Spinal cord tracts

peripheral nervous system

2 types of info

Afferent (sensory)

Somatic

-info taken from skin, retina or ear(membranous labyrinth)

Visceral

-info taken from thoracic and abdominal organs, olfactory epithelium or from taste buds

Efferent (motor)

Somatic

- to skeletal muscle

Visceral

-to cardiac muscle- to smooth muscle- to exocrine glands

spinal pathways

All spinal pathways involve a sequence of neurons

Excitability is transmitted from one neuron to the next in the sequence

Pathways are either ascending (carrying information from receptors to the brain) or descending (conveying information from the brain to spinal cord neurons)

spinal pathways

Tract is a bundle of functionally related axons in the CNS

Tracts are found in the white matter (white cos myelinated neurons for insulation)

Tracts carrying sensory information are ascending, and motor commands are descending

And divides into dorsal, lateral, and ventral funiculus (or columns)

Nerve tracts are named according to their origin spinothalamic tract etc.

These funiculi have a common origin, destination, and function

pic shows I = dorsal funiculus, II is lateral funiculus, III is ventral funiculus = white matter

in the grey matter we have dorsal horn lateral horn and ventral horn

ascending tracts - just need to know spinothalamic tract not the others

each tract is named from origin

Begin with the primary afferent neuron- terminate in the brain

Information about pain is carried in – spinothalamic tract

Spinocerebellar tracts in which funiculus? lateral and entral part of the column

spinothalamic pathway (pain and temperature)

Information about pain is carried in – spinothalamic tract

Begin with the primary afferent neuron (first-order neuron) - info arriving from environment, taken by first order neuron, synapsing in spinal cord, then second info transfered to the brain

Terminate in the brain

most correct term is pseudounipolar - but one axon

spinothalamic pathway cont

Pain is transmitted from primary afferent axons to the spinal

cord dorsal horn (marginal nucleus or nucleus proprius)

Throughout the length of the spinal cord, projection neurons are concentrated in the marginal nucleus and nucleus proprius of the dorsal horn

Primary order neurons synapse with secondary neurons

Axons of second-order neurons cross the midline and join other axons that also carry pain sensation

These axons form the Spinothalamic tract

spinothalamic patway cont

The spinothalamic tract to reach the contralateral thalamus (third- order neuron)

Thalamic projection neurons send axons to the somesthetic area of the cerebral cortex

Axons terminating in the lateral thalamus mediate discriminative aspects of pain

Axons terminating in the medial thalamus mediate the motivational-affective aspects of pain (emotional aspects and attention to and memory of pain

spinocervicothalamic pathway

Spinocervicothalamic tract (spinocervical) - stops in cervical region

Important (most) in cats and dogs - carnivores

Concerned with the transmission of superficial pain and tactile sensations, considered the primary conscious pathway in carnivores

(Spinoreticular tract is primarily concerned with deep pain and visceral sensations)

synpases at lateral cervical nucleus then it will cross

First order neurons: Spinal Ganglion

Second-order neurons: Marginal Nucleus or Nucleus Proprius

Axons of these second-order neurons ascend ipsilaterally to the upper cervical spinal cord to synapse on third-order neurons located in the lateral cervical nucleus

Axons from third-order neurons in the lateral cervical nucleus cross the midline and ascend to the contralateral thalamus to terminate on fourth- order neurons

Axons of the fourth- order neurons project to the cerebral cortex

motor systems

Motor system function

Maintain posture and gait

Provide a stable platform for movement

Voluntary movement and locomotion

Visceral motor function

Somatic motor activity is regulated by separate groups of nerve cells, designated the lower and upper motor neurons

descending tracts

Nerve tracts are named according to their origin

Caudally projecting neurons generally terminate on interneurons

Rubrospinal tracts

Vestibulospinal tracts

Reticulospinal tracts

Tectospinal tracts

Corticospinal tracts

Corticobulbar tracts

Motor systems often functionally grouped into two categories:

Ventromedial motor system- axial and proximal limb muscles, especially extensors

Dorsolateral motor system- Distal limb muscles, especially flexors

descending motor tracts

Rubrospinal , Vestibulospinal, Reticulospinal, Tectospinal tracts

Caudal projection tracts that originate from brainstem nuclei

These four tracts are often referred to as the extrapyramidal motor system (pass through the medulla oblongata outside the ventral pyramids- called pyramids cos more tracts passing that region)

It is the most important system in domestic species

Rubrospinal tracts- voluntary and skilled movements in non-primate mammals, very important in quadrupeds

Vestibulospinal tracts- subconscious posture control- keeping the body subconsciously upright against gravity

Reticulospinal tracts- subconscious posture control- muscle tone

Tectospinal tracts- essential for automatic orientation of the head and eyes to auditory and visual stimuli

descending tract rubrospinal tract example

dorsal lateral spinal chord of white matter

all of them originate brain stem

corticospinal tract - descending motor tract

Originates in the cerebral cortex

Fine motor skills requiring concentration and conscious thoughtprimates and carnivores

Voluntary skilled movements are mainly dependent on this tract

Most elaborate and dexterous voluntary movement

Axons pass through the ventral portion of the medulla oblongata in a region characterised by pyramid section, hence named the pyramidal system

pic shows extrapyrimadal is more developed in domestic specieis like horse e.g. rubrospinal tract more important

spinal cord tract functions

Connects with spinal nerves, through afferent & efferent axons in spinal roots

Communicates with the brain, by means of ascending and descending pathways that form tracts in spinal white matter – serving as a link

Gives rise to spinal reflexes, pre-determined by interneuronal circuits- integrating reflex activity

lower motor neurons

Located within the ventral column of the gray substance of the spinal cord and within the somatic motor nuclei of cranial nerves that contain somatic efferent components

Their axons are conveyed within the spinal and relevant cranial nerves to the skeletal muscles, where each terminates on a group of muscle fibers

LMNs provide the efferent limbs of simple reflexes, but are mostly directed by upper motor neurons

Their axons project into the peripheral nervous system via cranial or spinal nerves to connect with muscle

When stimulated LMN induces muscle contraction

‘Workers ‘

upper motor neurons

UMNs are completely contained within the central nervous system

The UMN cell body is located in the motor nucleus of the brain stem or the motor cortex of the forebrain

Their axons connect to the lower motor neurones either by synapsing on them directly or indirectly, via interneurons

Upper motor neurons do not project onto muscle fibers

UMNs initiate, regulate, modify and terminate the activity of the LMN

UMN may inhibit or facilitate LMN

‘managers’

LMNs make connection to the muscle

UMNs connect from the brain

they synapse

upper and lower motor neurons

the alpha motor neuron is the principal or most common type of lower motor neuron (LMN)

Disease of lower motor neurons causes stereotypical clinical signs

Upper motor neurons lie completely in the central nervous system and control lower motor neurons

Signs of upper motor disease differ from signs of lower motor neuron disease

reflexes

A reflex is an inherent, subconscious, relatively consistent response to a particular stimulus

Reflex arc involves sensory input, connection in the CNS to the UMN, the LMN, neuromuscular junction and muscle

There are number of different types of reflexes; somatic; autonomic etc.

Reflexes utilise inputs from exteroceptors, interoceptors or proprioceptors

They use cranial or spinal nerves and may affect striated or smooth muscle

Understanding reflexes and their modifiability is important in clinical settings

what five basic components do all reflexes contain?

Sensory receptor

Sensory neuron

Synapse in CNS

Motor neuron

Target organ (Effector organ)

spinal cord-medulla spinalis-reflexes

Examples of spinal reflexes, involving spinal nerves and the spinal cord, include:

Myotatic reflex (muscle stretch): muscle stretch is resisted by reflex contraction of the muscle

Withdrawal (flexor) reflex: limb flexes to withdraw from a noxious stimulus

Panniculus (cutaneous trunci) reflex: pricking skin triggers contraction of cutaneous trunci (panniculus) m.

Perineal (anal constriction) reflex: mild compression of the skin of the perineum or anus with forceps causes contraction of anal sphincter and flexion of the tail

Myotatic reflex (muscle stretch): muscle stretch is resisted by reflex contraction of the muscle

Patellar reflex- the most reliable pelvic limb reflex

Stretch stimulus on the tendon (1) travels via the afferent neuron (2) to the spinal cord

Impulse is then transmitted to the efferent neuron (3), which stimulates the quadriceps muscle (4)

Myotatic reflex (muscle stretch)- Patellar reflex

Motor component of this reflex arises from L4-L6 in the spinal cord, which is the origin of the femoral nerve

The femoral nerve innervates the quadriceps muscle- effector muscle of this reflex

Simultaneously, it also causes reciprocal inhibition

Therefore, this reflex evaluates the integrity of the spinal cord segments (L4–L6 and sensory/afferent and motor/efferent) as well as the femoral nerve

spinal cord reflexes

Withdrawal or flexor reflex: limb flexes to withdraw from a noxious stimulus

Test for primarily for the sciatic nerve and its spinal cord segments (L6, L7, and S1)

Elicited by applying a noxious stimulus to the distal part of the limb

Observe withdrawal ( flexion) of the entire limb

Initiated by free nerve endings- axons enter dorsolateral fasciculusbifurcate

Collateral branches enter gray matter to synapse on interneurons and projection neurons (other limbs will extend to compensate if standing

Panniculus (cutaneous trunci) reflex

pricking skin triggers contraction of cutaneous trunci (panniculus) m.

The sensory pathway from the skin enters the spinal cord and ascends bilaterally to the C8 to T1 spinal cord segment, where it synapses with the lateral thoracic nerve, resulting in a contraction of the cutaneous trunci muscles bilaterally

Perineal (anal constriction) reflex

mild compression of the skin of the perineum or anus with forceps causes contraction of anal sphincter and flexion of the tail

Evaluates S1 to S3 spinal nerves and, peripherally, the pudendal nerve

brain and spinal reflexes

Most spinal reflexes can be overridden at least temporarily by higher brain centers

Impulses may be sent down descending pathways to the efferent neurons supplying the involved muscles to override the input from the receptors, actually preventing the muscle from contracting despite a harmful stimulus