Introduction to the nervous system

Question 1

What is the function of the nervous system

Answer 1

The nervous system is designed to detect features of the internal and external environments, to process information, and to use it to direct behaviour and body processes. There are three basic mechanisms that work together to achieve this: perception, information transfer and processing, and output to the body.

Question 2

Describe the two different anatomical parts of the nervous system

Answer 2

CNS- consisting of the brain and spinal cord

PNS- this includes all the nerves arriving from and going to the brain and spinal cord.

Question 3

What can the peripheral nervous system be further divided into

Answer 3

Somatic division- consists of sensory and motor supply to the skin, muscles and joints.
Autonomic division- supplies smooth muscle and glands together with some specialised structures such as pacemaker cells of the heart. One of its main functions is to control the internal environment.
Has sympathetic and parasympathetic arms.

Question 4

Describe the differences between the sympathetic and parasympathetic arms of the autonomic motor division

Answer 4

Sympathetic- spinal nerves only

Parasympathetic- spinal nerves and cranial nerves.

Question 5

Describe the difference between afferent and efferent axons

Answer 5

▪ Afferent Axons – Towards the CNS from the PNS. o E.G. Sensory neurones, both somatic and autonomic. ▪ Efferent Axons – Away from the CNS to the PNS. o E.G. Motor neurones, both somatic and autonomi

Question 6

What are interneurons

Answer 6

Interneurons – CNS neurones that synapse with other CNS neurones within the brain and spinal cord.

Question 7

Describe, simply, the afferent pathway

Answer 7

Sensory, from periphery into brain.

Question 8

Describe, simply, the efferent pathway

Answer 8

Motor, from brain to periphery.

Question 9

Describe the coverings of the CNS

Answer 9

The brain and spinal cord are supported and protected by the bones of the skull and vertebral column respectively. Additionally, three membranous layers (meninges) envelop the brain and spinal cord within their bony surroundings; the dura mater is the outermost layer, the middle layer is the arachnoid mater, and the innermost layer is the pia mater. The meninges play a key role in protection.

Question 10

Describe the gross anatomy of the brain

Answer 10

Frontal lobe runs until central sulcus.
Parietal lobe sits behind frontal lobe.
Occipital lobe at back of the head.
Temporal lobe- sits on each side of the brain.
Base of brain-cerebellum 
Behind cerebellum is the brainstem. 
Brainstem goes into spinal cord.

Question 11

What does every hair in the body have

Answer 11

Its own sensory fibre

Question 12

What is the purpose of folds in the brain

Answer 12

Brain is a finite size

Cortex is folded to increase surface area for a greater density of neurones.

Question 13

What is the corpus collosum

Answer 13

Fibre tract that allows for communication between the two hemispheres.

Question 14

As an adult, what can a tumour in the pituitary gland result in

Answer 14

Acromegaly- large hands and feet.

Question 15

How many cranial nerves are there

Answer 15

12- mainly control face and neck

Question 16

Describe the meninges

Answer 16

Three concentric membranous layers envelope the brain and spinal cord. The outermost layer, the dura mater is a tough, fibrous, loose-fitting membrane. The arachnoid mater is a translucent membrane which loosely surrounds the brain. The subdural space is a virtual space that separates the dura and arachnoid mater, and is transversed by veins en route to venous sinuses. The pia mater is microscopically thin and closely adherent to the surface of the brain. Between the pia and arachnoid mater lies the sub-arachnoid space containing cerebrospinal fluid and large blood vessels. Between the pia and arachnoid mater lies the subarachnoid space containing CSF and larger blood vessels.

Question 17

What are the three parts of the CNS

Answer 17

1. Cerebral Cortex (Cerebrum) 2. Cerebellum – Coordination and movement. 3. Brain Stem – Regulates vital functions.

Question 18

Describe the sensory and efferent pathways involved in regulating blood pressure

Answer 18

Sensory endings in aorta (baroreceptors) detect changes in BP- send afferent signal to the hypothalamus. Efferent signal to change vascular tone and increase/decrease BP accordingly.

Question 19

What is meant by a subdural haematoma

Answer 19

Haemorrhage in between the dura mater and arachnoid- can increase intracranial pressure.

Question 20

What is meant by grey mater

Answer 20

Where cell bodies of neurones are found- processing occurs

Question 21

What is meant by white mater

Answer 21

Where the axons of those cell bodies are found- sending out of information occurs.

Question 22

What is meant by a ganglion

Answer 22

It is where a bunch of cell bodies are found.

Question 23

Describe the basic structure of the spinal cord

Answer 23

The spinal cord and its associated spinal nerves provide sensory, motor and autonomic innervation for the limbs and trunk. it occupies the vertebral canal within the vertebral column and is continuous rostrally with the medulla oblongata. The cord is approximately cylindrical and arranged segmentally with a central cellular area (grey mater) surrounded by nerve fibre tracts (white mater). The tracts carry information between different levels of the spinal cord (thereby permitting reflex actions),and also to and from the intraspinal structures.

Question 24

How many laterally paired spinal nerves are there

Answer 24

31

Question 25

How do the spinal nerves attach to the spinal cord

Answer 25

Via dorsal and ventral roots which contain primary afferent and efferent neurones respectively.

Question 26

What is the margin between the CNS and PNS in the spinal cord

Answer 26

The CNS ends at the margins of the spinal cord.

The dorsal and ventral roots that emerge from the spinal cord are part of the PNS.

Question 27

Describe the different roles of the dorsal horn, ventral horn and lateral horn layers in the grey mater

Answer 27

Dorsal horn- involved in sensory pathways and are the target sites for some sensory afferent nerves, particularly for pain, temperature and crude touch.
The ventral columns are made up of pools of motor neurones innervating the skeletal muscle. Medial motor columns supply proximal muscles. Lateral motor columns supply distal muscles.
Interomediolateral- cell bodies of preganglionic sympathetic neurones are found

Question 28

How can the ventral and dorsal root be distinguished

Answer 28

The dorsal root has a ganglion.

Question 29

What distinguishes a neurone form all the other cells in the nervous system

Answer 29

The axon.

Question 30

What determines the length of the axon

Answer 30

how far the axons have to travel to reach their target cell

Question 31

What is meant by a fascicle

Answer 31

A package of axons- both afferent and efferent

Question 32

Describe the packaging of axons into nerves

Answer 32

Spinal nerves contain both afferent and efferent axons
They are bundled into fascicles surrounded by perineurium.
The whole nerve is in a tough epineurium capsule.
Individual axons are also wrapped with myelin and endoneurium, though some are unmyelinated, e.g. nociceptive (pain) neurons

Question 33

Describe the difference between axons that transmit dull pain and the axons that transmit sharp pain

Answer 33

Dull pain- axons not likely to be myelinated.

Sharp pain- axons likely to be myelinated.

Question 34

What is the diameter of the synaptic cleft

Answer 34

10nm

Question 35

What is the dark smudge on the electron micrograph of a synapse

Answer 35

The post-synaptic receptors.

Question 36

How do we see neural cells today

Answer 36

Fluorescently stain each neurone in transgenic mice- see their function.

Question 37

Describe the different types of cells found in the brain

Answer 37

Glia account for 50% of all cells in the brain
Originally thought to be “packing” material, now known to have vital roles
Astrocytes form the blood-brain-barrier and recycle neurotransmitters
Microglia act as immune cells
Oligodendrocytes (Schwann cells in PNS) produce myelin sheath
Astrocytes are the most abundant neuroglia.

Question 38

What is the purpose of the blood-brain barrier

Answer 38

The blood-brain barrier exists to maintain the environment of the brain in a steady state, protected from extracellular ion changes, peripheral hormones such as adrenaline, infection and drugs. It also prevents neurotransmitters from the CNS entering the peripheral circulation.

Question 39

Explain the role of the foot processes of astrocytes in maintaining the blood brain barrier

Answer 39

Astrocytes have foot processes which adhere to capillary endothelial cells such that they are entirely enclosed. Astrocytic foot processes also secrete factors that help to maintain the tight junctions between endothelial cells.

Question 40

Describe axon regeneration in the peripheral nervous system

Answer 40

Injury leads to stimulation of phagocytes, which remove debris which would otherwise inhibit regrowth
May be compromised by aberrant axon sprouting and non-specific target reinnervation – can lead to neuropathic pain

Question 41

Describe axon regeneration in the CNS

Answer 41

Glia exert inhibitory influence on regeneration
Glial scars form in place of injured brain tissue
Absence of guidance cues that stimulate axon growth
Inhibitory molecules in the CNS (not found in the PNS – e.g. differences in myelin). Some loss of intrinsic axon growth capabilities by the neurones.

Question 42

What are the two pathways for sensory perception

Answer 42

Dorsal column-medial lemniscus pathway
Conveys fine touch, vibration and two-point discrimination
Spinothalamic pathway
Conveys pain and temperature sensation

Question 43

What is the key difference between the two sensory pathways

Answer 43

In the dorsal tract, the information is conveyed up the cord the same side that it enters, whereas in the spinothalamic tract, immediately cross and travel upwards on the opposite side of the cord.
Dorsal tract switches to the contralateral side in the medulla.

Question 44

What is the point at which the spinothalamic tract crosses to the contralateral side known as

Answer 44

Decussation.

Question 45

Describe the sequence of neurones between the peripheral receptor and the cerebral cortex

Answer 45

First-order neurons (primary afferent neurons) enter the spinal cord via the dorsal roots of spinal nerves. They make synaptic contact with second order neurons either in the spinal cord grey mater or medulla. The cell bodies of the second-order neurons lie in the cord or medulla. Their axons decussate and ascend to the thalamus where they synapse with third-order afferent neurons.
Third-order neuron cell bodies lie in the thalamus. The axons project to the somatosensory cortex in the parietal lobe.

Question 46

What are the two different motor pathways

Answer 46

Lateral corticospinal tract

Vestibulospinal tract.

Question 47

Describe the lateral corticospinal tract

Answer 47

Axons originate from cell bodies in the precentral gyrus (motor cortex), travel down to medulla, decussate, travel down to spinal cord on ipsilateral side of target.

Question 48

Describe the vestibulospinal tract

Answer 48

Receives input from vestibular system in the ear- allows you to know which orientation you are in. Loss leads to difficulty to maintain balance and posture (creates micro twitches that allow you to keep your balance). The whole tract is ipsilateral. Important as you want sensation in one ear to control movement in same side, otherwise you would make the change worse and fall over.

Question 49

Describe reflexes

Answer 49

o Only the somatic sensory inputs to, and motor outputs from the SPINAL CORD are required (not communication with the sensorimotor cortex, i.e. the brain). o Reflexes require that both the sensory and motor neurones are intact between the PNS and the SPINAL CORD (not the brain necessarily, this is important clinically).

Question 50

Describe conscious registering

Answer 50

Conscious Registering – Pre-mediated/Deliberate movements. o Ascending tracts – Requires a sensory stimulus which activates sensory neurones in the grey matter of the spinal cord that then transmits an action potential up the white matter in an axon towards the sensory cortex of the brain (afferent axon). o Descending tracts – Neurones in the motor cortex of the BRAIN extend axons downwards to synapse with the spinal motor neurones and transmit action potentials for voluntary movement.

Question 51

How do sensory neurones stimulate reflex activity

Answer 51

They stimulate reflex activity via spinal cord motor neurones

Question 52

What does reflex activity indicate

Answer 52

No peripheral nerve damage.

Question 53

What does loss of sensation and voluntary movement in one limb indicate

Answer 53

Injury must be to region(s) in RIGHT hemisphere containing both motor and sensory neurons/axons – but only from arm.
Injury likely to be close to the sensorimotor cortex (in reality sensory and motor cortices, anatomically close but distinct) for the arm.
Hemisphere opposite to the side affected.

Question 54

if both the limbs on one side of the body lose sensation and voluntary movement, what does it indicate

Answer 54

Injury likely to be in right brain sensorimotor cortex as only left limbs affected, but spread across regions for both arm and leg.
(in reality sensory and motor cortices, anatomically close but distinct)

Causes? Middle aged - Brain tumour? Stroke?