Introduction to the nervous system Flashcards
What is a reflex?
The simplest form of motor act - rapid, automatic and stereotyped response to a given stimulus. Coordinated by involuntary
What is a reflex arc?
The neurons forming the path taken by nerve impulses responsible for a reflex make up a reflex arc.
What is a motor system?
Refers to the neural pathways that control the sequence and patterns of muscle contractions
Briefly describe the hierarchical organisation of motor control
neural circuits in the spinal cord controlling many reflexes > postural control by the brainstem > goal-directed movements involve primary motor & cerebral cortex > basal ganglia and cerebellum influence motor cortex by way of the thalamic nuclei. Higher than all is the association cortex and limbic system.
Describe the types of voluntary movement
Well learnt movements such as driving, and goal-directed movements with a degree of conscious attention e.g. hitting a cricket ball.
Describe the types of involuntary movement
postural control and reflex actions
What are the roles of reflexes
Homeostasis, protection and control of movement
What is the simplest structure for a reflex
A monosynaptic reflex such as the stretch reflex involves on afferent sensory neuron carrying information to the CNS, one synapse, and one efferent motor neuron transmitting nerve impulses from the CNS to an effector. In many cases reflexes are modulated by activity in the CNS
How do reflexes get more complicated?
More interneurons are added between the sensory and motor neurons, from disynaptic reflexes such as the withdrawal reflex to trisynaptic and polysynaptic reflexes such as the scratch reflex.
Describe an example of a stretch (myotactic) reflex
The knee-jerk or tendon tap reflex. A sharp tap to the patellar tendon stretched the quadricep, stimulating the dynamic nuclear bag receptors of the muscle spindles, thus An increased rate of firing of group Ia afferents of the quad. Afferent fibres branch as they enter the spinal cord, some enter the grey matter and make monosynaptic contact with motor neurons to the quad, causing a synchronised discharge and quad contraction. Other branches make synaptic contact with inhibitory neurons to the flexor muscles of the knee joint (reciprocal innervation)
What causes a loss of the knee jerk reflex?
damage to the lower lumbar dorsal roots of the spinal cord
What is the Golgi tendon reflex?
Complements the tonic stretch reflex in posture maintenance. Example: the afferent fibre from golgi tendon organ in the rectus femoris muscle branches as it enters the spinal cord. Both branches are disynaptic, one exciting interneurons that inhibit the rectus femoris, and the other exciting interneurons that stimulate motor neurons activating hamstring muscles
How does the golgi tendon reflex maintain posture?
During maintained posture e.g. standing, the rectus femoris will tire, and tension in the patellar tendon monitored by the golgi tendon organ will decline. As such the activity of the afferent Ib fibres from the GTO will decline, removing the normal inhibition of the rectus femoris and causing it to contract more strongly.
What is the flexion reflex
A complex reflex involving many interneurons and proprio-spinal connections. Simply, nociceptors stimulate the right sensory neurons to branch and stimulate interneurons in reciprocal inhibition, such that antagonistic muscle pairs are stimulated and inhibited to withdraw a body part away from the pain. Longer latency than the stretch reflex and response is proportional to pain.
What is the crossed extensor reflex?
Describe golgi tendon organs
Mechanoreceptors in tendons immediately beyond their attachment to the muscle. Usually 10-15 muscle fibres to one GTO. When tension is applied, impulses are carried to the CNS by group Ib fibres.
Do active recall on how muscle spindles work because it can’t be that important and I ain’t typing all that champ
Describe the general organisation of the brain
Each cerebral hemisphere has four lobes, occipital, parietal, temporal and frontal
What can be seen in a midsagittal cross section of the brain?
The corpus callosum. Below is the septum pellucidum separating the two cerebral ventricles filled with CSF. Below this is the thalamus and hypothalamus. Posterior and ventral to this the mid-brain merges into the pons (connects cerebellar hemispheres). Below and behind this is the medulla, followed by the spinal cord
Describe what can be seen in an oblique cross-section of the brain
see image on onenote
Describe the meninges
Spinal and cranial meninges are continuous. Dura mater is tough layer nearest the skull. The arachnoid mater and soft pia mater lie beneath, separated by the CSF, which circulates and diffuses waste products into the venous sinuses.
What are the three layers of protection for the CNS
Bone, meninges and cerebrospinal fluid from the outside in
Describe cerebrospinal fluid
An ultrafiltrate of plasma produced in the choroid plexus. Leaves the foramina at the base of the brain and spread over the brain in the subarachnoid space and down the length of the spinal cord. It’s absorbed into the blood via a pressure gradient from the arachnoid villi
What are the functions of CSF?
Acts as a cushion/buffer. Provides buoyancy to the brain. Excretion of waster products one way to venous system. Transports hormones and nutrients to other parts of the brain.
