maintained. Greater intensities of light cause the pupil to constrict; lower intensities cause it to dilate. This pathway begins at the optic nerve, which carries visual afferent information via the optic tract to the pretectal olivary nucleus. Here, interneurons project bilaterally (splits and some decussate) to the Edinger-Westphal nucleus, where they synapse onto preganglionic parasympathetic fibres of the oculomotor nerve. These fibres project to the ciliary ganglion where they synapse onto postganglionic ciliary fibres which innervate the iris sphincter muscle of the iris to mediate contraction.
The accomodation reflex facilitates near and far vision, allowing one to focus on objects of different distances. This is a similar pathway to the light reflex, adjusting pupil constriction to increase depth of focus of the eye, but also involves changes in lens shape which is necessary to adjust the refractive power of the eye. This pathway also involves optic tract projections to the pretectal and subsequently Edinger-Westphal nuclei and ciliary ganglia. However, as well as adjusting iris sphincter muscle tone, projects onto the ciliary muscles can cause them to contract, lessening the tension exerted through zonular fibres and increasing the curvature of the lens due to its intrinsic elasticity (accomodating near vision).
Explain the retinotopic organisation of the entire visual pathway.
1) Retina and optic tracts- The transduction of light into action potentials is done by a chain of three retinal neurons- a photoreceptor, followed by a bipolar cell, and finally a ganglion cell. Horizontal cells bridge several photoreceptors to bipolar cells, and amacrine cells bridge several bipolar cells onto ganglion cells. Each bipolar cell has a particular receptive field of photoreceptors. Photoreceptors directly adjacent to the bipolar cell have direct synaptic connections, and constitute the center of the receptive field. Photoreceptors surrounding these central ones are referred to as the receptive field surround, and input to bipolar cells via horizontal cells (indirect). Detection of difference in luminance between the centre and surround receptive fields is further refined by the ganglion cells- they have the same concentric centre-surround field organisation as the bipolar cells, but they will also detect light falling into the surround receptive field which will cancel out the effect of off-centre ganglion cells, giving a finer tuning of firing action (detecting luminance contrast between centre and surround receptive fields).
Each retina is split into two parts- the nasal hemiretina and temporal hemiretina (nasal = medial retina, receives light from the lateral visual fields (100 degrees laterally); temporal = lateral retina, receives light from the medial visual fields (60 degrees medially)). Projections from the nasal hemiretina decussate at the optic chiasm and join with the projections from the temporal hemiretina of the contralateral eye to form the optic tract. So, the optic tract contains fibres from the ipsilateral temporal hemiretina and the contralateral nasal hemiretina. The optic tracts bilaterally travel to the lateral geniculate nuclei (LGN). The partial decussation means that all information presented in the left visual field will be interpreted by the right side of the brain, and all information presented in the right visual field will be interpreted by the left side of the brain.
2) LGN and optic radiations- The LGN is a laminated structure within the thalamus which has 6 layers (magnocellular layers 1-2; parvocellular layers 3-6; and koniocellular layers (interlaminar) 1-6). All neurons of the LGN have the exact same centre-surround organisation as the neurons of the retina. Each layer corresponds to specific distant layers in V1 where these pathways project. This gives segregated retinal-geniculate-cortical pathways which are crucial for the visual processing. Each layer of the LGN therefore represents a map of the contralateral visual hemifield, but each layer only receives a monocular input (coming either from the contralateral nasal hemiretina, or from the ipsilateral temporal hemiretina). Layers 1, 4, and 6 receive input from the contralateral eye, and layers 2, 3, and 5 from the ipsilateral eye. The most significant projection from the LGN of the thalamus is to the primary visual cortex (Brodmann’s area 17) via the optic radiations.
3) V1- Projections from the upper visual field travel through optic radiations known as Meyer’s loop (through the temporal lobe) from the LGN to V1. Projections from the lower visual field travel through optic radiations through the parietal lobe from the LGN to V1. At V1, all information in the upper visual field is represented below the calcarine fissure, and all information form the lower visual field is represented above the calcarine fissure. Additionally, vision from the macula is represented at the apex of the occipital lobe, followed by binocular vision, and finally monocular vision. The central vision (20 degrees, including 5 degrees from the fovea), is over-represented in V1 (similar to the sensorimotor homunculi)- this represents 1% of the retina, but 50% of V1 (cortical amplification).
The cortex is organised in 6 layers (from most superficial (I) to deepest (VI)). It receives information into layer IV-C, and sends information to higher hierarchal processing centres from the more superficial layers. Projection to the deeper structures (subcortical or back to LGN) comes from the deeper layers. Magnocellular layers from the LGN project to layer IV-Cα; Parvocellular layers from the LGN project to layer IV-Cβ. Koniocellular layers from the LGN project to superficial layers I-III. Feedback projections back to the LGN arise from layer VI; feedback to subcortical structures arise from layer V. Neurons of V1 can be divided into simple cells, complex cells, and hypercomplex cells. Simple cells respond to edge and orientation of light by converging inputs from LGN neurons with aligned centre-surround receptive fields. Complex cells integrate responses from many single simple cells, responding to specific orientations independent of their location in the visual field (therefore they are sensitive to orientation, but also detect motion direction). Hypercomplex cells are orientation and motion direction detecting cells, but also respond to length.
Input from the left and right eyes remain segregated in alternating eye-specific columns within cortical layer IV (the aforementioned LGN layers on page 9). These columns of layer IV are known as ocular dominance columns, where all neurons are monocular. Outside of layer IV, neurons have binocular receptive fields (integrate information from both eyes, particularly at the interfaces between the columns). In addition to this, all cortical neurons of one column (through all V1 layers) have the same preferred orientation (they have a columnar organisation, too). Every 1mm, of cortex, there is repetition of all orientations (so that you can respond to all directional orientations for each point in space). To add another level of complexity, “blobs” are columns which are sensitive to particular wavelengths (colours), but do not have orientation selectivity. Blobs run through layers 2, 3, 5 and 6. Interblobs are found between blobs and are specialised in form detection. Cortical modules are the repeated units in V1 which are repeated every 1mm- these ensure that every part of V1 has the machinery necessary to analyse each small region of visual space, with selectivity for orientation, motion, colour, and form.
Information coming from the ganglion cells of the retina to the layers of the LGN, and subsequently the layers, columns and cortical modules of V1 is segregated into parallel pathways at every step. Specific parts of V1 which are selective for colour, orientation, motion, and form, project first to V2, and subsequently to extrastriate cortical areas such as V5 and V4.
4) Extrastriate cortical areas- There are two main pathways for visual processing which project from V1- the dorsal pathway and ventral pathway. The dorsal pathway projects to V5 and then the parietal lobe, and neurons of this pathway are selective for direction and speed of movement. This is important for spatial vision and relative position of objects. The ventral pathway (ventral stream) projects to V4 and subsequently the inferior part of the temporal lobe, and has high resolution for object recognition and form (neurons selective for shape, colour, and texture). Within the ventral pathway, specific areas respond to particular objects/categories of objects (objects, faces, places). For instance, the PPA (parahippocampal place area) responds to places more than objects/faces; the FFA (fusiform face area) responds mostly to faces.
